A level physics nuclear question (3)

It should be well known that 2 or 3 neutrons are released by each fission of U235

But only one neutron absorbed into a U235 nucleus will cause it to fission (and release 2 or 3 new neutrons)

Reactors are surrounded by a neutron shield to stop neutrons that have escaped the reactor from causing a health hazard. This is necessary cos some neutrons escape the reactor..

So the chain reaction can go 3 ways, if exactly one of the neutrons from a previous fission causes a new fission the rate of fissions will be constant.

If more than 1 neutron from each fission goes on to cause a new fission the rate of fissions will increase

If less than 1 neutron from each fission goes on to cause a new fission the rate of fissions will decrease.

Tbh not knowing how to answer this question is telling you that you need to go back and revise your notes.

For a nuclear reactor in which the fission rate is constant, which one of the following statements is correct?
A There is a critical mass of fuel in the reactor.
B For every fission event, there is, on average, one further fission event.
C A single neutron is released in every fission event.
D No neutrons escape from the reactor.


Can someone pleas explain why the answer is B and also why the other options are wrong please?

Okay so:
A - Critical masses would be something more likely encountered with nuclear weapons than reactors - so we can rule that one out.
C - Fission events typically release multiple neutrons - which then go on to cause more fissions.
D - Typically some neutrons do escape from reactors - even with all the shielding that is employed - though this is often within the designed tolerances and is safe.

Thank you ! But why isn’t critical mass normally associated with reactors ?

From my understanding of it, nuclear reactors are designed to work at a subcritical state - where the neutrons are more controlled in a sense.

Where as the dangerous uncontrolled chain reactions is good for nuclear weapons.

Your teacher may be able to explain it better than me.