Prominent among those who envisioned fuel breeding as a promising concept were Enrico Fermi and
Walter Zinn,the first director of Argonne National Laboratory. Late in 1944, Zinn, at the urging of
Fermi, began planning a smallscale proof-test facility for proving the validity of the breeding
principle and for evaluating the feasability of using a liquid metal as a coolant.

Their approach to the technology was simple: minimize the fraction of neutrons lost by parasitic
capture to the coolant, moderator, and fuel; and maximize productive captures in massive uranium
blankets. Although sophisticated neutron cross-section data were lacking at that time (circa 1943)
enough information did exist to support the conclusion that parasitic neutron losses could be
sharply reduced if the average neutron energy remained high. Such a requirement imposed a variety
of constraints, among the most important of which was the complete absence of conventional coolant-
moderator materials. Attention was accordingly drawn to liquid metal coolants, in particular to NaK
(the sodium-potassium eutectic mixture). NaK had the obvious advantages of being a liquid at room
temperature and having excellent heat transfer properties. Furthermore, NaK was considered nearly
ideal from the viewpoint of neutron economy; it was both a poor moderating and a poor absorbing material.
(The American Society of Mechanical Engineers - June 15,1979)