Conceptual design of ASTRID fuel sub-assemblies

•The fuel sub-assembly design for the ASTRID CFV core is described.•Innovative design choices have been made to comply with the GEN IV objectives.•The heterogeneous and the large fuel pins contribute to a low sodium void worth.•The upper neutron shielding is removable from the S/A head before washing. The French 600MWe Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID) project has reached the end of its Conceptual Design phase. The core design studies are being conducted by the CEA with support from AREVA and EDF. Innovative design choices for the core have been made to comply with the GEN IV reactor objectives, marking a break with the former Phénix and SuperPhénix Sodium Fast Reactors. The main objective to improve safety compared with current GEN II or III reactors led to a core design that demonstrates intrinsically safe behaviour. A negative sodium void worth is achieved thanks to a new fuel sub-assembly design including (U,Pu)O2 and UO2 axially heterogeneous fuel pins, a large cladding/small spacer wire bundle, a sodium plenum above the fuel pins, and upper neutron shielding with both enriched and natural boron carbide (B4C) which also maintain a low secondary sodium activity level. As these Na-bonded B4C pins can lead to the retention of unacceptable amounts of sodium, the whole upper neutron shielding has been made removable on-line through the sub-assembly head just before the washing operations. Finite elements calculations have been performed to increase the stiffness of the stamped spacer pads in order to analyse its effect on the core mechanical behaviour during hypothetical radial core flowering and compaction events. More generally, all design choices for ASTRID have been made with the permanent objective of minimising the sub-assembly height to decrease the overall costs of the reactor and the fuel cycle. This paper describes the fuel sub-assembly design for the ASTRID CFV v4 core at the end of the Conceptual Design phase and studies performed to optimise the performances. Focus is placed on innovations and specificities in the design compared with former French SFRs.