In fact, the Germans thought that heavy water would be the only moderator that would work and this hampered their efforts. Fermi et al. were able to ascertain t

In fact, the Germans thought that heavy water would be the only moderator that would work and this hampered their efforts. Fermi et al. were able to ascertain that a certain purity of graphite could instead be used, since heavy water was difficult to obtain and expensive at the time. By utilising material from a specialty graphite manufacturer they were able to achieve criticality with Chicago Pile-1