Engineering Fundamentals CBT

Printout of CBT Content for Reference Purposes Only

Reference CBT:

Basic Atomic and Nuclear Physics V 1.0

1019164

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 ▪ PO Box 10412, Palo Alto, California 94303-0813 ▪ USA

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Engineering Fundamentals CBT:

Printout of CBT Content for Reference Purposes Only

Reference CBT:

Basic Atomic and Nuclear Physics V 1.0

1019164

June 2009

EPRI Project Manager

Ken Caraway

iv

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

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Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc.

Copyright © 2009 Electric Power Research Institute, Inc. All rights reserved.

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PRODUCT DESCRIPTION

Summary

This document provides a printout of the CBT content for use as a reference document only.

Students are encouraged to use the CBT as animations, flash video, and interactive features are

intended to enhance their learning experience.

NOTE: The CBT should be used to validate information as errors may have been introduced

when converting the graphics, equations, etc.

Abstract

The Basic Atomic and Nuclear Physics Version 1.0 module of Engineering Fundamentals

provides a basic overview of this topic applicable to all engineering disciplines beginning their

career in the nuclear power industry.

Description

The Basic Atomic and Nuclear Physics module covers basic atomic structure, fission,

radioactivity, reactor operation, and nuclear safety. This course will help new engineers

understand how their work might impact reactor operations and nuclear safety. This module is

intended for use as orientation training for new engineering support personnel.

Software Requirements

Windows 2000 SP2, Windows XP, Windows Vista

Application, Value and Use

Allows engineering support personnel to review the content when they desire and at their own

pace

Uses interactive features and graphics to illustrate key concepts & enhance training

Keywords

Training

Fundamentals

Atomic structure

Nuclear physics

Radioactivity

Fission

Reactor operation

Reactor systems

Nuclear safety

ACKNOWLEDGEMENTS

EPRI would like to acknowledge the following individuals for their active participation and significant contributions toward the development of this training course:

Ken Caraway EPRI Jack Feimster Exelon Corporation Nate Granger Wolf Creek Nuclear Operating Corporation Beth Hughes Handshaw, Inc. Don Lesnick Exelon Corporation Joe Montague Dominion Henry Nicholson Duke Energy Corporation Liz Sisk EPRI Terry Stuchlik Wolf Creek Nuclear Operating Corporation

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CONTENTS

1 INTRODUCTION TO BASIC ATOMIC AND NUCLEAR PHYSICS ......................................1-1

2 BASIC ATOMIC STRUCTURE ..................................... ERROR! BOOKMARK NOT DEFINED.

3 THE FISSION PROCESS AND NEUTRON INTERACTIONSERROR! BOOKMARK NOT DEFINED.

4 RADIOACTIVITY .......................................................... ERROR! BOOKMARK NOT DEFINED.

5 REACTOR OPERATION .............................................. ERROR! BOOKMARK NOT DEFINED.

6 NUCLEAR SAFETY ..................................................... ERROR! BOOKMARK NOT DEFINED.

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1 INTRODUCTION TO BASIC ATOMIC & NUCLEAR PHYSICS

Introduction

Welcome to the Basic Atomic and Nuclear Physics course. In this course, you will learn about basic atomic structure, radioactivity, fission, reactor operation, and nuclear safety. Regardless of your discipline, you can make changes that affect reactor operations and nuclear safety in your day-to-day job. Changes that may affect operations include instrumentation and control changes, electrical and/or mechanical system alignment or availability, and introduction of foreign material (items unintended for system use) including coatings inside containment. You must be aware of how all equipment and systems in the plant might be impacted by what you and others do.

After completing this lesson, you will be able to:

Describe how a nuclear power plant generates electricity

Describe the design differences between the two types of reactor systems typically used in the United States

If you are not familiar with the navigation features used in this course, click the About tab to review the navigation information.

Nuclear Steam Supply Systems

First, let’s take a high-level look at how a nuclear power plant works. The purpose of a nuclear power plant is to produce steam that is used to generate electricity. The underlying concept behind this Nuclear Steam Supply System (NSSS), as it is called, is straightforward. Nuclear fission indirectly creates heat, which is then used to generate steam. From this point, a nuclear power plant functions very much like a fossil fuel generating plant. The steam flows through turbines, rotating a shaft, which then turns the generator to produce electricity.

Each component and system in a nuclear power plant is designed and must be operated to ensure that fission remains a safe, economic, reliable source of power. Although the basic premise is simple, the NSSS is a complex system requiring a fundamental understanding of certain aspects of nuclear physics.

The diagram below shows the main components of a PWR NSSS system. Roll your mouse over each component to identify its function. (Note: This function will not work in this Word document.)

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Fuel

Let's take a closer look at how an NSSS works. The fuel for an NSSS is typically enriched uranium that fissions easily with proper geometry and moderation. For U.S. nuclear plants, fuel is typically contained in small, cylindrical pellets, which measure approximately 0.4 inch in diameter by 0.5 inch long. Pellets have small diameters so that heat can be removed effectively. One pellet of uranium can generate as much electricity as 4 barrels of oil or 1.3 tons of coal. Clearly, the reactor fuel has high power density (power per unit volume) compared to fossil fuels.

The pellets are stacked one on top of another to form a 12-foot long column and placed into a sealed tube. The tube is called a fuel rod and is cladded with a wall thickness typically about 0.04 inches thick. The cladding is made of a zirconium alloy that transmits heat, absorbs few neutrons, and is non-corrosive, characteristics important for maintaining the integrity of the tubes while efficiently transferring the heat and neutrons produced by fission. The cladding provides the first layer in the defense-in-depth design for keeping radioactive fission products contained. A set of fuel rods is bundled in a square lattice called a fuel assembly. Assemblies are placed side-by-side in a nearly cylindrical array inside a large steel vessel called the reactor vessel, which is part of the Reactor Coolant System boundary - the second barrier to radioactivity. The total number of fuel assemblies depends on the type and power level of the reactor.

Moderator and Coolant

Interactions of free neutrons with nuclei in the fuel can result in fission. Fission occurs when a free neutron collides with a nucleus, is absorbed by the nucleus, and causes the nucleus to split into two fragments and emit additional neutrons. During fission, energy is released in the form of heat. If the newly emerged neutrons cause additional fissions, a chain reaction results. Self-sustained fission chain reactions generate a steady supply of heat, the source of nuclear power in an NSSS.

However, neutrons born from fission typically have high kinetic energy and tend not to cause fission. Therefore, to facilitate fission of uranium, neutrons must be slowed down. This is achieved by using a moderator, which surrounds the fuel rods. Fast neutrons bounce off nuclei of similar mass in the moderator. With each collision, the neutrons lose some of their kinetic energy and start slowing down.

In U.S. reactors, the moderator is water, which also serves as the coolant. The heat from fission transfers from the fuel rods to the coolant, which is used for steam generation, but also keeps the fuel from overheating. Reactors that use "light" water (no neutron in the hydrogen atom) as moderator and coolant are called light water reactors, or LWRs. In other countries such as Canada, other media are often used for the coolant and moderator, including deuterium (known as "heavy water"). In those cases, the moderator and coolant may be located in physically separate volumes.

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Controlling Fission Rate

While continuous fission is necessary for power generation, the rate at which power is produced must be controlled. Too many fissions can generate too much heat, while too few can produce an insufficient power output. The number of fissions per second—the fission rate—can be controlled by introducing or withdrawing a substance that absorbs neutrons. Of the several neutrons generated in each fission, only one is needed to cause another fission and maintain a steady chain reaction. Therefore, by using a neutron absorber, you can "collect," or absorb, extra neutrons, which slows the fission rate. If power production is low, you can remove the neutron absorber so more neutrons are available, which quickens the fission rate.

One method for absorbing neutrons is to use control rods. They are made of a substance such as silver, indium, hafnium, cadmium, or boron that readily absorb neutrons, making them unavailable for future fissions within the fuel. Control rods are partially or fully inserted or withdrawn from the reactor core as needed to start up the reactor and to increase or decrease power. They can be inserted completely to shut down the reactor for refueling, or in case of an automatic scram or emergency condition. The control rods are either moved through the reactor vessel from the top, as in a Pressurized Water Reactor (PWR), or from the bottom, as in a Boiling Water Reactor (BWR). Other prevalent methods for controlling the fission rate are:

Using boric acid in PWRs - Boric acid is added to or diluted from the moderator before the moderator enters the reactor. Like control rods, boric acid absorbs neutrons.

Increasing pump speed on BWRs - Increasing the pump speed sweeps steam bubbles from the core more rapidly. This increases neutron moderation, and thus increases the population density of available slow neutrons.

Shielding

The nuclear reactor consists of the reactor vessel and everything inside it, including the core, support structures, and other components. The core is the heart of the reactor, the part that generates heat. It consists of the fuel assemblies, the coolant, the moderator, and the control rods. Fission in the reactor core not only creates heat, it produces significant amounts of nuclear radiation and radioactive byproducts harmful to people and the environment. To safeguard workers inside the plant, a concrete biological shield surrounds the vessel, while other individual components that may contain radioactive materials are shielded by the reactor building wall or other concrete structures.

In addition to the fuel cladding and reactor coolant system boundary (includes the reactor vessel) preventing the release of radioactive materials into the environment in the event of an accident, the NSSS is completely enclosed in a steel-lined, concrete containment barrier, which in many nuclear plants is the large dome visible on the site. Extensive additional systems and design features, which vary by reactor type, contain and prevent accidental releases of radiation.

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Steam Generation

Once fission starts to heat up the coolant in the reactor, what happens next? Depending on the type of reactor, the heated water is pumped to a heat exchanger, as in this example, or is allowed to boil within the reactor itself.

Click each picture below to view a larger version.

In a Boiling Water Reactor, or BWR, bulk boiling occurs in the upper part of the reactor and nucleate boiling in the lower portion of the reactor core.

In a Pressurized Water Reactor, or PWR, high pressure keeps the water in the reactor (called the primary coolant) from bulk boiling by raising its boiling point (localized nucleate boiling can still occur). The pressure is controlled by a pressurizer. This highly pressurized and heated primary coolant is pumped to a heat exchanger called a steam generator. The steam generator is a large, cylindrical steel vessel containing water at a lower pressure (called the secondary coolant). The high pressure primary coolant flows through tubes in the steam generator and heats up the surrounding lower-pressure secondary coolant, causing it to boil. While remaining isolated in the primary coolant loop, after transferring heat energy to the secondary coolant, the primary coolant flows back into the reactor to be reheated.

Power Reactor Types

As you've already seen in this lesson, PWR systems and BWR systems generate steam in different ways.

In the table below, click each button on the left to learn about more differences between the two systems.

Design

Steam Production

Reactivity Control

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Creating Electricity

From the point after the steam is created, a nuclear power plant is very similar to a fossil fuel power plant. The steam that has been created in the steam generator (PWR) or in the reactor (BWR) will then flow to the steam turbines. The turbines then turn a shaft as the hot steam expands through them. Large nuclear power plants typically have one high pressure and three low pressure turbines. The turbine shaft is coupled to an electrical generator, which converts the mechanical energy of the turbines to electrical energy. About one-third of the fission heat is converted to electricity in U.S. nuclear generating stations.

Cooling

After passing through the turbines, the steam is exhausted into a condenser, which consists of hundreds of tubes designed to remove the latent heat of vaporization. The steam condenses and is termed condensate, as it passes around the tubes, which are filled with cool circulating water. The condensate is pumped back to the steam generator in a PWR, called the secondary loop, or to the reactor in a BWR. The steam cycle is now complete.

The heat transferred from the steam to the water inside the condenser tubes must be removed. To remove it, the heated condenser water is pumped to a cooling tower or a cooling canal, where it cools by evaporation. At that point, it is either reused for cooling, as in a closed cooling cycle with a cooling tower, or it is discharged to an impoundment or stream according to state and federal requirements on thermal discharge. In some locations, state and federal environmental regulations allow the cooling water from the condenser to be pumped directly to a body of water, such as a river or ocean.

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Review Question 1

Match the nuclear power function in Column 2 with the component in Column 1.

Moderator A. Rotated by expanding steam to produce mechanical energy Control rods B. Controls the reactor power and also provides a mechanism for rapid shutdown

of the reactor Biological Shielding C. Slows down fast neutrons Steam generator D. Prevents the release of radioactive materials into the environment in the event

of an accident Turbine E. Converts the secondary coolant in a PWR into steam

The correct matching sequence is CBDEA.

Review Question 2

In a BWR, bulk boiling occurs in the upper region of the core, not in separate steam generators. True or False?

The correct answer is True. In a BWR, bulk boiling occurs in the upper region of the core, while in a PWR, bulk boiling occurs in the steam generator.

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Conclusion

You have now completed the Introduction to Basic Atomic and Nuclear Physics lesson, and have learned about the fundamental concepts of nuclear power generation.

Now that you have completed this lesson, you can do the following:

Describe how a nuclear power plant generates electricity

Describe the design differences between the two types of reactor systems typically used in the United States

In the next lesson, you will learn about the basic atomic structure and the properties of a nucleus.

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2 BASIC ATOMIC STRUCTURE

Introduction

In the last lesson, you learned how a nuclear power plant creates electrical energy. In this lesson, you will learn about the characteristics that make an atom the key to nuclear energy. You will learn about the structure and properties of an atom, as well as the forces that influence it.

After you have completed this lesson, you will be able to:

Define the following terms: nucleus, proton, neutron, electron, isotope, atomic mass unit (u), electron volt (eV)

Given a nuclide symbol, identify the number of neutrons, number of protons, number of nucleons, chemical element, and mass number

Define the ground and excited states of a nucleus and, in general terms, explain how a nucleus changes energy levels

Describe the Line of Stability

Nuclear Energy

At the heart of every atom is an extremely dense, positively-charged nucleus, 20,000 to 200,000 times smaller than the overall size of the atom but containing essentially all of its total mass. The nucleus is a tiny conglomeration of particles that is in a constant tug-of-war between two competing forces—one pushing the nucleus apart (Coulombic repulsion), the other keeping it together (nuclear force).

Within this miniscule piece of matter is the door to massive energy—nuclear energy. Understanding the nucleus, its components, its properties, the forces influencing it, and the units and measurements used to describe it is the key to unlocking the door to nuclear power.

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The Atom

An atom is too tiny to be seen very clearly, even with an electron microscope. For this reason, models that explain physically observed atomic phenomena are used to describe an atom. The simplest of these, the Bohr model of an atom, describes a small nucleus where over 99.9% of the atom's mass is concentrated, surrounded by electrons that orbit the nucleus in shells of discrete radii. Imagine a penny in the center of a baseball stadium, and you have an idea of the nucleus's relative size compared to the rest of the atom. In fact, as a whole, an atom is mostly empty space.

Below is a picture of an atom. Click each labeled part to learn more.

Nucleus: The nucleus is the center of an atom that is comprised of a tightly-packed cluster of particles called neutrons and protons. It contains 99.9% of its mass and is comprised of neutrons and protons held together in a very small volume by nuclear force. Neutron: A neutron is a sub-atomic particle with no charge that has about the same mass as a proton. Proton: A proton is a positively-charged particle that, along with neutrons, comprise a nucleus. Protons have a positive charge exactly equal in magnitude to the charge of an electron. Electron: An electron is a negatively-charged particle with little mass that orbits the nucleus. An atom with an equal number of electrons and protons has a neutral charge. Electron Shell: An electron shell is the orbital area around the nucleus that corresponds to energy levels of electrons.

Atomic Elements

The number of protons in a nucleus determines the atomic element. There are currently 113 confirmed elements. An atom with only one proton is hydrogen (H). In fact, it is the only element that may not have any neutrons. An atom with two protons is helium (He), with three, lithium (Li), and so on. You can use the periodic table to look up information about elements. Click each element below to see a diagram of its structure.

Isotopes are atoms of the same element (same number of protons) that may have different numbers of neutrons. Every element can exist in the form of several different isotopes. Hydrogen isotopes, for example, include an atom with just one proton in the nucleus, another with a proton and a neutron, and yet another with one proton and two neutrons.

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Standard Notation

The standard notation of an atom is shown below.

Click each part of the notation to learn more.

When interpreting standard notations of atoms, you should remember the following:

The mass number (A) is the sum of the atomic number (Z) and the number of neutrons (N).

The number of neutrons (N) is usually excluded from the notation. It can be calculated by subtracting the atomic number (Z) from the mass number (A).

The atomic number (Z) may be left off of the notation because an element, as specified by its symbol, uniquely determines the number of protons.

Don't confuse the mass of an atom with its atomic number (Z).

X is the chemical symbol for the element. A, the mass number, is the total number of protons and neutrons. Z, the atomic number or charge number, is the number of protons. N is the number of neutrons.

Standard Notation, Continued

Remember, you can calculate the number of neutrons by subtracting the atomic number (Z) from the mass number (A).

Try calculating the number of neutrons in each element below. Then, click the element to show the answer.

Suppressing the atomic number and the neutron number in the notation, the above isotopes can also be represented as 1H, 6He, 12C and 235U. They are also sometimes written in X-A notation, as in H-1, He-6, C-12 or U-235.

The hydrogen atom has 0 neutrons. The helium atom has 4 neutrons. The carbon atom has 6 neutrons. The uranium atom has 143 neutrons.

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Sub-Atomic Notation

Sub-atomic particles have similar representations.

Click each particle below to view its notation.

Proton

This notation shows that the mass number for a proton is 1 and the charge for a proton is +1.

Neutron

This notation shows that the mass number for a neutron is 1, and the neutron has no charge.

Electron

The mass number of an electron is 0 and carries a -1 charge. Nuclei frequently interact in some way with many of the other particles they encounter. Balance equations use "isotope" notation to depict the reactants and the products of these nuclear reactions. The total mass numbers and the charge (atomic) numbers are unchanged in nuclear reactions and must balance on both sides of the equation.

Excitation and Energy

Like an orbital electron, a nucleus can exist at various, discrete levels of energy, each isotope having a characteristic set of levels. The lowest energy state is called the ground state. Higher energy states are called excited states. A nucleus gains excitation energy in many nuclear reactions and jumps to a higher energy level. An excited nucleus will release excess energy as radiation to return to its ground state. Like the energy levels, the excitation energy and the emitted radiation energy are discrete and will equal the difference in energy between the initial and final levels.

Click the Forward button in the bottom-right corner of the graphic at right to progress through each slide in the animation.

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Electron Volts

The energy involved in a nuclear reaction is so small that usual energy units such as Joule and Btu are cumbersome. Instead, the electron volt, eV, is used to describe energy at atomic and sub-atomic levels. One eV is equal to the amount of energy gained by an electron when accelerated through a potential difference of one volt.

Nuclear reaction energy is typically of the order of keV or MeV.

Atomic Mass

The mass of an atom is so small that it is usually expressed in terms of the convenient atomic mass units or u (amu). One u is defined as 1/12 the mass of a carbon-12 atom or 1.66054 x 10-24 grams. The mass of H, the smallest atom, is 1.0078250 u. The mass of a C-12 atom is 12 u. The tables below show the atomic mass of sub-atomic particles and a comparison of the atomic mass of the nuclei of three atoms.

Click each table to learn more.

Notice the masses of nuclear particles. Which has the greater mass, a proton or a neutron? The neutron is more massive. At less than one billionth of a gram, a proton or a neutron is still about 1,840 times more massive than an electron. The mass of an atom overall is only very slightly more than the mass of its nucleus, since essentially all of the mass is concentrated in the nucleus. Compare the mass and radii of three different nuclei—hydrogen, boron, and uranium. The size of a nucleus varies with the number of nucleons (protons and neutrons) it has. A nucleus with a small number of nucleons is called a light nucleus. One with a large number is a heavy nucleus.

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Since like charges repel, why aren't the positively-charged protons in a nucleus sent off in every direction? The nucleus, with its compact collection of nucleons, is held intact by a specific force. Which of the following forces do you think keeps a nucleus intact? A. Electrostatic attraction B. Magnetism C. Gravity D. Nuclear force

The nuclear force, or "strong force," is the attractive force between the nucleons that holds them very tightly together to form the nucleus. It impacts protons and neutrons equally and is independent of charge. Although it is the strongest force known, the nuclear force acts only over very short distances and has no effect outside the nucleus.

Binding Energy and Mass Defect

The nucleus is not easily broken apart. The stability of the nucleus is maintained by its binding energy. Nuclear binding energy is the energy that binds all the nucleons in the nucleus and, therefore, is also the energy required to break the nucleus into individual nucleons. Every time a nucleus fissions and splits into two, it is releasing a small part of its binding energy. The major component of the binding energy is the potential energy associated with the nuclear force between neighboring nucleons. Binding energy can also be explained by examining the mass defect.

The energy equivalent of this value can be determined by using Einstein's energy-mass equivalence equation, E=mc2.

A nucleus has less mass than the total masses of its parts (mass defect), but it has binding energy equivalent to the difference. Since energy and mass are interchangeable, nothing is really missing!

Binding energy can also be expressed in terms of its average value for any individual nucleon. Binding energy per nucleon is an index of a nucleus's stability: the larger its value, the more tightly bound the nucleons and the more stable the nucleus. The graphic at right shows an example of how binding energy is calculated for a deuterium (2H) atom.

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Line of Stability

Every isotope has unique characteristics, including atomic mass, binding energy, atomic percent abundance, and radioactivity data for radioactive isotopes. Information about isotopes can be found in the Chart of Nuclides (CN), (http://atom.kaeri.re.kr/). Below is a condensed graphical representation of the most commonly used properties of isotopes. Navigate to the website for a larger view of the chart.

Below is an image of the Chart of Nuclides. Click each section to learn more.

Nuclides to the right and left of the line of stability are unstable and strive to become stable through radioactive decay. Elements with Z>83 (bismuth) have no stable isotopes. Radioactivity explains much of what happens in a nuclear reactor. You will learn more about radioactivity in the next lesson. Nuclides to the left of the line of stability generally have an excess of protons compared to the stable nuclides. Stable nuclides (blue squares) are clustered in a narrow band called the Line of Stability. They have a balanced number of neutrons and protons because at small numbers, the nuclear force is strong enough to counteract the Coulombic repulsion. However, with more protons in the nucleus, the Coulombic repulsion is more difficult to counteract. Therefore, larger stable nuclei tend to have more neutrons than protons. Neutrons increase the distance between protons, acting as buffers among them. Nuclides to the right of the line usually have an excess of neutrons and are considered neutron-rich.

Review Question 3

Below is a diagram of the standard notation of an atom. Match the description in Column 2 with the letter in Column 1.

X A. The chemical symbol for the element A B. The number of neutrons Z C. The atomic number, or the number of protons N D. The mass number, or total number of protons and neutrons The correct matching sequence is ADCB.

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Review Question 4

An excited nucleus will release excess energy as radiation to return to its ground state.

True or False? The correct answer is True. An excited nucleus will release excess energy as radiation to return to its ground state.

Review Question 5

What is the force trying to pull a nucleus apart?

A. Nuclear force B. Coulombic repulsion C. Binding energy D. Kinetic energy

The correct answer is B. Coulombic repulsion is the electrostatic force that makes particles with like charge repel each other along a straight line between their centers.

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Conclusion

You've now completed the Basic Nuclear Structure lesson and have learned about the properties and characteristics of an atom.

Now that you've completed this lesson, you can:

Define the following terms: nucleus, proton, neutron, electron, isotope, atomic mass unit (u), electron volt (eV)

Given a nuclide symbol, identify the number of neutrons, number of protons, number of nucleons, chemical element, and mass number

Define the ground and excited states of a nucleus and, in general terms, explain how a nucleus changes energy levels

Describe the Line of Stability

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3 THE FISSION PROCESS AND NEUTRON INTERACTIONS

Introduction

Now that you've learned about the structure and properties of an atom, you'll learn about the process that makes nuclear power possible - fission. A nuclear reactor is designed to sustain the fission process in a safe and controlled manner while transferring the heat energy and containing the radioactive fission products. This lesson will describe the neutron interactions that initiate and sustain the process, and how the process is controlled.

After you have completed this lesson, you will be able to:

Describe the fission process

Describe the life cycle of a neutron

Define criticality and reactivity

Describe how a chain reaction is controlled and maintained

State the difference between fuel and non-fuel absorption (poisoning) and a scattering (moderation) type of neutron interaction

The Fission Process

When a nucleus absorbs a neutron, it gains “excitation” energy from the neutron’s mass and kinetic energy. In certain heavy isotopes, this added excitation energy may put the “compound” nucleus over its critical energy threshold. This may induce the nucleus to split, or fission, into two lighter radioactive nuclei (also called fission fragments, fission products), or daughters.

During fission, some of the total mass of the original heavy nucleus and absorbed neutron is lost in the reaction. In accordance with Einstein's equation (E = mc2), this lost mass “defect” is converted to about 200 MeV of energy. Fission also releases two to three other (fission) neutrons along with heat energy and radiation.

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Neutron Moderation

Neutrons born from fission typically have high kinetic energy (are "fast"). A very small fraction of the high energy neutrons cause “fast fission” in U-238. As neutrons slow down, the probability of a neutron causing fission (cross section) in certain heavy nuclei increases. This probability is greatest when neutrons slow down to energies that are in thermal equilibrium with their surroundings (i.e., the fuel nuclei).

As you learned earlier in this course, U.S. reactors incorporate a moderator to slow down fast neutrons and facilitate fission. The moderator, which is water in the U.S., surrounds the fuel rods to slow neutrons down to thermal equilibrium with the U-235 fuel material, in order to undergo thermal fission. When small hydrogen atoms collide with a neutron, they “scatter.” Much of the neutron’s energy is lost, similar to when two billiard balls have an elastic collision. While not absorbed, the “moderated” neutron loses energy and slows down toward thermal energy levels.

Click the Forward arrow in the bottom-right corner of the graphic at right to progress through each slide in the scattering animation.

Fission Fuels

Because they can fission with low energy thermal neutrons, certain isotopes of Uranium and Plutonium (i.e., U-235, Pu-239, and Pu-241) are used in U.S. reactors. These isotopes, displayed in red in the table at right, are referred to as fissile. Because they readily fission, they make excellent fuels.

U-235 Enrichment

Uranium is the most widely used isotope. It occurs naturally all over the world; however, less than 1% of natural uranium is fissile U-235. In order to use its fissile quality to sustain fission reactions, the concentration of U-235 atoms in U.S. reactor fuel designs is increased to approximately 4-5% U-235 by weight through enrichment. The balance of these fuel rods is composed almost entirely of 95-96% U-238 isotopes.

Plutonium Generation

While most of the fission energy comes from enriched U-235 fuel, in U.S. reactors, a significant amount of energy comes from Pu fission. Over time, non-fissile isotopes, such as U-238 (called fertile isotopes, displayed in blue in the table at right), capture neutrons and decay to fissile isotopes, such as Pu-239. Since approximately 95% of the fuel is U-238 conversion to fissile Pu isotopes, or breeding reaction, is natural.

Click here to see a diagram of a breeding reaction.

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Sustaining the Fission Chain Reaction

As the 2-3 neutrons created in the fission process are slowed down, they can fission other heavy nuclei in the reactor. But, every neutron does not necessarily cause fission in the fuel. Some neutrons escape from the reactor, and others may be absorbed by other reactor materials (such as the moderator, cladding, control rods/absorbers, or structural materials). The ratio between one generation of neutrons (n1) and the previous generation (n0) is denoted by k, the multiplication factor, as shown below.

Criticality

In order for the production of neutrons and a fission reaction to be sustainable, it is imperative that k = 1 where the neutron population (or power) stays constant and the reactor is exactly critical. When k is greater than 1, the number of neutrons grows with time and the reactor is supercritical. When k is less than 1, the number decreases with time and the reactor is subcritical.

Click here to see the multiplication factor equation from the previous page.

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Neutron Multiplication - Six Factor Formula

From fast, high-energy birth, there are six major factors that affect neutron multiplication (k).

Six Factor Formula: Since n1/n0 is the multiplication factor, k, the equation can be re-arranged and written in its usual form as k = η ε ρ f Pf Pt. Thus, this expression for k, which is the product of the six factors defined above, is called the six-factor formula.

ε, Fast fission factor:

This factor determines how many additional fission neutrons are generated by fast (non-thermal) fissions. In addition to the generation of fissile plutonium, a very small number of “fast” fissions occur in U-238, while the neutrons are still fast or at high energy.

Pf, Fast non-leakage probability:

Some fast neutrons escape through the boundaries of the reactor core before they start to slow down. Pf is the probability that fast neutrons do not escape. In other words, 1-Pf

is the probability of escape. So there are n0 ε Pf fast neutrons that remain inside the

reactor and try to become thermal neutrons.

ρ, Resonance escape probability:

Probability a neutron is not absorbed as they slow down to thermal energy. As the remaining neutrons continue to slow down, some will fall into energy resonance with the surrounding material and be captured. Those that escape resonance capture will slow down and reach thermal equilibrium with the fuel material.

Pt, Thermal non-leakage probability:

Probability a neutron does not leak out of the core while thermal. A small fraction of the thermal neutrons will escape from the reactor. The others will get absorbed in fuel or other reactor materials (moderator and control rods).

f, Thermal utilization factor:

This is the fraction of thermal neutron absorptions that occur in the fuel as opposed to in any material in the reactor core. Since that will include absorptions in the control rods or control poisons, f is primarily the factor that can be controlled by the reactor operators. f is typically in the range of 0.75 ± 0.5 for an LWR.

η, Reproduction factor:

The remaining neutrons that cause fission produce the next generation (n1) of two to three fission neutrons for every neutron absorbed by the fuel.

Delayed Neutrons

Most, but not all, of the n1 generation neutrons come directly from fission. Fission can be thought of as a two-stage process. In the first stage, called the prompt stage, the nucleus splits emitting fragments, neutrons, energy, and radiation. In the second, known as the delayed stage, several seconds after fission on average, a small number of the fission products release an additional delayed neutron. U.S. reactor designs use delayed neutrons to sustain criticality. This time delay using neutrons from previous generations provides an inherent safety control mechanism for the reaction.

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Reactivity

In addition to the delayed neutrons, other methods are used to control reactivity in LWRs. Change in the reactor that affects neutron multiplication is called reactivity (ρ). Reactivity is a measure of the departure of a reactor from criticality and is described as the percentage difference in the value of k from one, or:

Reactivity can be positive, zero, or negative. When the multiplication factor, k, is exactly one, the reactor maintains a steady, self-sustaining chain reaction. If k is less than 1, and the reactor must be made critical, the difference, k-1, must be compensated for by using positive reactivity. Thus, the magnitude of Δk, which is equal to (k-1), is a measure of the compensation required to make a non-critical reactor become critical.

Positive (or withdrawing control rods) reactivity (e.g., creating Pu-239) increases neutron multiplication and moves the reactor towards supercritical. Negative (or inserting control rods) reactivity (e.g., depleting U-235 fuel) takes away neutrons and moves the reactor towards subcriticality.

Another example of reactivity change is referred to as the Reactivity Coefficient. An example is the Moderator Temperature Coefficient, expressed as reactivity per degree change in moderator temperature. Temperature affects water density and changes the number of moderating atoms in a given volume. This changes the multiplication factor. In BWRs, the control of water flow changes the amount of steam production and density of the coolant.

Controlling Fission

Materials that absorb or capture neutrons are used in the reactor to control the chain reaction. Control rods are inserted or withdrawn from the reactor to control the power level or shut down the reaction. The insertion of a control rod is another example of adding negative reactivity. Conversely, removing the control rods removes negative reactivity. Control rods are positioned in terms of resulting reactivity changes per distance moved. This facilitates operator-generated reactivity changes through control rod movement.

In PWRs, the neutron poison, boron, is dissolved throughout the reactor core in the moderator/coolant. This helps control the neutron population. The boron absorbs neutrons, thus reducing the possibility of fission. This is another example of negative reactivity. However, as the boron burns out, it introduces a positive reactivity effect.

Some fuel assemblies may also have an additional burnable poison to compensate for the high positive reactivity of the new fuel in the core after refueling. This neutron absorbing material (e.g., gadolinia or boron) may be part of the fuel material, coated externally on the fuel pellets, or inserted as discrete rods in the fuel assembly.

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Neutron Life Cycle

Throughout this lesson, you've learned that in a fission chain reaction, neutrons are born, live for a short while, may beget new neutrons, and then die. What important events mark the life cycle of fission neutrons inside an LWR? The graphic below shows everything that can happen to such neutrons from the time they are born from thermal fissions in fissile fuel nuclei to when they complete their life cycle and produce the next generation of fission neutrons.

Review Question 6

A nuclear reactor is designed to do which of the following?

A. Sustain the fission process B. Shut down the fission process C. Transfer heat energy D. Contain the radioactive fission products

The correct answers are A, C, and D.

A nuclear reactor is designed to sustain the fission process in a safe and controlled manner, while transferring the heat energy and containing the radioactive fission products.

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Review Question 7

The ratio between one generation of neutrons and the previous generation is known as which of the following?

A. Criticality factor B. Multiplication factor C. Energy-mass equivalence D. Neutron life cycle

The correct answer is B.

The ratio between one generation of neutrons and the previous generation is known as the multiplication factor.

Review Question 8

Which of the following are examples of adding negative reactivity in the reactor?

A. Inserting a control rod B. Removing a control rod C. Introducing a poison D. Burning out a poison

The correct answers are A and C.

Negative reactivity takes away neutrons and moves the reactor towards subcriticality. Examples include inserting a control rod and introducing a poison.

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Conclusion

You have completed this lesson and learned about the neutron interactions that initiate and sustain the fission process, and how the process is controlled.

Now that you've finished this lesson, you can:

Describe the fission process

Describe the life cycle of a neutron

Define criticality and reactivity

Describe how a chain reaction is controlled and maintained

State the difference between fuel and non-fuel absorption (poisoning) and a scattering (moderation) type of neutron interaction

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4 RADIOACTIVITY

Introduction

Nuclear reactors use and generate a large amount of radioactive material. Radioactivity is the state describing unstable isotopes, which spontaneously emit sub-atomic particles and energy (radiation) from their nuclei. In this lesson, you will learn about radioactivity and radioactive decay, and how they affect nuclear power plant operation.

After you have completed this lesson, you will be able to:

Define the following terms: radioactivity, radioactive decay, radiation, radioisotope, half-life, and decay constant

Describe the three major modes of radioactive decay: alpha, beta, and gamma-ray emission

Determine the number of atoms of a specific radioisotope and their activity after a specified time using the radioactive decay law and half-life

Fission Products and Radioactivity

When they fission, fuel nuclei do not split evenly. As can be seen in the graph to the right, the fission products come out as a wide variety of neutron-rich isotopes ranging from 76 to 160 in mass number. The excess neutrons give the fission fragments too much energy and make them unstable. Unstable nuclei naturally release energy by means of radioactive decay, otherwise known as radioactivity or activity. The most common forms are Alpha and Beta, which are particles similar to neutrons, and Gamma, which are similar to X-rays.

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Alpha Decay

Typically, heavy radioactive nuclei such as U-238 and Pu-239 emit alpha particles. Alpha particles resemble the nucleus of a Helium atom with two protons and two neutrons. They are relatively big with a lot of mass and energy.

Because the alpha is such a relatively large particle, a lot of energy is released when heavy nuclei undergo alpha decay. However, because of their size, alpha particles are easily stopped by thin layers of shielding. A sheet of paper or the outer layers of dead skin will stop them.

Beta and Gamma Decay

The mid-sized fission fragment nuclei typically decay by beta particle emission. The beta particle emission is the decay of a nucleon, either an electron or an anti-electron (positron) from the nucleus. Electron emission is known as beta minus decay and positron emission as beta plus decay. The daughter products of the beta decays are also usually radioactive, and thus lead to a decay chain that will terminate ultimately in a stable isotope. It is because of these radioactive fission products that used fuel removed from the reactor must be isolated and shielded in order to minimize radiation exposure to the public and environment.

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Decay Rate

Like all radioisotopes, fission products decay at different rates and by different modes. One isotope may take billions of years to decay while another decays in seconds. Radioactive decay does not make nuclei disappear; they simply change to a more stable configuration. The decay rate of a fission fragment is determined by its half-life (t1/2), the time for half the atoms to decay. Half-life is an inherent, unchangeable characteristic of a radioisotope. The decay rate or activity (A) for a radioisotope depends on and is proportional to the current population of available atoms, N(t). This relationship is represented by the decay constant (λ) where λ = A/N. The initial (time = 0) activity A and population N0 of a radioisotope decreases exponentially over time. This radioactive decay law is expressed as:

or because they are proportional

There is a unique relationship between decay rates and half-life. In the radioactivity equation, if t = t1/2, the activity must decrease to half the original value or A(t)/A0 = ½. Therefore:

This means radioactivity decreases faster for isotopes with larger decay constants, which implies shorter half-lives. The shorter the half-life, the larger the decay constant and the faster N(t) or A(t) decreases. More highly radioactive fission products decay away more quickly. Fission products with longer half-lives typically have lower energy radioactivity. It is important to understand radioactivity and its implications, to ensure on-site work is performed with adequate protective measures.

Decay Heat

In addition to shielding requirements, another feature of nuclear power arising from fission product radioactivity is that, even after the reactor is shut down and the fission chain reaction has stopped, the fuel continues to generate decay heat. Decay heat generation impacts spent fuel handling and storage, waste management, and reactor safety at a nuclear plant. A typical LWR produces over 220 MWt of decay heat immediately upon shutdown. Since much of this early decay heat is from short-lived fission products, the heat production rate decreases sharply in the first few hours. But, following a reactor shutdown, adequate cooling must be available. Otherwise, the decay heat is sufficient to boil the core dry and melt the fuel. Reactor designs have specially engineered safeguards to provide such cooling.

You will learn more about decay heat in the next lesson, Reactor Operation.

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Review Question 9

Which of the following determines a fission fragment's decay rate?

A. Radioactivity

B. Decay heat

C. Beta particle emissions

D. Half-life

The correct answer is D.

The decay rate of a fission fragment is determined by its half-life, the time for half the atoms to decay.

Review Question 10

After the reactor is shut down and the fission chain reaction has stopped, the fuel is safe for handling with special precautions and safety measures (e.g., shielding and cooling).

True or False?

The correct answer is True.

Even after the reactor is shut down and the fission chain reaction has stopped, the fuel continues to generate decay heat, which impacts spent fuel handling and storage, waste management, and reactor safety at a nuclear plant. When the fuel is handled, it must be moved using proper shielding and tools.

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Conclusion

You have completed the Radioactivity lesson. You have learned what radioactivity and radioactive decay are and how they affect the operation of a nuclear power plant.

Now that you've completed this lesson, you can:

Define the following terms: radioactivity, radioactive decay, radiation, radioisotope, half-life, and decay constant

Describe the three major modes of radioactive decay: alpha, beta, and gamma-ray emission

Determine the number of atoms of a specific radioisotope and their activity after a specified time using the radioactive decay law and half-life

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5 REACTOR OPERATION

Introduction

So far in this course, you've learned about atomic structure, the fission process, and radioactivity. In this lesson, you'll learn how it all works together in the reactor to create and control nuclear energy.

At the end of this lesson, you will be able to:

Describe how temperature, pressure, voids, fuel constituents, control rods, boron concentration, and fission product poisons affect the fission process

Describe how reactor operation affects temperature, pressure, voids, fuel enrichment (burn-up or depletion), boron concentration, and fission product poisions

Describe how criticality and/or reactivity affect startup, power operations, and shutdown

Describe how changes in various balance of plant parameters affect reactor operations in a PWR and a BWR

Define decay heat and describe its affect on plant operations

Describe the significance of loss of coolant accidents (LOCA) in all plant conditions

Reactivity Feedback Mechanisms

The control of reactivity is an important function in reactor operations. Physical mechanisms, inherent to the design, that are affected by and in turn affect reactivity are called reactivity feedback mechanisms. Feedback can be positive or negative: positive feedback increases reactivity and negative feedback decreases reactivity. The following mechanisms affect reactivity in reactor operation.

Click each mechanism below to learn how it affects reactivity.

Temperature feedback: Temperature feedback results from temperature changes in the water (moderator) and the fuel. Temperature affects both the density of the moderator and the neutron absorption cross sections of the fuel, either of which can impact reactivity. Pressure feedback and void feedback: Pressure feedback and void feedback, of primary concern in BWRs, is due to the change in moderator density as a result of changes in the number of steam bubbles (or voids) in the coolant during power operation. Fuel enrichment: Fuel enrichment is affected by U depletion and Pu production, which alters the number of fissile atoms in the fuel. U-235, initially loaded in the reactor fuel at the beginning of the cycle, is depleted by neutron absorption, resulting in removal of positive reactivity. Pu-239 is produced from neutron absorption in U-238 and increases positive reactivity. Control rods: Control rods contain materials that absorb neutrons. Inserting more control rods into the core is an addition of negative reactivity, while withdrawal of control rods from the core provides a removal of negative activity. Burnable poison rods: Burnable poison rods also capture neutrons that would otherwise be absorbed in the fuel. They are placed in the reactor at the beginning of core life to compensate for the excess fuel necessary for the reactor to remain critical over the irradiation cycle. The material used to absorb the neutrons is gradually depleted (burned), resulting in a negative reactivity removal.

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Boron: Boron, in the form of boric acid, is used as a soluble poison in PWRs. The high positive reactivity of fresh fuel early in core life is offset by the negative reactivity of high concentrations of boric acid. As the fuel depletes over core life, the plant operators reduce the boron concentration. Fission product poisons: Fission product poisons capture neutrons that would otherwise be absorbed in the fuel, and therefore lower the multiplication factor. The production of fission product poisons is then a source of negative reactivity. Of primary concern are Xe-135 and Sm-149 due to their high neutron absorption cross sections.

Moderator Temperature Feedback

In LWRs, the moderator (water) and coolant are the same. Moderator temperature changes generally cause changes in reactor power. An increase in reactor power generally results in an increase in the moderator (coolant) temperature, which decreases the moderator density. In an under-moderated condition, the further reduction in moderator density decreases its effectiveness in neutron moderation, resulting in decreased reactivity (a negative effect). LWRs are designed to be under-moderated such that the moderator temperature feedback is negative.

Some PWRs experience an over-moderated condition early in core life due to the high boron concentration in the core. In an over-moderated system, a reduction in moderator density from increased coolant temperature reduces the number of boron atoms that act as absorbers, which increases reactivity (a positive effect). Thus, a temperature increase will continue to add positive reactivity, which increases temperature even more. This undesirable effect can be overcome by other feedback mechanisms, such as fuel temperature feedback.

Reactivity increases with moderator density until neutrons become thermal. After that, reactivity decreases with moderator density because neutron absorption increases.

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Temperature, Pressure, and Void Feedback

Fuel Temperature Feedback

Increasing the fuel temperature in an LWR increases resonance radiative capture in U-238, which constitutes over 96% of the uranium fuel, thereby decreasing reactivity. A decrease in fuel temperature has the opposite effect, resulting in an increase in reactivity. Opposing this effect is the possibility that the fuel temperature increase may also increase absorption in resonances in the fission cross section of the fuel. However, this effect is relatively small compared to resonance capture in U-238.

Pressure and Void Feedback

The effect of the presence of steam bubbles, or voids, in BWRs is similar to the moderator temperature effect in that the moderator density is reduced. In a BWR, increasing pressure decreases the presence of voids. Conversely, decreasing pressure increases the presence of voids. In under-moderated systems, the presence of voids effectively removes moderation of neutrons and lowers the reactivity. In an over-moderated system, just the opposite is true. The presence of voids removes excess moderator, lowering absorption and increasing reactivity.

U Depeletion and Pu Production

Positive reactivity is supplied by the U-235 that is blended into the U-238 in the fuel rods. The term "burn-up" is used to represent the consumption of the uranium and is quoted as MWd/Tonne U (megawatt days/metric ton of uranium). As the operating cycle progresses, the U-235 that is initially loaded in the core gets depleted by fission. In a reactor that is operating at steady power output, a good approximation is that the U-235 concentration, hence reactivity, decreases linearly with burn-up, a reduction in positive reactivity over the life of the core.

As the reactor operates at power, Plutonium (Pu) isotopes are produced from neutron absorption in the U-238 contained within the fuel rods. This increases the reactivity of the core because some of the fissile Pu isotopes (Pu-239 and Pu-241) undergo fission and produce energy. By the end of an irradiation cycle, about 40% of the reactor power is estimated to be due to the fission of these Pu isotopes. The Pu isotopes build up to equilibrium values with irradiation. The net contribution to reactivity from Pu production is positive, but is not large enough to offset the positive reactivity decrease caused by U-235 depletion. Overall, over the fuel cycle, the positive reactivity of the core decreases.

Click the Play button in the graphic at right to view U depletion.

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Fission Product Poisons

Fission products capture neutrons and lower reactivity. Many of these isotopes have fairly low absorption cross sections; however, two isotopes that occur near the second peak of the fission yield curve, Xe-135 and Sm-149, have very high absorption cross sections. Their accumulation in the fuel rods can cause major changes in reactivity. To fully understand their effects, let's look at their production and decay modes and how their concentration might vary with time in a reactor:

Xe-135 is produced in two ways: directly as a fission product, and also by the decay of I-135 (half-life = 6 h), which is produced by fission. Xe-135 has an incredibly high thermal neutron absorption cross section, which is the reason for its nuisance factor. It decays with a half-life of 9.1 h to Cs-135, which is practically stable. In a reactor operating at a steady power level, the Xe concentration is held at an equilibrium value by its destruction through radioactive decay, and through neutron absorption and conversion to Xe-136, which has a much smaller absorption cross section. If reactor power is decreased or the reactor shuts down, I-135 continues to decay to Xe-135, but Xe-135 destruction by neutron absorption stops. Since I-135 has a shorter half-life than Xe-135, this results in an initial increase in the Xe level and an accompanying addition of negative reactivity. After the relatively short-lived I-135 has decayed away, the production of Xe-135 stops, and the Xe-135 concentration decreases exponentially by radioactive decay. Xe can cause operational problems because of the large reactivity swings it can cause over relatively short periods of time. Click here to see an animation of Xe buildup after reactor shutdown.

Sm-149 is the decay product of Pm-149, a daughter of the fission product Nd-149. The half-lives of Nd and Pm are 2 h and 54 h respectively. Sm-149 is a stable isotope and is destroyed only by neutron absorption. Its thermal neutron cross section is also very high and thus, its build-up in a reactor adds significant negative reactivity. Since Sm is stable, it simply accumulates to some maximum value following power reduction or reactor shutdown. When the reactor is restarted and increases in power, the built-up Sm is destroyed by neutron absorption. Click here to see an animation of Sm buildup after reactor shutdown.

Reactivity Control in a PWR

Positive reactivity largely comes from the fuel or from the removal of negative reactivity through inherent or operator-initiated processes meant to offset the depletion of the positive reactivity in the fuel. Reactivity in a PWR is controlled in three ways.

Click the graphic at left to view a larger image of a PWR control assembly, then click each method of reactivity control below to learn more.

Control rods: Most control rods are made of an alloy of 80% silver, 15% indium, and 5% cadmium, provide quick power control. The control rods consist of rodlets (individual rods) connected to a “spider,” which form a single control rod assembly. Each assembly is moved by a separate control rod drive mechanism that drives each control rod assembly into the top of the core. About one-third of the fuel assemblies in the core will contain a control rod assembly. Burnable poison rods: Some fuel assemblies may also have burnable poison rods, usually made of gadolinia (Gd2O3) that are placed in unrodded fuel pin tubes. The gadolinia absorbs neutrons, thus compensating for the high positive reactivity of the core in the fuel cycle. That absorption of neutrons burns out the gadolinia, thereby reducing the negative reactivity during the cycle. Boric acid: Soluble boric acid serves as a chemical shim (changing boron concentration) in the primary coolant to provide longer-term control. Increasing the boron concentration via boration or decreasing the boron concentration via dilution provides somewhat uniform control of reactivity, both vertically and radially, in the core.

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Reactivity Control in a BWR

Reactivity in a BWR is also controlled in three ways.

Click the graphic at left to view a larger image of a BWR control assembly, then click each method of reactivity control below to learn more.

Control rods: Control rods, which enter the BWR core from the bottom, are used for large changes of power and to compensate for fuel depletion. The control rods are made of B4C (boron) powder packed inside thin rods. The rods are canned into a cruciform (cross-shaped) configuration. Each single cruciform control blade is associated with four neighboring fuel assemblies. They are operated using hydraulic controls and a locking piston arrangement that prevents accidental removal. Burnable poison rods: Some fuel assemblies may also have burnable poison rods, usually made of gadolinia (Gd2O3) that are placed in unrodded fuel pin tubes. The gadolinia absorbs neutrons, thus compensating for the high positive reactivity of the core in the fuel cycle. The absorption of neutrons burns out the gadolinia, thereby reducing the negative reactivity during the cycle. Recirculation flow: This method provides up to 25% power changes. Increasing the flow will decrease the number of steam bubbles within the core (void fraction), which will increase reactivity via increased moderation, thereby increasing power. Decreasing the recirculation flow will increase the number of steam bubbles in the core, displacing the water that was used for neutron moderation. The decreased moderation will decrease the positive reactivity, causing the power to decrease.

Reactivity Coefficients

Short term changes in reactivity, such as those due to temperature, are often quantified in terms of feedback reactivity coefficients. The reactivity coefficient is the change of reactivity per unit change in the parameter that alters the reactivity. Common reactivity coefficients allow quantification of the reactivity effects attributable to moderator temperature, fuel temperature, voids, and pressure. For example, a fuel temperature coefficient (αT) is defined as:

where Δρ is the change in reactivity and ΔT is a unit change in the fuel temperature.

If a reactor had a positive fuel temperature coefficient, an increase in fuel temperature would result in an increase in reactivity, and an associated increase in power. The power increase would then cause a further increase in fuel temperature, and so on. A positive fuel temperature coefficient would be detrimental to reactor operations.

Conversely, if a reactor has a negative fuel temperature coefficient, then an increase in fuel temperature would result in a reduction in reactivity, and a corresponding reduction in reactor power. The reduction in power would lower the fuel temperature, which would increase the reactivity and bring the reactor back to a constant power level.

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Reactor Startup

At the beginning of the startup, the control rods are fully inserted in the core, providing a large amount of negative reactivity to overcome the large amount of positive reactivity contained in the fuel. As the rods are withdrawn, the number of neutrons available to cause fission increases, which increases the number of fissions. The control rods are withdrawn incrementally until the positive reactivity in the core overcomes the remaining negative reactivity of the control rods. Removal of the control rods continues until the startup rate desired by the plant operators is achieved.

In this condition, the reactor is slightly super-critical, and will remain super-critical until the fuel temperature and moderator temperature begin to increase. As described earlier, those temperature increases will insert negative reactivity, which will offset the reactivity increase caused by the withdrawal of control rods, and will ultimately return the startup rate to zero. The reactor will then be critical at a much higher rate of flux than was present prior to rod withdrawal.

Achieving 100% Power

To achieve 100% power from startup condition in a PWR, the boron concentration in the reactor coolant system is reduced, which reduces the negative reactivity in the core, allowing the neutron flux, or power to increase. This increase will continue until the negative reactivity feedback increase from the moderator temperature and fuel temperature overcomes the reduction in boron concentration. The operators and reactor engineers are able to calculate the combined effects of the boron, moderator temperature, and fuel temperature to determine the power level at which the reactor will be controlled.

To achieve 100% power from a startup condition in a BWR, the control rods are withdrawn from the core, allowing the positive reactivity of the core to increase the neutron flux. At about 70% reactor power, variable speed recirculation pumps are used to increase the flow of water through the core. This increased flow replaces the steam voids in the core with water, which increases the moderation of the neutrons, adding positive reactivity to reach 100% power.

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Reactor Power Plant Operation

A reactor is critical when its reactivity is zero. If the change in a system parameter results in the addition of reactivity (positive or negative) then a corresponding change must occur in some other parameter to compensate for this reactivity insertion.

The effects on reactor operations caused by changes in the balance of plant parameters can be viewed from the perspective of the changes in moderator temperature and pressure. For example, a turbine load increase will increase the amount of thermal energy removed from the reactor coolant system, causing the reactor coolant temperature to decrease. With a negative moderator temperature coefficient, the drop in coolant temperature will insert positive reactivity. The positive reactivity insertion from the moderator temperature coefficient will cause the reactor power to increase, which will increase the fuel temperature. With a negative fuel temperature coefficient, this fuel temperature increase will insert an amount of negative reactivity that balances with the positive reactivity added by the moderator temperature. The reactor will continue operation with a reduced coolant temperature at an increased power.

In both a PWR and a BWR, this scenario can be created by increased removal of thermal energy via any of the following processes:

Feedwater termperature decrease

Feedwater flow increase

Steam flow increase

Void Fraction

Unlike in a PWR, the boiling water in the core of a BWR produces steam voids that affect reactivity. The effects on reactor operation caused by changes in balance of plant parameters can then be viewed from the perspective of the changes in the fraction of voids in the core. In a BWR, a decrease in the void fraction results in increased moderation of the neutrons because more water occupies the space between the fuel rods. An increase in neutron moderation causes an increase in flux due to an increase in power/reactivity.

Any of the following plant parameter changes will cause a decrease in the void fraction, thereby increasing power:

Feedwater temperature decrease

Feedwater flow increase

Steam temperature decrease

Steam pressure increase

Steam flow decrease

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Reactor Shutdown

To shutdown a PWR from a 100% power condition, the operators increase the boron concentration of the reactor coolant system. This addition of negative reactivity causes the reactor power to start decreasing. As the power decreases, the fuel temperature and moderator temperature also decrease, providing a feedback of positive reactivity. The amount of boron added to the coolant can be varied by the operators to reduce power to a pre-determined level or to completely shut down the reactor. Again, the combined effects of the boron, moderator temperature, and fuel temperature must be calculated to determine the power level at which the reactor will be controlled.

The build-up of Xe-135 during the power reduction must also be considered. As the power decreases, less Xe-135 is burned-up by the reduced neutron flux, but the I-135 continues to decay to Xe-135 at a rate commensurate with production at 100% power. In short, Xe-135 starts to add negative reactivity to the core while power is being reduced. The plant operators will have to compensate for this negative reactivity addition by reducing the boron concentration or withdrawing control rods if the power decrease is to be stopped before the reactor reaches zero power.

To shutdown a BWR from 100% power, control rods are inserted to increase the addition of negative reactivity to decrease the neutron flux to zero power. The core recirculation flow can also be reduced, which promotes the generation of steam in the core. The increased voiding displaces the water that was moderating the neutrons. Reducing the moderation results in a reduced number of thermal neutrons for fissions, so reactor power decreases.

Decay Heat

As you learned in the Radioactivity lesson, one unique feature of nuclear power is that after the reactor is shut down and the fission chain reaction has stopped, the fuel continues to generate some heat because of the decay of fission products. The amount of decay heat decreases with time since it originates from radioactive decay processes. Still, over the first several days following a reactor shutdown, adequate cooling must be available to remove the decay heat to reduce the potential for core damage. Reactor designs have specially engineered safeguards to provide such cooling.

Since much of this early decay heat will be from short-lived fission products, the heat production rate decreases sharply in the first few hours. For example, about 16 MWt is generated after one day of reactor shutdown, and it drops more slowly to about 9 MWt after the first five days. Without cooling, such thermal power is sufficient to boil the core dry and melt the fuel. Click here to view the graph from the Radioactivity lesson that illustrates the decay heat generation rate as a function of time.

Decay heat generation impacts spent fuel handling and storage, waste management, and reactor safety at a nuclear plant. The production of decay heat long after the reactor has been shutdown requires cooling systems to provide cooling water flow to the core during accident conditions as well as during normal shutdown conditions. Nuclear power plants use decay heat removal systems to reject that decay heat from the core.

Click the graphic below for operating experience related to decay heat.

At Three Mile Island Unit 2 in 1979, a loss of forced cooling water flow through the core allowed the core temperature to increase even though the reactor had been shut down for several hours. The thermal energy generated from radioactive decay increased the temperature in the core to a value that allowed the fuel cladding to burn and the fuel to melt. That reactor has not been re-started.

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Loss of Coolant Accidents

Loss of coolant accidents (LOCA) can also occur with severity during routine shutdown conditions. The risk for a LOCA while shutdown is greatest when the reactor coolant system inventory has been reduced during a refueling outage, which reduces the volume of water available to remove decay heat. A loss of forced cooling flow through the core in a reduced inventory condition can result in rapid temperature increase of the coolant and subsequent degradation of the fuel.

To reduce the risk associated with loss of coolant during reduced inventory conditions, the nuclear power plants ensure a sufficient number of pumps with redundant power supplies are available to provide core cooling water flow. In addition, some activities are prohibited until after the core has been shutdown for a time period that allows the decay heat generation to be reduced to an acceptable level.

Spent Fuel

Fuel assemblies that are discharged from reactors during refueling are called spent fuel. During the course of the irradiation, much of the U-235 is depleted by fission and transmutation to U-236. A small fraction of the U-238 changes to plutonium isotopes. Even smaller amounts of other transuranic isotopes, such as Np, Am, and Cm, are also produced by neutron transmutation. Among the Pu isotopes that are produced, Pu-239 and Pu-241 are fissile and contribute to energy production.

The generation of decay heat continues long after the fuel assemblies have been removed from the core, so the spent fuel assemblies are stored in spent fuel pools immersed in water to remove the decay heat. The water in the spent fuel pools is circulated through coolers to remove the decay heat. After allowing several years for the decay of the radioactive isotopes in the spent fuel, the spent fuel assemblies can be removed from the pools and stored in dry casks.

Fuel pellets that developed cracks during irradiation

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Review Question 11

How does U-235 depletion affect core reactivity?

A. It decreases linearly with burn-up as a result of U-235 buildup

B. It is not affected by U-235 depletion

C. It increases linearly with burn-up as a result of U-235 depletion

D. It decreases due to U-235 depletion and increases due to Pu production

The correct answer is D.

Core reactivity decreases due to U-235 depletion and increases due to Pu production.

Review Question 12

Match the type of reactor in Column 2 with the reactivity control method in Column 1.

Control rods A. PWRs only

Burnable poison rods B. BWRs only

Recirculation flow C. Both PWRs and BWRs

Soluble boric acid

The correct matching sequence is CCBA.

5-21

Review Question 13

Why are changes in the isotopic composition of fuel with burn-up a concern?

A. Because Pu production contributes to nuclear energy generation

B. Because composition changes can alter the neutron economy in the reactor and

consequently affect its operation

C. Because of the large amount of radioactivity emitted and its implications in handling

and disposal of the spent fuel

D. Because of the potential of accidental release of radioactive material from spent fuel

All of these are reasons why changes in the isotopic composition of fuel with burn-up are a concern.

Conclusion

Great work! You've completed the Reactor Operation lesson and have learned how the fission process is used to create energy in a nuclear power plant.

Now that you've completed this lesson, you can:

Describe how temperature, pressure, voids, fuel constituents, control rods, boron concentration, and fission product poisons affect the fission process

Describe how reactor operation affects temperature, pressure, voids, fuel enrichment (burn-up or depletion), boron concentration, and fission product poisions

Describe how criticality and/or reactivity affect startup, power operations, and shutdown

Describe how changes in various balance of plant parameters affect reactor operations in a PWR and a BWR

Define decay heat and describe its affect on plant operations

Describe the significance of loss of coolant accidents (LOCA) in all plant conditions

6-1

6 NUCLEAR SAFETY

Introduction

Because the possibility for nuclear power plant accidents exists, every measure is taken in the design and operation of nuclear plants in the U.S. to prevent accidents and to assure safety. In this lesson, you will learn about causes of nuclear power plant accidents and the measures taken to prevent them.

At the end of this lesson, you will be able to:

Identify the types of reactivity control accidents

List the causes of fuel failures

Describe the consequences of fuel failures

Describe three methods used to ensure nuclear safety

Nuclear Safety

Two significant accidents have occurred in commercial nuclear power plants.

Click each power plant location below to learn about the accident and why it occurred

Chernobyl: In 1986, the Chernobyl nuclear power plant in the Ukraine experienced a steam explosion (not a nuclear explosion), which destroyed the reactor, killed 31 people, and caused significant health and environmental consequences. The accident was the result of major design weaknesses in the reactor, as well as human error. This type of reactor is an older design. The accident would not have happened in a U.S. licensed reactor because it would not have met design requirements. Three Mile Island:

In 1979, at the Three Mile Island (TMI) nuclear power plant in Pennsylvania, a cooling malfunction caused the majority of the core to melt in an LWR reactor. The reactor was severely damaged, but radiation was contained within the containment as designed, and there were no adverse health or environmental consequences. The accident was attributed to a mechanical failure and a series of compounded human errors. In addition, poor valve indication design was a factor.

Every measure is taken in the design and operation of nuclear plants in the U.S. to prevent accidents and to assure safety. The underlying philosophy is to demand very high standards in the design, operation, maintenance, construction, testing, and reliability of systems and components, to employ redundant safety systems, and to emphasize conservative decision-making and sound risk assessment and management.

6-3

Defense-in-Depth

Defense-in-depth is a design and regulatory philosophy that protects the health and safety of the public from the uncontrolled release of radioactivity. Defense-in-depth is a hierarchical set of different, independent levels of protection, as shown in the graphic below. This strategy worked at Three Mile Island. At Chernobyl, however, this strategy did not work because there was a weak reactor confinement system, no containment structure, and an unstable reactor design.

Safety Features

In addition to defense-in-depth, there are various features in nuclear power plants that promote safety.

Click each feature below to learn more.

High standards for human performance and corrective action: The first line of defense against accidents is the actual workforce at a nuclear plant. The workforce must have a diligent attention to reducing human errors, and must promptly write up and address equipment and process issues in a corrective action program. Simply put, one should never rely on the built-in barriers as the first line of defense, but the people and leadership of the plant. Inherent safety features: Inherent safety features incorporated into the design of nuclear reactors result from basic physics and properties of matter and do not require operation of any piece of equipment. An example is the negative fuel temperature coefficient. Active and passive safety systems:

An example of an active system is Emergency Core Cooling Systems that supply water for cooling the reactor and remove decay heat.

An example of a passive system is the gravity-driven fall of a control rod in a PWR.

Redundant and diverse systems:

A redundant system has identical back-up components that perform the same safety task.

Diverse types of systems can act independently to provide similar service.

6-5

Reactivity Accidents

Any change that will increase the number of neutrons is called a positive reactivity addition. Any change that will decrease the number of neutrons is called a negative reactivity addition. Reactivity control accidents are positive reactivity additions that typically result in rapid power increases and potentially damaging fuel temperatures.

The Chernobyl accident, as shown in the graphic at right, started as a reactivity control accident.

A reactivity control accident in an LWR is highly improbable due to negative reactivity feedbacks, shutdown mechanisms, and strict regulatory and operational procedures. In the normal operation of a reactor, prompt criticality is avoided by the contribution of delayed neutrons. Control rods are not absolutely fail-safe; control rod release mechanisms can fail or the motion can be blocked. In such cases, redundancy is always provided to greatly reduce the risk of such events. In addition, it is a fundamental tenet of design of a core that with the most reactive rod stuck out, the reactor will still achieve subcriticality on any trip (scram for a BWR.)

Other Types of Accidents

Other types of reactivity control accidents that are considered in design analysis include the following.

Click each type of accident to learn more

Rod ejection accident: A control rod is accidentally ejected from the operating reactor by a failure of the rod housing, which could be caused, for example, by boric acid induced corrosion. Redundancy in the reactor trip system is the method of mitigation and thorough vessel head inspections are the means of prevention. Cold water injection accident: Negative moderator temperature feedback assures a lowering of reactivity as the moderator heats up. However, injection of a slug of cold water into the core will increase the reactivity, as in the inadvertent startup of one of the loops in a PWR that was idle for an extended period. Core reload error: Placement of partially burnt and new fuel assemblies in the core during refueling must exactly follow the core design. Human errors in either loading or in new assembly fabrication have resulted in reactivity problems following refueling, and can also result in fuel defects occurring due to high local power peaks. Boron dilution in PWR: Water with low boron concentration injected into the primary loop or during refueling operations when water is added in large quantities to the refueling pools has occurred because of human error.

6-7

Fuel Failures

Failure of fuel rod cladding integrity is a common scenario for release of radioactive fission products into the primary water. The common causes are listed below.

Click each cause to learn more

Debris-induced clad failure Over the life of a plant, small pieces of hardware may break off or be left in piping during maintenance operations. They may be carried through the primary flow loop and become trapped and vibrate against the cladding, causing it to fret and open a crack. This occurrence is commonly known as a failure of the FME (foreign materials exclusion) practices. Manufacturing defects and design limitations Defects in fuel manufacture (such as end cap weld contamination causing weld corrosion) can result in failure of the cladding. Power-induced failures Localized increase of power in a portion of the fuel rod may be caused by various human errors in power ramp-ups or control rod alignments. Chemistry-induced failures Cracking or oxide loading of cladding can result from improper water chemistry controls or monitoring. Additionally, poor handling of materials during maintenance outages can introduce harmful chemicals into the NSSS.

Radioactive materials in gaseous and/or solid form escape the fuel cladding and enter the primary water, increasing the probability and magnitude of radioactivity releases to the containment and personnel who may be working there. Fuel failures contribute to increased personnel exposures, radioactive waste, and maintenance outage time, as well as decreased productivity. They also add cost because finding failed fuel during a refueling is a very time-consuming task requiring special equipment and perhaps even extra vendor expertise. This is necessary, so the assembly is not reloaded into the core. The chances of a "hot particle" exposure are also elevated when fuel defects occur.

The Human Factor

Most accidents are the result of human error. In a study done several years ago and published in Nuclear News, the bulk of the most significant events studied by the NRC showed that about 70 percent were primarily due to “latent engineering error.” Key players in the industry say the following about nuclear safety:

Dr. James Rhodes, past Institute of Nuclear Power Operations (INPO) chairman, president, and CEO, said that nuclear safety is always the top priority and that the sharing of safety information between nuclear facilities is vital to the success of the industry.

Chris Poindexter, Chairman and CEO, Constellation Energy Group, said that companies with the highest safety standards are the most competitive.

INPO describes the safety culture in their criteria for power station performance as: Individuals at all levels of the organization consider nuclear plant safety as the overriding priority. Their decisions and actions are based on this priority, and they follow up to verify that nuclear safety concerns receive appropriate attention. The work environment, the attitudes and behaviors of individuals, and the policies and procedures foster such a safety culture.

How best to predict and prevent human error is the greatest challenge in today's nuclear power stations. Individual accountability in your role as an engineer is an integral part of nuclear safety and no less important than other aspects of nuclear safety.

6-9

Review Question 14

What is the first line of defense against nuclear power plant accidents?

A. Defense-in-depth

B. Standards for human performance and corrective action

C. Active safety systems

D. Redundant systems

The correct answer is B. The first line of defense against nuclear power plant accidents is always the high standard for performance and corrective action of the plant's workers.

Review Question 15

In which type of accident does radioactive material in gaseous and/or solid form escape the fuel cladding and enter the primary water?

A. Fuel failure

B. Reactivity accident

C. Rod ejection accident

D. Core reload error

The correct answer is A. A fuel failure results in radioactive material in gaseous and/or solid form escaping the fuel cladding and entering the primary water.

6-11

Conclusion

You have completed the Nuclear Safety lesson, and have learned about the types of accidents that can occur in nuclear power plants, and the measures you can take to avoid them.

Now that you've completed this lesson, you can:

Identify the types of reactivity control accidents

List the causes of fuel failures

Describe the consequences of fuel failures

Describe three methods used to ensure nuclear safety

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