 Before the MSRE development began, tests had shown that salt would not permeate graphite in which the pores were on the order of a micrometer.Graphite with the desired pore structure was available only in small, experimentally prepared pieces, however, and when a manufacturer set out to produce a new grade (CGB) to meet the MSRE requirements, difficulties were encountered.The fuel was 7LiF-BeF2-ZrF4-UF4 (65-29.1-5-0.9 mole %).