Science Questions

How do you control a nuclear reaction?

Sat, 17th Jul 2010

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Question

John Reid asked:

How do nuclear reactors such as those that power ships and power stations limit the chain reaction that occurs in plutonium fuelled bombs?

John Reid

Answer

Chris -   They use what are called control rods - these are dense materials which, when dropped down into the reactor, soak up some of the neutrons that are produced by the nuclear chain reaction.  The consequence of that is that there are fewer neutrons left to bust open other uranium or fissionable nuclei, and as a result, the chain reaction is slowed down.  By putting the fuel rods in, or drawing them out, you can speed up or slow down the chain reaction, and therefore, you can affect how much energy actually comes out of the reactor.

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John Reid asked the Naked Scientists: Dear Chris, How do nuclear reactors such as those that power ships and power stations limit the chain reaction that occurs in plutonium fuelled bombs? John D.M. Reid What do you think? John Reid , Tue, 11th Aug 2009

Usually with bars full of materials (boron, cadmium, ecc.) which absorbs neutrons. Those bars are held up when the reactor works; when the reaction increases too much, they are lowered down the reactor so they absorb more neutrons and the reaction slows; if something goes wrong, they authomatically goes completely down for gravity (security mechanism), absorb (almost) all the neutrons and the reaction switches off. Of course things in details are a little bit complex, but essentially is that. lightarrow, Tue, 11th Aug 2009

Nuclear fission reactors use different 'grades' forms and isotopes of Uranium or Plutonium to those used in fission weapons.  For example, reactor grade Uranium is around 3-4% U-235, whereas weapons grade is around 90% U-235.

The reactions in a fission power plant are controlled (moderated, as light arrow describes) whereas the reactions in a fission bomb are not.  The cores of reactors are also designed so that in combination with the specific grades and isotopes of fuel used, they cannot reach the degree of super-criticality required for the run-away uncontrolled reactions you get in bombs, even if the core melts and puddles together. LeeE, Wed, 12th Aug 2009

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