 In these early days we explored all sorts of power reactors , comparing the advantages and disadvantages of each type . The number of possibilities was enormous , since there are many possibilities for each component of a reactor — fuel , coolant , moderator . The fissile material may be 233U , 235U , or 239Pu ; the coolant may be : water , heavy water , gas , or liquid metal ; the moderator may be : water , heavy water , beryllium , graphite — or , in a fast- neutron reactor , no moderator . I have calculated that , if one counted all the combinations of fuel , coolant , and moderator , one could identify about a thousand distinct reactors . Thus , at the very beginning of nuclear power , we had to choose which possibilities to pursue , which to ignore . 
