[John Reid]

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?
 Â 
John D.M. Reid

What do you think?

[lightarrow]

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?

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.

[LeeE]

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.