Nuclear chain reactions with graphite moderation

The fission reaction occurs in a controlled way in the nuclear pile. Here rods of ordinary which has been enriched with are built into a structure with a moderator such as graphite or D2O. The neutrons that are emitted at high speeds from the fission of 23 5u are slowed to thermal speeds by the moderator. The thermal neutrons suffer three important fates some continue the chain to produce the fission of more others are captured by and some are absorbed by the control rods of the reactor. The neutron flux in the reactor is monitored constantly. Moving the absorbing control rods into or out of the pile reduces or increases the neutron flux. In this way suflicient neutrons are permitted to maintain the chain reaction at a smooth rate, but enough are absorbed to prevent an explosion. 

In about June 1942 everybody became convinced that a chain reaction in a graphite moderated pile was feasible with ordinary uranium. As a result emphasis began to shift from problems of nuclear physics to problems of engineering and arrangement. The work on nuclear physics continued to go on but the principal (and often emotional) interest was shifted to questions of cooling and general engineering. 

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