# Pre-design of a target assembly for minor actinides transmutation

Abstract : Minor actinides (MA) transmutation options for critical fast reactors are divided in two different approaches, the homogeneous one in which MA are diluted in the driver fuel and the heterogeneous one in which Ma are concentrated in UO$_2$ based fuel in sub-assemblies located at the periphery of the core. This latter option, named minor actinides bearing blankets has a small impact on core behavior, at the expense of lower transmutation performances due to the lower flux level experienced by the targets. As such, there is an incentive to maximize the volume fraction of minor actinides loaded in the target assemblies in order to achieve optimal transmutation performances. However, a high MA fraction lead to an increase in gas especially Helium production and negatively impacts fuel swelling and pressurization of the fuel pin along with its thermo-mechanical behavior. Consequently, an iterative optimization process must be carried out during the pre-design step of such an assembly to optimize both the neutronic and mechanical performances. Using a conservative approach, we assumed that all produced gases were released in the pin free space. This production was evaluated using depletion calculations and the corresponding pin internal pressure and resistance criterion were computed. Centerline temperature of the hottest pin was also evaluated. Sensitivities to technological constraints considered into the model were also computed.Several options were evaluated to develop a suitable assembly smear density decrease, plenum size increase, cladding thickness increase and modification of the minor actinides volume fraction. We found that an optimum existed at 47 % of fuel volume fraction, corresponding to a situation with wider pins, thicker cladding and increased gas expansion plenum height compared to standard fuel assemblies. The associated transmutation performances were in the range of -8.2 kg/TWhe, or a 33 % increase in the minor actinides consumption compared to standard fuel assembly design. A pressure drop model was also implemented and it was verified that the pressure drop remained below the one of a standard fuel assembly. The new assembly design impacts on decay heat and neutron source were also assessed and it was shown that a design margin existed for optimization with regards to these impacts.
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Cited literature [14 references]

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• HAL Id : hal-02441960, version 1

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T. Kooyman, L. Buiron, B. Valentin, G. Rimpault, F. Delage. Pre-design of a target assembly for minor actinides transmutation. 14th Exchange Meeting on partitioning and transmutation - Actinide and Fission Product Partitioning and Transmutation, Oct 2016, San Diego, United States. ⟨hal-02441960⟩

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