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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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