【重水反应堆】...目前天然铀反应堆有两种石墨反应堆和重水反应堆....

【重水反应堆】...目前天然铀反应堆有两种石墨反应堆和重水反应堆....

【答案】 在商业运行的反应堆类型中使用天然铀的确实只有石墨反应堆和重水反应堆,因为石墨和重水对中子的吸收较低,用天然铀也能够达到临界,而且经济性相对也可接受.
　　理论上讲,慢化剂应选择核轻（轻核慢化效率高,不需要很大的堆芯体积）、对中子吸收能力弱（中子利用率高,对铀的富集堆要求低）的材料.那么铍也是很好的慢化剂材料,理论上也可以使用天然铀,但是成本太高了.
　　这道题建议选择对.具体可参考维基百科“Neutron moderator”词条,下边是截取的两段：
　　Some moderators are quite expensive,for example beryllium,and reactor-grade heavy water.Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium.This is difficult to prepare because heavy water and regular water form the same chemical bonds in almost the same ways,at only slightly different speeds.
　　The much cheaper light water moderator (essentially very pure regular water ) absorbs too many neutrons to be used with unenriched natural uranium,and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors,increasing overall costs.Both enrichment and reprocessing are expensive and technologically challenging processes,and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material,causing proliferation concerns.Reprocessing schemes that are more resistant to proliferation are currently under development.

我也是这个答案