Nuclear criticality safety analysis of a spent fuel waste package in a tuff repository

Preclosure analysis of conceptual waste package designs for a nuclear waste repository in tuff

“…The actual reactivity change that results from accounting for fuel burn-up is, of course, depen dent on many parameters including the fuel type, geometry, initial enrichment, and burn-up. The signif :ance of taking credit for fuel burn-up is demonstrated by the results of Weren et al (1983) and Petrie et al (1975). These studies showed, re spectively, a reduction of approximately 0.01 Ak per 1000 megawatt days per metric ton of uranium (MWD/MTU) for PWR assemblies and a 0.188 Ak reduction for a fresh-to-21,000 MWD/MTU burnup B&W 15 x 15, 3.1-wt% U-235 initial enrichment assembly, where Ak is the change in the criticality coefficient.…”

“…For various dry and flooded configurations of spent fuel canisters emplaced in the tuff reposi tory, recent criticality calculations show that k,,, is always below 0.95 for spent fuel depleted to < 1.4-wt% (weight percent) U-235 (Weren et al, 1983). The calculations indicate that k," will be less than 0,95 unless, as required in NRC 10 CFR 60, two un likely, independent, and concurrent changes have occurred in the conditions essential to nuclear crit icality safety.…”

“…The canisters contained fuel elements from six PWR or 18 BWR spent fuel assemblies consolidated (with no poison rods) in cylindrical space frames with triangular holes. * An analytical effort by Westinghouse Electric Cor poration's Waste Technology Services Division in Pittsburgh, PA, was subcontracted by LLNL to de termine if these designs conformed to the 10 CFR 60 criticality safety requirements (Weren et al, 1983). The analytical study was limited to a few scenarios and configurations over long time peri ods in order to identify any initial or rearranged configurations and any unlikely events which * The final reference design is 70 cm in diameter and contains six PWR or 14 BWR assemblies in square holed space frames.…”

“…Each of these config urations was described in Weren et al (1983). Con figurations 1, 2, 6, and 10 were evaluated in this study.…”

“…However, for DHLW and CHLW, the fissile material content of the waste forms' proposed to date is sufficiently low that a criticality accident is not of concern during the containment period (Westinghouse, 1983, p 60). Criticality analysis has not been conducted for long term conditions for glass waste forms.For various dry and flooded configurations of spent fuel canisters emplaced in the tuff reposi tory, recent criticality calculations show that k,,, is always below 0.95 for spent fuel depleted to < 1.4-wt% (weight percent) U-235 (Weren et al, 1983). The calculations indicate that k," will be less than 0,95 unless, as required in NRC 10 CFR 60, two un likely, independent, and concurrent changes have occurred in the conditions essential to nuclear crit icality safety.…”