This is interesting and probably why MNS graphite would be ideal:
"In the first attempt to produce a self sustaining nuclear reaction, nuclear graphite impurity may have prevented success. The second try was performed with AGX graphite and went critical. The Pile constructed was called the Chicago Pile-1. Interestingly, German scientists reached the conclusion that graphite is preventative when combined with natural uranium for a sustained nuclear chain reaction. However, the scarcity of pure nuclear graphite was at play. The purest nuclear graphite the Germans had access to was a product from the Siemens Plania company. Siemans Plania exhibited a neutron absorption cross section of approximately 7.5 mb. The nuclear graphite used in Chicago Pile-1 portrayed average thermal absorption cross sections of 6.68 mb, 5.51 mb, and 4.97 mb."
"Finally, maintaining purity may be considered a safety precaution concerning the integrity of nuclear graphite. Measuring differences in thermal neutron absorption encountered between graphite standards and graphite test bars placed in a reactor is a method for measuring purity and is known as units of in-hours (”delta-in-hours” or DIH). The DIH measurement may be performed at a reactor and is pertinent to graphite utilized as moderator or reflector components of nuclear and test reactors and for thermal columns. "
"The main reason graphite has been used to date in nuclear reactors, despite the fact that it's not as good a moderator as water (hydrogen), is that it has very low neutron absorption cross section, unlike hydrogen. If graphite is pure enough (without a significant amount of neutron absorbent impurities) and present in sufficient quantity, makes it possible to achieve criticality using just natural uranium as fuel. The latter is not possible with light water. Heavy water would be needed instead, which is much more expensive than graphite."
(20min delay)