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.
This percentage is too low to
sustain a chain reaction when combined with most practical moderators.
Hence either, the probability of fission
must be enhanced or the moderator effectiveness must be enhanced.
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.