In order for fission to cause a chain reaction, what is required?

Question

critical mass

a decaying element

partial accelerator

a high mass star

Answer 1

Hi again Mallory P.,

This one is a toss-up! Chain reaction is commonly interpreted to imply a continuously increasing rate of fission events. And that can only occur when critical mass has been achieved (one big vote FOR critical mass). However, the fission decay rates increase as you approach the critical mass, which is the result of emitted neutrons triggering subsequent fissions. So there are sequential fission events chained together, you might say (one vote AGAINST critical mass).

Now, with respect to "a decaying element", the element to undergo fissioning might not be spontaneously decaying, but might be triggered to do so by an externally supplied neutron. Does this count as "a decaying element" or not? It wasn't before it was triggered (one vote AGAINST decaying element), but it certainly is once it absorbs a neutron and fissions (one vote FOR decaying element). And also, the best way of generating thermal neutrons is through radioactive decay. But that doesn't have to be decay of the element to undergo a fission chain reaction (pushes the "decaying element" definition even more distant). So this is also a somewhat ambiguous choice.

All in all, I'm guessing that your instructor is looking for "critical mass" as the answer. But, you now see perhaps some of the pitfalls of trying to create unambiguous questions!

-- Cheers, -- Mr. d.