Critical Mass (1 Viewer)

[Viper]

Hey again,

Another quick one (i hope). I was reading up on Quanta to Quarks, and it occured to me that im not 100% sure of what the critical mass is.

I understand that the sample of the radioactive substance must be greater than the critical mass for fission to occur, but what is Critical Mass?

Cheers

 

[BlackJack]

Critical mass is the mass of your hot stuff when the fission chain reaction is exactly 1 (theoretic).
In other words on average, exactly one neutron from each fission go on to produce another fission.

 

[stag_j]

Yes. What blackjack said. Think about it as if there isnt 1 neutron released from each fission then eventually there will cease to be any reactions occuring. Obviously, in order for a nuclear weapon or sustained nuclear reaction (power generation) you need a critical mass of more than 1.
However if the mass is too great then there will be an explosion before the maximum potential has been reached, so there will be some loss of efficiency.
Ideally you want the critical mass to be reached in the smallest possible time...

 

[Viper]

So, would i be right in saying that the critical mass is the molecular mass of the radioactive isotope? (More Nuetrons than protons?)

Cheers

 

[sunken eyes]

dont think so. like CM for u-235 is like 35 kilos right? not the molecular mass. its just a specific mass of fuel where fission will become self sustaining for reasons listed above.