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9. Nuclear Fission Reactions - nuclear power as an energy resource - problems with nuclear power

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KS4 science GCSE Physics Revision Notes

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What exactly is 'nuclear power'? What is a nuclear reactor? What is nuclear fission? What processes go on that release so much energy in nuclear fission? What is a chain reaction? What causes the chain reaction in nuclear fission?  Why is nuclear power controversial? What does a moderator do in a nuclear reactor? Why are uranium-235 and plutonium good nuclear fuels? Why is it all about neutrons? What are the arguments for nuclear power? What are the arguments against using nuclear power? What are the problems in running nuclear powers stations? What does decommissioning a nuclear power station involve? These revision notes on nuclear fission, nuclear reactors and nuclear power stations should help with GCSE/IGCSE physics courses and A/AS level physics courses

RADIOACTIVITY and NUCLEAR PHYSICS INDEX

 

9. Nuclear fission reactions, nuclear power energy resources

  • When large atomic nuclei are hit with slow moving neutrons they can become highly unstable if the neutron is absorbed by the nucleus.

    • The larger unstable nucleus breaks into two smaller 'daughter' nuclei and also release more neutrons, as well as beta and alpha particles and gamma.

    • The neutrons must be slow moving to be absorbed by a uranium or plutonium atom.

    • The heavier resulting nucleus is unstable and breaks apart - nuclear fission, with the formation of several smaller atoms, neutrons and lots of nuclear energy released, which mainly ends up as heat energy.

  • The two smaller atoms formed are themselves may be unstable too and hence radioactive.

    • The fission products are often isotopes if elements with the wrong neutron/proton ratio for nuclear stability.

    • Therefore highly radioactive waste' products are formed e.g. in a nuclear reactor.

    • The nuclear fission equations below are a gross simplification of the process!

    • Lots of fission products are possible e.g. uranium ==> lanthanum + bromine

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    • Note the production of more neutrons that you started with (2-3 per uranium atom fission).

    • After the uranium nucleus absorbs the neutron its split into two smaller nuclei (often radioactive themselves), sometimes called 'daughter nuclei'.

  • The overall process is called nuclear fission and because it is accompanied by an enormous release of energy, it forms the basis of nuclear power stations.

  • (c) doc bThe radioisotope uranium-235  is particularly useful for energy generation by nuclear fission.

  • Much of the energy released is initially the kinetic energy of the fission fragments, but collisions, radioactive decay etc. result in most of it changing to heat and some as electromagnetic radiation.

  • The nuclear fission process of energy release is carried out under controlled conditions in a nuclear reactor.

    • The nuclear reaction must be well sealed with thick walls of concrete and steel.

    • Heat exchange pipes of water or carbon dioxide convey the heat from the reactor to the boilers which generate the steam to drive the steam turbines and generators.

  • The heat energy from nuclear fission processes in the nuclear reactor, via a heat exchange system, can be used to boil water to make steam to drive a turbine and electrical generator in a nuclear power station.

    • This in a nutshell is how a nuclear power station works, its just the nuclear reactor replaces a coal, gas or oil fired furnace in a conventional fossil fuel power station.

  • The energy release is much greater than for exothermic chemical reactions eg 1g uranium nuclear fuel releases the same amount of energy as 1 tonne of coal (= 1000kg =  1000000g), a million x energy density factor!

  • One consequence of fission is that more neutrons are formed, these in turn 'split' other atoms making even more neutrons (2-3 neutron per fission), which, will then facilitate even more energy releasing nuclear fission reactions.

    • This is called a chain reaction and leads to acceleration in the atom 'splitting' and hence an even greater energy release which must be controlled.

    • If fission is uncontrolled a nuclear explosion results.

    • The energy release in nuclear reactions is much greater then the energy release in chemical reactions.

    • A fission bomb based on uranium-235 was dropped on the Japanese city of Hiroshima in 1945, Nagasaki was hit by a plutonium based fission bomb later in the same year.

    • Both of these 'fission bomb' nuclear weapons relied on preventing fission happening until the bomb mechanism explodes a neutron source and uranium/plutonium metal into each other, then fission happens in a totally uncontrolled way within a deadly split second.

  • BUT, we need controlled fission:

    • Surrounding the uranium fuels rods with a moderator material e.g. graphite or 'heavy water' to absorb some of the neutrons, but that is not usually enough for full control.

    • Thankfully in nuclear reactors, control rods of an excellent 'moderator' like boron can lowered into the reactor core to absorb neutrons and slow down fission to keep the chain reaction and energy release under control.

      • If heat production become to low, the boron moderator can be raised out of the reactor to allow more fission to take place.

  • Note again, the balancing of nuclear equations for, in this case, fission reactions (somewhat simplified!) ...

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  • uranium splits into lanthanum and bromine nuclei releasing three more neutrons

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  • uranium split into molybdenum and lanthanum plus two neutrons to continue the chain reaction, and a good balancing challenge too!

  • When the spent uranium nuclear fuel is removed from a nuclear reactor, it is sent for reprocessing.

    • This is an extremely complicated and costly process.

    • However, one idea was to extract plutonium metal, which is formed as a natural 'nuclear consequence' of the nuclear reactions in a uranium nuclear reactor.

    • Plutonium, like uranium, is a readily fissile material i.e. it readily undergoes fission under neutron bombardment to release considerable quantities nuclear energy.

  • Nuclear power is controversial and many issues are involved.

    • The public perception of nuclear power is often very negative and not worth the risk i.e. it is seen as very dangerous from radioactive leaks, overheating or explosions of reactors, unsafe storage of harmful radioactive waste, and to be fair, these fears are not without foundation, all have happened, and, rightly, these concerns must be addressed by scientists, design engineers and politicians etc.

    • Arguments for nuclear power:

    • It contributes little to the greenhouse effect and global warming since there is no emission of carbon dioxide from fossil fuels, so climate change isn't an issue for nuclear power.

    • Unlike nuclear power, burning fossil fuels produces pollution effects from harmful gases e.g. nitrogen oxides are bad for the lungs, sulfur oxides produce acid rain causing environmental damage e.g. acidified lakes, building corrosion.

    • Fossil fuels are a finite resource and can't last for ever and there is probably enough uranium ores to last for many years even though these are also finite resources. A little uranium can go a long way!

    • Generally speaking it is a reasonably safe and reliable way of generating electricity and don't forget the workers lives lost in the extraction of oil & gas and coal mining to feed fossil fuel power stations.

    • Arguments against nuclear power:

    • Leakage of radioactive material into the environment.

    • The difficulty of safely disposing of highly radioactive nuclear waste for a long period of time.

    • The huge capital cost of building a nuclear power station and the equally huge cost of decommissioning spent reactors. The latter is not well understood by the public taxpayer since the energy companies in the UK don't appear to want have anything to do with decommissioning!

    • The length of time for construction, maybe 10-20 years and decommissioning takes decades too!

    • Some of these points are further elaborated on below:

  • It is no simple decision to build a nuclear power station.

    • Theoretically it provides a reliable large scale source of electricity - unlike e.g. wind power or solar power which can be unreliable and cannot provide the same concentration of power.

    • No carbon dioxide is produced, so it doesn't contribute to global warming and climate change in the way that fossil fuel power stations do.

    • It benefits a large number of people and provides lots of jobs in construction and when its finished and operating.

    • BUT, running a nuclear power station is not without risks e.g. increase in background radiation expose to local inhabitants, risk of accident, (see below).

    • So, to make any major infrastructure planning decision to build a nuclear power station you have to weigh up the benefits, drawbacks like ....

      • cost and operating problems because of the complex technology and high safety standards,

      • risks e.g. increased radiation exposure, accident etc.

  • Problems with nuclear power (despite not contributing to global warming).

    • Nuclear power contributes very little to global warming since carbon dioxide is not form in the process, as is the case with burning fuels. This is one of the principal arguments for developing nuclear power projects.

    • Also, the nuclear fuel like uranium are not that costly, at least compared to the huge capital cost of building and running a nuclear power station.

    • Not only is building/running nuclear power plants very costly, BUT the cost of decommissioning them is also very high.

      • Nuclear power plants cost billions of £/$ to build and cost £/$ billions to decommission!

      • Energy companies in the UK are doing their best to avoid paying decommissioning and it looks like the UK taxpayer will pay the huge bill!

      • Also, nuclear fuels based on uranium, plutonium and thorium, are nonrenewable because there is only a limited amount of suitable ores around the world.

  • Decommissioning a nuclear plant involves taking everything apart and disposing of all the waste materials like concrete, nuclear reactor components (most radioactive) etc. and may take many years (decades!) to fully dismantle and safely deal with all the waste.

  • The nuclear waste problem

  • Unfortunately, even before decommissioning, when the nuclear power plant is operating, considerable amounts of highly radioactive waste are formed which must be disposed of safely (NOT easy, you can't just dump it like any old refuse!). The waste must be added to in the long-run from future decommissioned nuclear reactors and associated buildings.

    • Reprocessing nuclear waste is technically complex, potentially dangerous and very costly.

    • Much of this waste from the working nuclear reactors or decommissioned nuclear power plants is dangerously radioactive for many months or years.

    • Some radioactive atoms take thousands of years to fully decay to stable isotopes which are safe and no longer radioactive.

    • This raises considerable technical issues for even temporary as well as long term safe storage and leaks seem to be inevitable.

    • Any leakage from this dangerous waste is potentially harmful to the environment and its inhabitants and must be stored in a secure location well away from people.

    • Currently, much of the dangerous radioactive waste is dealt by a process of vitrification. The waste is mixed with ceramic producing materials and melted to form a sort of glass. The liquid glass is poured into steel cylinders, cooled and securely sealed. The containers are then stored in a suitable waste dump which doesn't have to be underground. The idea is that its quite difficult for the radioactive materials to diffuse out of the glass.

    • The radioactive waste can also be stored in thick sided metal containers and bury them deep underground with tons of protective concrete so no dangerous radiation ever reaches the surface.

    • Potential for a major catastrophe: As was seen at Chernobyl in the Ukraine (from operational mistakes, April 1986) and Fukushima in Japan (from earthquake, March 2011), dangerous explosions occurred releasing considerable amounts of dangerous radioactive materials into the environment.

  • See health and safety, half-life of radioisotopes and background radiation


RADIOACTIVITY and NUCLEAR PHYSICS INDEX

1. Atomic structure, fundamental particles and radioactivity

2. What is radioactivity? Why does it happen? What radiations are emitted?

3. Detection of radioactivity, measurement, dose units, ionising radiation sources, background radiation

4. The properties and dangers of alpha, beta & gamma radioactive emission

 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation

6. Half–life of radioisotopes, how long does material remain radioactive? Uses of decay data & half–life values

7. Nucleus changes in radioactive decay? how to write nuclear equations? Production of Radioisotopes

 8. Nuclear fusion reactions and the formation of 'heavy elements'

 9. Nuclear Fission Reactions, nuclear power energy resources


Advanced Chemistry Page Index and Links

(c) doc b(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

 (c) doc b five word-fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!

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