Main characteristics and options of reactor families
Commercial reactors operate with natural uranium or moderately enriched uranium. Using natural uranium as fuel restricts the choice of moderator to graphite and heavy water. The use of enriched uranium opens a large choice of coolant (liquid, vapor or gas) and moderators. Some combinations are more successful than others: water, either heavy or ordinary, has been very successful since it is an efficient coolant and a good moderator. Light water reactors (PWRs and BWRs) represent the majority of the contingent of today Generation II reactors and their Generation III successors in a near future.
P.Reuss/Précis de neutronique EDP Sciences 2003