Nuclear Power Plant

D. Sakthivadivel Teaching Fellow

Institute for Energy Studies Department of Mechanical Engineering

CEG, Anna University Chennai - 25

World Energy Scenario

There are 435 nuclear power reactors in 31 countries.

30 more are under construction.

They account for 16% of the worlds electricity.

They produce a total of 374,411 MW (billion watts) of electricity.

435 commercial reactors in 31 countries, producing

16% electricity

Indian Energy Scenario

History and Basics

Henri Becquerel discovered radioactivity in 1896

Becquerel named the emission of invisible radiation from uranium ore radioactivity.

Radioactive materials was the name given to materials that gave off this invisible radiation.

When he exposed a light-tight photographic plate to a radioactive mineral, then developed the plate. (A) A photographic film is exposed to an uranite

ore sample. (B) The film, developed normally after a four-day exposure to

uranite. Becquerel found an image like this one and deduced that the mineral

gave off invisible radiation that he called radioactivity.

Ernest Rutherford later discovered that there were three kinds of radioactivity.

Alpha particles () is a helium nucleus (2 protons and 2 neutrons)

A beta particle () is a high energy electron

A gamma ray () is electromagnetic radiation with a very short

wavelength.

Radiation passing through a magnetic field shows that massive, positively charged

alpha particles are deflected one way, and less massive beta particles with their

negative charge are greatly deflected in the opposite direction. Gamma rays, like

light, are not deflected.

Radioactivity is the spontaneous emission of particles or energy from an atomic

nucleus as it disintegrates.

Radioactive decay is the spontaneous disintegration of decomposition of a nucleus.

Nuclear Equations

The two subatomic particles that occur in the nucleus, the proton and the neutron,

are called nucleons.

The number of protons is the atomic number which determines the identity of the element.

The number of protons and neutrons determines the atomic mass of the element.

Different isotopes of an element have the same atomic number (same number of protons) but different atomic masses (different number of neutrons)

The three isotopes of hydrogen have the same number of protons but different

numbers of neutrons. Hydrogen-1 is the most common isotope. Hydrogen-2, with

an additional neutron, is named deuterium, and hydrogen-3 is called tritium.

Neutrons and protons are called nucleons because they are in the nucleus.

Just like any other chemical reaction, we use symbols to show a nuclear reaction

As an example, when uranium 238 emits an alpha particle, it loses 2 protons and 2 neutrons.

Nuclear reactions must balance just like any other chemical reaction, but we must also be aware of balancing protons and neutrons

The Nature of the Nucleus.

Protons and neutrons are held together by a nuclear force when they are

very close together.

The shell model of the nucleus visualizes the protons and neutrons

moving on energy levels or shells, much like the electrons move in

shells.

HeThU 42234

90

238

92

Radioactive Decay Series

Radioactive decay produces a simpler and more stable nucleus.

A radioactive decay series occurs as a nucleus disintegrates and achieves a

more stable nuclei

There are 3 naturally occurring radioactive decay series.

Thorium 232 ending in lead 208

Uranium 235 ending in lead 207

Uranium 238 ending in lead 206

Nuclear Energy

The energy that exists within the nucleus of an atom.

Nuclear Fission the release of energy from the splitting of atoms!

Nuclear Fusion the combining of two smaller atoms into one larger atom.

Bombs & power plants

Big, unstable isotopes are struck by

neutrons, which splits the isotopes

nuclei

More neutrons shoot out to strike

nearby isotopes, causing a chain

reaction.

Sun and stars, some weapons

2 small (light) isotopes are forced

together

H + H = He

Need temps > 100,000,000C

Releases more E than fission

Nuclear Fission Nuclear Fusion

Nulear Change

What is Radiation?

Radiation particles given off by unstable atoms.

3 Types:

Alpha ()

Travels few inches

Blocked by paper (skin)

Beta ()

Travels few feet

Blocked by aluminum, glass

Gamma ()

Travels far

Blocked by lead (steel & concrete).

Nuclear Chain Reactions

A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus.

This nucleus in turn produces neutrons, and the process repeats.

If the process is controlled it is used for nuclear power or if uncontrolled it

is used for nuclear weapons

Controlled Nuclear Fission Reaction

Nuclear Fission from Slow Neutrons and Water

Moderator

Inside a Nuclear Reactor

Steam outlet

Fuel Rods

Control Rods

Fuel Rods

35,000 70,000 fuel rods

3% Uranium-235 pellets

In water (moderator)

Control Rods

absorb extra neutrons

Control the chain reaction

Nuclear Basics Power plants produce radioactive wastes

mostly spent fuel rods (3-4 years)

each reactor produces about 20-30 tons yearly

Currently stored in pools on site

some remain dangerous for tens of thousands of years

Half life time is a time needed for one-half of the nuclei in a radioisotope to

decay and emit their radiation to form a stable isotope

Half-time Emitted

Uranium 235 710 million yrs alpha, gamma

Plutonium 239 24,000 yrs alpha, gamma

Measurement Methods

Film badges

Workers who are exposed to radioactivity carry film badges

The film is exposed and the optical density of the film shows the

workers exposure levels during the time the film badge was worn.

Ionization counter

Measure ions that are produced by radiation

Scintillation counter

Measures the flashes of light that occur when radiation strikes a

phosphor.

Geiger counter

Measures pulses of electrons released from the ionization of gas

molecules in a metal cylinder

Each pulse of electrons is heard as a pop or click

This is a beta-gamma probe, which can measure beta and gamma

radiation in millirems per unit of time.

The working parts of a Geiger counter

Radiation Units

Curie (Ci)

Measurement of the activity of a radioactive source.

Measured as the number of nuclear disintegrations per unit of

time

A curie is 3.70 X 1010 nuclear disintegrations per second.

Rad

Measures the amount of energy released by radiation striking living tissue

Short for radiation absorbed dose

One rad releases 1 X 10-2 J/kg

Rem

Short for roentgen equivalent man

This takes into account the possible biological damage to humans of certain types of radiation.

CONTROL RODS

Control rods made of a material that absorbs neutrtons are inserted into the

bundle using a mechanism that can rise or lower the control rods.

The control rods essentially contain neutron absorbers like, boron, cadmium

or indium.

STEAM GENERATORS

Steam generators are heat exchangers used to convert water into steam from

heat produced in a nuclear reactor core.

Either ordinary water or heavy water is used as the coolant.

STEAM TURBINE

A steam turbine is a mechanical device that extracts thermal energy from

pressurized steam, and converts it into useful mechanical.

Various high-performance alloys and super alloys have been used for steam

generator tubing.

COOLANT PUMP

The coolant pump pressurizes the coolant to pressures of the order of 155bar.

The pressure of the coolant loop is maintained almost constant with the help

of the pump and a pressurizer unit.

FEED PUMP

Steam coming out of the turbine, flows through the condenser for

condensation and recirculates for the next cycle of operation.

The feed pump circulates the condensed water in the working fluid loop.

CONDENSER

Condenser is a device or unit which is used to condense vapor into liquid.

The objective of the condenser are to reduce the turbine exhaust pressure

to increase the efficiency and to recover high quality feed water in the

form of condensate & feed back it to the steam generator without any further

treatment.

COOLING TOWER

Cooling towers are heat removal devices used to transfer process waste heat to

the atmosphere.

Water circulating through the condenser is taken to the cooling tower for

cooling and reuse

Nuclear Fission We convert mass into energy by breaking large atoms (usually Uranium) into

smaller atoms. Note the increases in binding energy per nucleon.

A slow moving neutron induces

fission in Uranium 235

Fission products

The fission products shown are just examples, there are a lot of different

possibilities with varying probabilities

Expanding Chain Reaction

The fission reaction produces more neutrons which can then induce fission in

other Uranium atoms.

Linear Chain Reaction Obviously, an expanding chain reaction cannot be sustained for long (bomb). For controlled nuclear power, once we reach our desired power level we want each fission to produce exactly one additional fission

Moderator Neutrons are slowed down by having them collide with light atoms (Water in

US reactors).

Highest level of energy transfer occurs when the masses of the colliding particles are equal (ex: neutron and hydrogen)

Control Rods

Control rods are made of a material that absorbs excess neutrons (usually

Boron or Cadmium).

By controlling the number of neutrons, we can control the rate of fissions

Basic Ideas

The Uranium is both the fuel and the source of neutrons.

The neutrons induce the fissions

The Water acts as both the moderator and a heat transfer medium.

Control rods regulate the energy output by sucking up excess neutrons

Feasibilities

Processing of Uranium

Each ton of Uranium ore produces 3-5 lbs of Uranium compounds

Uranium ore is processed near the mine to produce yellow cake, a material rich in U3O8 (Triuranium octoxide).

Only 0.7% of U in yellow cake is 235U. Most of the rest is 238U which does not work for fission power.

World Distribution of Uranium

Enrichment To be used in reactors, fuel must be 3-5% 235U.

Yellow cake is converted into UF6 and this compound is enriched using gaseous diffusion and/or centrifuges.

There are some reactor designs that run on pure yellow cake.

A nuclear bomb requires nearly 100% pure 235U or 239Pu. The 3% found in reactor grade Uranium CANNOT create a nuclear explosion!

Fuel Pellets The enriched UF6 is converted into UO2 which is then made into fuel pellets.

The fuel pellets are collected into long tubes. (~12ft).

The fuel rods are collected into bundles (~200) rods per bundle.

~175 bundles in the core.

Cladding The material that the fuel rods are made out of is called cladding.

It must be permeable to neutrons and be able to withstand high heats.

Typically cladding is made of stainless steel or zircaloy.

Controlling the chain reaction

depends on Arrangement of the fuel/control rods

Quality of the moderator

Quality of the Uranium fuel

Neutron energy required for high probability of fission

Two common reactor types: Boiling

Water Reactor and Pressurized Water

Reactor.

BWR: P=1000 psi

T=545F

PWR P=2250 psi

T=600F

PWR is most common and is basis of

marine nuclear power.

CANDU (CANada Deuterium

Uranium) Reactor

1 Nuclear fuel rod

2 Calandria

3 Control Rods

4 Pressurizer

5 Steam generator

6 Light water condensate pump (secondary

cooling loop)

7 Heavy water pump (primary cooling loop)

8 Nuclear fuel loading machine

9 Heavy water (moderator)

10 Pressure tubes

11 Steam

12 Water condensate

13 Reactor containment building

Reactor is inside a large

containment building

Other Options

Other countries use different reactor designs.

Some use heavy water (D2O) as a moderator. Some use Graphite as a

moderator.

Some are designed to use pure yellow cake without further enrichment

Liquid metal such as sodium or gasses such as Helium are possibilities to

use for coolants

Breeder Reactors

A big problem with nuclear power is the creation of Plutonium in the

reactor core.

This is a long lived radioactive element that is difficult to store.

Q: Why not use it as a fuel too?

Basic Idea

Process that creates the Pu.

During fission use one of the extra neutrons to create a Pu atom

Somewhat difficult in that we want fast neutrons to breed the 239Pu out of the 238U, but we want slow neutrons to induce the fission of 235U.

Requires a different design of reactor.

Doubling time: Time required to produce twice as many 239Pu atoms as 235U destroyed. A good design will have a 6-10 doubling time.

There are no currently operating breeder reactors.

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ADVANTAGES

Nuclear power generation does emit relatively low amounts of carbon dioxide

(CO2). The emissions of green house gases and therefore the contribution of

nuclear power plants to global warming is therefore relatively little.

This technology is readily available, it does not have to be developed first.

It is possible to generate a high amount of electrical energy in one single plant

DISADVANTAGES

The problem of radioactive waste is still an unsolved one.

High risks: It is technically impossible to build a plant with 100% security.

The energy source for nuclear energy is Uranium. Uranium is a scarce

resource, its supply is estimated to last only for the next 30 to 60 years

depending on the actual demand.

Thank You