•Most practical reactors are
"thermal" reactors, that is, they utilize the higher thermal cross sections.
•Possible fuels include some of the various
isotopes
of uranium (U) and plutonium (Pu). The only naturally occurring fuel with
suitable properties
of significant quantities is U-235, hence most reactors use this fuel.
•Naturally occurring uranium is composed of 0.7% U-235.
The rest is U-238.
•One group of reactor types (PWR, BWR, HTGR) enrich the fuel (a costly task) and use a cheap moderator (ordinary water or graphite).
•In another class, natural uranium (relatively cheap) is used with an excellent but expensive moderator (heavy water). This is the CANDU approach.
•Which is better?
There is no simple answer. Both work. In engineered systems, there are always tradeoffs and the final design has to be viewed in the overall context of the end-use environment.