# A level physics nuclear question (3)

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?
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.
Original post by Joinedup
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.

Thank you do taking the time to answer and I definitely will look over my notes! Quick question though now it’s obvious why C and D are wrong but Why is A wrong I thought at critical mass there is a stable rate of decay ?
Original post by 1234kelly
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.
Original post by StevenWong1
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 ?
Original post by 1234kelly
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.
Original post by StevenWong1
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.

That makes sense Thanks for the help again!