Analysis of sodium void reactivity of control rod regions in Phenix sized core with Monte Carlo Code MVP

The cause of the abnormal power change phenomenon of Phenix core is studied under the assumption that a negative reactivity insertion is induced by a gas blow into the control rod assemblies. The size and layout of the core analyzed are set to simulate the Phenix core while material compositions of the assemblies are assumed to be a typical MOX fuel core and the control rod channel is assumed to be a simple sodium plenum zone at intact. The calculation has been conducted by using three-dimensional continuation energy Monte Carlo code MVP with JENDL-3.3 library so that the neutron moderation and leakage through the void channel can be evaluated exactly. The result of the analysis shows the negative void reactivity is about -80 cents when the sodium below the level of 1/3 of core height from the core bottom of the seven control assemblies including the center shutdown system is voided while it is about -1 cents when the sodium between the core bottom to the core top is voided in the same assemblies. The void reactivity for the region above the level of 2/3 of core height is again about -80 cents. The negative reactivity of -80 cents is consistent to the power decrease of about 40 % at the beginning of the event if zero-power reactor model can be applied to the transient of this phenomenon. The successive power swing observed can be explained by the reactivity change if the gas flows up through the control assemblies in which the cross sectional area of the gas is reduced by a half at the upper level of the assembly due to the absorber pins. (authors)