Question

Why does a fast fission reactor require enriched uranium?

Answer 1

Water has disadvantage as a moderator. It can absorb neutron and remove it from the reaction. It does this just enough that the concentration of uranium - 235 is too low to sustain the chain reaction. The neutrons lost through absorption in water and uranium - 238 which result in too few left in the fuel. The most common solution of this problem is to used slightly concentrate the amount of uranium - 235 in the fuel to produce enriched uranium.

If the fuel is enriched, a thersold will be reached where there are fissile atom in the fuel to maintain a chain reaction even with fast neutrons.