Why is enrichment important for a fission chain reaction ?

Question

- keeps neutrons from escaping the sample
-keeps neutrons from being absorbed by other isotopes
-allows neutrons to be absorbed by other fissionable nuclei
-provides enough fuel to make enough energy .

Answer 1

-allows neutrons to be absorbed by other fissionable nuclei

Answer 2

I disagree with the answer by anonymous. If a sample is enriched, say we're talking uranium, then it's the uranium that is enriched from say 3% or so to something larger like 50%. Therefore, a neutron has a larger chance of entering a U-235 atom than some other atom, thus the chances of a chain reaction with U-235 is enhanced many fold. I think the correct answer is -keeps neutrons from being absorbed by other isotopes BECAUSE the fissionable material is enhanced several fold which helps maintain a chain reaction AND by enriching the fissionable material it makes the other isotopes, fissionable or not, be there in much smaller amounts. In fact, by enriching the fissionable material it stand to reason that other materials there, even if they are fissionable (and most aren't) there won't be enough of them to sustain a chain reaction. The bottom line is that if there is a competition for the available neutrons between the U-235 (the fissionable material) and some other element, it stands to reason that by enriching the U-235 it increases the chances that U-235 gets the neutron rather than much much smaller amount of other elements capturing the neutron.