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Criticality Consequence Calculation Involving Intact PWR MOX SNF in a Degraded 21 PWR Assembly Waste Package

John A. McClure
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The purpose of this calculation is to evaluate the transient behavior and consequences of a worstcase criticality event involving intact pressurized water reactor (PWR) mixed-oxide (MOX) spent nuclear fuel (SNF) in a degraded basket configuration inside a 21 PWR waste package (WP). This calculation will provide information necessary for demonstrating that the consequences of a worst-case criticality event involving intact PWR MOX SNF are insignificant in their effect on the overall radioisotopic inventory and on the integrity of the repository. This calculation includes results obtained by maximizing postulated rates of reactivity insertion to assure no synergistic reactions could occur among waste packages from hypothetical criticality events. Another variable, potentially influencing the criticality consequences, is the exit area of the leakage path(s) from the WP. If the leakage area through the WP is sufficiently small, inflow rates will be restricted, lengthening the time required for flooding the WP and thus delaying potential criticality events, which require flooded conditions. However, if a criticality event does occur, a limited leakage area will reduce the exit flow volume. The immediate effect on the system from a limited leakage area is to reduce the negative reactivity effect of voiding the WP because the water/vapor escape rate may be lowered. This, in turn, leads to higher heat output, higher internal pressure, and higher temperatures. The higher pressure and density of the water vapor will increase the mass flow out of the WP, so that eventually the negative reactivity from voiding the system becomes dominant, and the criticality event shuts down.
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