Nuclear Power Principles of Operation

1. Nuclear PowerPrinciples of Operation Keith Moloney

2. 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

3. 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 Containment Turbine Hall

4. 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

5. 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

6. 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

7. 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

8. Nuclear Power Plant • uses the heat released by a nuclear fission chain reaction to generate electricity • Key Components • Reactor core where fission occurs producing heat • Coolant loop which removes heat from core • Steam generator to make high pressure and temperature steam • Steam turbine which used energy of steam rotates a shaft • Electrical generator • Condenser which dumps waste heat either to cooling tower or the sea

9. Reactor Core • Fuel rods contain uranium oxides enriched to 2-5% 235U • Moderator slows down neutrons so they can be absorbed by 235Uwater • Control rods absorb neutrons are removed/inserted into core to control the rate or completely stop the chain reaction • Coolant captures the energy released and removes it from the reactor • Pressure Vessel encloses all of the above which are at high pressure and temperature

10. 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

11. 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

12. 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

13. 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

14. 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 Blades Turbine Hall

15. 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

16. Nuclear Waste Uranium Cycle • 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

17. 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 • Causedbydesign 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

18. 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

19. 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 World Electricity generation

20. 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.

21. 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