• 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 the production of beta and alpha particles and gamma radiation.

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

  • The heavier resulting nucleus is unstable and spontaneously 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

  , as well as many other elements.

  • uranium-235  + neutron  ===>   lanthanum-145  +  bromine-88  +  three neutrons

  • 

  • Note the production of more neutrons that you started with (2-3 per uranium atom fission) and these go on to split more of the uranium atoms causing a chain reaction, which must be controlled to operate a nuclear reactor safely!

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

  • In the process, mass is lost - the nuclear fission products have less mass than the uranium.

  • The law of conservation of energy is not contravened, the mass loss is converted into energy e.g. thermal energy and electromagnetic radiation according to Einstein's famous law E = mc² !!!

• 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.

• The 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.

Energy store changes for nuclear power station:

nuclear potential energy store (in uranium or plutonium fuel rods)

==> thermal energy store of steam (thermal energy store transfer from hot gases of furnace to water)

==> kinetic energy store of turbine (mechanical energy transfer)

==> kinetic energy store of generator (mechanical energy transfer)

==> electrical energy output (to power line system)

• 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 the coolant, water or carbon dioxide, convey the heat from the reactor to the boilers which generate the steam to drive the steam turbines and generators.

  • BUT, the coolant cannot do this directly, for a good reason.

  • There is a heat exchanger system from the coolant to the pipes conveying steam to the turbine.

  • The coolant passes through the highly radioactive reactor and could become contaminated with radioisotopes.

  • Therefore the two fluid systems ensure that no radioactive materials (ideally) never leave the nuclear reactor itself.

• 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 - fortunately, in the case of Chernobyl, the reactor overheated and a steam explosion occurred, but it resulted in a huge amount of radioactive contamination.

  • 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.

    • Ideally, you just want one of the two or three neutrons produced to go and initiate a fission reaction.

    • 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!) ...

• uranium-235  + neutron  ===>   lanthanum-145  +  bromine-88  +  three neutrons

• 

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

• -

• uranium-235  +  neutron  ===>  molybdenum-95  +  lanthanum-139  + two neutrons + seven beta minus particles

• 

• 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.

  • Nuclear power stations can produce a high base load of electricity from relatively little fuel.

  • 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 and storing it for a long period of time due to the big half-lives of some of the radioisotopes produced in the nuclear reactor core.

  • 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.

    • The dangers of a major accident at a nuclear power station cannot be overstated - such events like the Russian Chernobyl nuclear power station accident released radioactive materials into the environment leading to long-term contamination - ecological disaster and people from the surrounding town and villages had to be moved away to a less contaminated area.

• 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 of £/$ 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 non-renewable 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, especially when dealing highly radioactive material in which there may be radioisotopes with half-lives of thousands of years - a long safe storage time required.

  • 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

Easier Foundation Tier Radioactivity multiple choice QUIZ

Harder Higher Tier Radioactivity multiple choice QUIZ

Worksheet QUIZ Question 1 on RADIOACTIVITY - absorption of alpha, beta and gamma radiation

Worksheet QUIZ Question 2 on RADIOACTIVITY - dangers & monitoring ionising radiation levels

Worksheet QUIZ Question 3 on RADIOACTIVITY - revision of atomic structure

Worksheet QUIZ Question 4 on RADIOACTIVITY - what happens to atoms in radioactive decay?

Worksheet QUIZ Question 5 on RADIOACTIVITY - uses of radioisotope and half-life data

ANSWERS to the WORD-FILL WORKSHEET QUIZZES

Crossword puzzle on radioactivity and ANSWERS!

[SEARCH BOX]

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*