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20/04/23

Moylish Pk. Limerick Ireland T. +353 61 293000 F. +353 61 293001 E. information@lit.ie

Nuclear PowerPrinciples of Operation

Keith Moloney

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Nuclear Power

• Nuclear Power is the generation of electricity using the energy released during a controlled nuclear reaction

• The heat released is used to produce steam which drives steam turbines which turn generators to produce electricity.

• The nuclear reaction is the splitting of the nucleus of particular fuel atoms– Uranium is by far the most common

• An enormous amount of energy is released from a small amount of fuel– 1kg of uranium can produce same amount of heat as 3000t of coal

• Nuclear Power Plants do not release CO2 into the atmosphere

• But they do produce long lived radioactivity waste– Accidents could contaminate vast areas and affect millions of people

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Typical Nuclear Power Plant

• Paluel Nuclear Power Plant - Normandy France– 4 Pressurized Water Reactors each generating 1330MW

– Annual Output 32000GWh – more than annual demand for Ireland

– Cooled using seawater from English Channel

Turbine Hall

Containment

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Atoms and Isotopes

• Everything is made of atoms which have a nucleus containing protons & neutrons– Protons have a positive charge

– Neutrons have no charge

• The number of protons is fixed by the element.– Carbon has 6, nitrogen 7, uranium 92

• Some elements have different versions called isotopes which have different numbers of neutrons– Uranium has two main isotopes

– Uranium 238 238U has 146 neutrons & 92 protons

– Uranium 235 235U has 143 neutrons & 92 protons

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Radioactivity• In the nucleus there are two different force acting

– Electrostatic force between positive protons tries to break nucleus apart

– Strong Nuclear Force between protons and neutrons holds it together

• If the forces aren’t balanced nucleus can break apart giving out energy as radiation – called radioactive decay– The radiation can be either an alpha or beta particle – harmless unless inside body

– Usually accompanied by gamma radiation, similar to high energy x rays, and can cause serious damage to cells in the body

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Fission

• Fission is a type of radioactive decay that doesn’t occur spontaneously– caused by deliberately bombarding particular isotopes with neutrons

• The nucleus first absorbs the neutron and then splits releasing a lot of energy– The daughter nuclei and neutrons travel very fast so they have lots of energy

– Gamma rays are emitted as well

• Only a very few isotopes can be split by fission - called fissile– Uranium 235 is fissile but uranium 238 isn’t

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Chain Reaction

• The neutrons released by splitting one atom can split another atom

• This releases more neutrons which in turn can split further atoms and so on causing a chain reaction– If splitting one atom causes the splitting of two or more subsequent atoms, the

reaction quickly gets out of control causing an explosion

• In a nuclear power plant the chain reaction is controlled – Only one neutron from each split atom splits another atom

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Nuclear Power Plant

• uses the heat released by a nuclear fission chain reaction to generate electricity

• Key Components1. Reactor core where fission occurs producing heat

2. Coolant loop which removes heat from core

3. Steam generator to make high pressure and temperature steam

4. Steam turbine which used energy of steam rotates a shaft

5. Electrical generator

6. Condenser which dumps waste heat either to cooling tower or the sea

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Reactor Core

1. Fuel rods contain uranium oxides enriched to 2-5% 235U

2. Moderator slows down neutrons so they can be absorbed by 235Uwater

3. Control rods absorb neutrons are removed/inserted into core to control the rate or completely stop the chain reaction

4. Coolant captures the energy released and removes it from the reactor

5. Pressure Vessel encloses all of the above which are at high pressure and temperature

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Boiling Water Reactor

• BWR is the simplest reactor

• Water is used as both moderator and coolant– Pumps circulate through core

• Water is allowed to boil and steam is fed straight to turbine– Condensed steam is pumped back to core

• Temperature and pressure of steam are relatively low– heat to electricity efficiency is also low

• Turbine sees radioactive steam

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Pressurized Water Reactor

• PWR is most common reactor– Water is used as moderator and coolant

• Steam generator inserted between core and turbine

• water in reactor is pressurized to ~ 100atm

• Higher pressure & temperature means better efficiency

• Turbine is separated from radioactivity

• Steam generator adds cost

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Gas Cooled Reactors

• Advanced Gas cooled Reactor (AGR)

• Use graphite as the moderator– Solid block with channel for fuel rods,

control rods and coolant

• Carbon dioxide as coolant– Pump through core and into steam

generator

• Can produce higher temperature steam than PWR– Use steam turbines from coal-fired plants

• More expensive to build than PWRs– 7 AGR plants in UK

– Last one completed 1988

– No new ones planned

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Steam Generator

• The steam generator is a heat exchanger – transfers heat from the primary coolant

loop to the secondary steam loop

– No mixing occurs

• Radioactive coolant is pump through thousands of tubes and heats flows through walls to boil water on other side

• Steam generator is ~ 20m tall and weights up to 800 tonnes

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Steam Turbine

• turbines expand the high pressure steam converting the pressure into kinetic energy

• turn the turbine blades which rotate the shaft which drives the generators to produce electricity

– done in a number of stages with each stage operating at a lower pressure– To maximize efficiency steam is condensed on low side– Waste heat must be dumped– Can use a cooling tower or a river or the sea

Turbine Hall

Turbine Blades

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Uranium Fuel• Uranium is as common element as tin

– found in low concentration in many types of rock and in oceans

• occurs as an oxide and is mined where it is found in high concentrations (>0.5%)– Producing 1t of uranium oxide leaves 1000t of tailings

• Naturally occurring uranium is only 0.7% 235U which isn’t high enough for chain reaction– Must be enriched to increase 235U concentration

– Complicated expensive process using gas centrifuges

• Enrich uranium is processed into fuel rods– Zirconium is used as it doesn’t absorb neutrons

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Nuclear Waste• Fuel rods need to be replaced after 3 years

– Most of 235U consumed and fission rate drops

• Rods are 94% 238U but remainder is highly radioactive by-products

• Most dangerous are the fission by-products– Have short half-lives and decay rapidly

releasing large amounts of heat and radiation

• Spent fuel rods must be stored at the plant in storage ponds with water cooling for up to 5 years– Then moved offsite for reprocessing or long

term storage

• Other by products have thousand year half-lives which means waste must be stored safely for thousands of years– No agreed way to do this

Storage Pond

Uranium Cycle

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Nuclear Safety

• Very low risk of a nuclear explosion at a power plant – 235U concentration is too low, weapons grade uranium is >90% 235U

• Main risk is the release of radioactive material into the environment – Accident at a power plant or leakage from a storage site

– Most likely causes are failure of cooling or control systems at power plant

• Once released radioactive material can spread huge distances, build up in the food chain and affect the health of millions of people– Large areas have to be evacuated indefinitely

• Relatively few major accidents in 60 years of nuclear power

• 1986 Chernobyl accident in Ukraine worst so far

• Caused by design flaws & human error

• Overheated causing steam explosion and fire

• radioactive material blown all over Europe on wind

• Long term deaths between 4000-100000

• Huge economic & psychological impact in Belarus

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Fukushima

• 2011 accident at Japanese plant was a major blow to nuclear industry as design and operation were state of the art.

• Plant shutdown safely during earthquake but power lines were cut– Back up generators kicked in to run cooling of reactors and storage ponds

• Was designed to survive 5.9m tsunami but was hit by 13m one– Generators were swamped and cooling was lost

• Reactors went into meltdown, hydrogen explosive destroyed containment– Radioactive material released but wind was blowing out to sea

• no deaths were directly caused by radiation• But 200,000 people evacuated from homes

• 2000 deaths could be caused by the stress

• Ongoing problems with leakage of radioactive material

• Major review of all Japan’s nuclear plants

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World Electricity generation

Status

• Nuclear power supplies 12-13% of the world electricity and 27% of the EU’s– France is 75% nuclear

• There are 437 operational reactors spread across 30 countries– US, France, Japan, Russia, S Korea, India, China

– Japan & Germany reviewing nuclear programs since Fukushima

• There are currently 67 reactors under construction– China (29), Russia(11) , India(7)

– UK have just announced 1st new plant

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Nuclear Summary

• Advantages– Could supply a large proportion of electricity needs with no greenhouse gas

emissions.

– Baring accidents, it has a limited environmental impact, much less than coal

– Reduces reliance on oil and gas which have a limited supply

• Disadvantages– Nuclear power plants are very expensive to build

– Accidents could release radioactive material ,contaminate vast areas and impact the health of millions of people

– Waste from reactors remains hazardous for thousands of years and must be stored securely.

– Uranium supply is limited and extracting it causes local environmental damage.

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Videos & Websites

• Westinghouse Animation

http://www.westinghousenuclear.com/docs/WhatIsNuclearEnergy/AP1000%20Loop%20Animation.swf

• UAE Nuclear Industry with good animation http://www.youtube.com/watch?v=VJfIbBDR3e8  

• Very basic BBC Science programme http://www.youtube.com/watch?v=MGj_aJz7cTs  

• CNN visit to US reactor http://www.youtube.com/watch?v=ZS8Zw1i7pic   

• Canadian Reactor http://www.youtube.com/watch?v=PKNbwcIaGng