Ph.D. Thesis Defense by John F. Zino
Monday, May 17, 1999

(Dr. Chris Wang, advisor)

"Design of a Pulse-Timing Based Subcritical Reactivity Measurement System for Spent Nuclear Fuel Assemblies"


This research investigated the concepts associated with crediting the burnup of spent nuclear fuel assemblies for the purposes of criticality safety. To accomplish this, a collaborative experimental research program was undertaken between Westinghouse, the University of Missouri Research Reactor (MURR) facility and Oak Ridge National Laboratory (ORNL). The purpose of the program was to characterize the subcritical behavior of a small array of fresh and spent MURR fuel assemblies using the 252Cf source-driven noise technique. An aluminum test rig was built which was capable of holding up to four, highly enriched (93.15 wt.% 235U) MURR fuel assemblies in a 2x2 array. The rig was outfitted with a 0.11 ?g 252Cf neutron source and up to four 3He neutron detectors which allowed researchers to perform active neutron noise measurements on the array of fuel assemblies. The neutron detectors were quenched with CF4 gas to allow improved discrimination of the neutron signals in very high gamma radiation fields (~8000 R/hr). In addition, the detector drywells were outfitted with 1 lead collars to provide additional gamma shielding from the spent fuel. Reactivity changes were induced in the subcritical lattice by replacing individual fresh assemblies (in a 4-assembly array) with spent assemblies of known, maximum burnup (143 Mw-D). The absolute and relative measured reactivity changes were then compared to those predicted by three-dimensional Monte Carlo calculations. The purpose of these comparisons was to investigate the accuracy of modern transport theory depletion calculations to accurately simulate the reactivity effects of burnup in spent nuclear fuel.

A total of seven subcritical measurements were performed at the MURR reactor facility on July 20th and 27th, 1998. These measurements generated several estimates of prompt neutron decay constants (?) and ratios of spectral densities through frequency correlations between the 252Cf source and different pairs of neutron detectors in the array. Agreement between measured and calculated subcritical reactivity was within 5% for both ? and spectral ratio values for a reference measurement using four fresh (unirradiated) fuel assemblies. Agreement between measured and calculated ? values was comparable for the single and double spent fuel replacement measurements however spectral ratios disagreed by as much as 12 to 20%. Investigation into the potential effects of spontaneous fission neutron sources in the spent fuel from 242Cm and 240Pu are believed to account for some of the differences. In addition, analyses revealed that the different approximations assumed in calculating the burnup estimates of fissile and fission product inventories of the spent fuel are believed to be primarily responsible for the differences. The insights gained in performing these measurements provide the basis for establishing the technical criteria needed for future designs of spent nuclear fuel burnup credit devices.