Two-region analysis of pulsing data in fast critical systems

Pulsed neutron die-away experiments were performed in three fast critical systems assembled in the Argonne Zero Power Reactor-III (ZPR-III). These experiments were part of a comprehensive study of the mockups of the proposed fast reactors FARET (Fast Reactor Test) and SEFOR (Southwest Experimental Fast Oxide Reactor). The two FARET systems pulsed were Assembly 46-B, a 68 l plutonium-uranium carbide core with stainless steel and sodium reflector; and Assembly 46-C, an 87 l plutonium-uranium oxide core with the same reflector materials. The SEFOR mockup was a 528 l plutonium-depleted uranium oxide core with a reflector of stainless steel, sodium and nickel. In all three cases, an acceptable analysis of the die-away curves required two exponentials plus background. From this data, it was possible to assign, by a two-region transfer function analysis, a prompt neutron lifetime to core neutrons and to the system as a whole. Further, the transfer function model used predicts that the die-away curves in the reflector will contain two exponential terms having the same two exponential decay constants as were found in the core, and that the coefficient of the faster decay will be negative. This was found to be the case within experimental uncertainty. (Auth.)