Pre-conceptual design of ASTRID fuel sub-assemblies

International audienceThe French 600 MWe Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID) project is currently reaching the end of its conceptual design phase (AVP2). 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 Phenix and SuperPhenix 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)O$_2$ and UO$_2$ 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 (B$_4$C) which also maintain a low secondary sodium activity level. As Na-bonded B$_4$C 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. To prevent unsafe reactivity insertions due to hypothetical radial core compaction, the stiffness of the embossed spacer pads has been improved significantly by optimising its geometry using finite elementcalculations. More generally, all design choices for ASTRID have been made with the permanent objective of minimising the sub-assembly height 4.50 m to decrease the overall costs of the boiler 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. Focus is placed on innovations and specificities in the design compared with former French SFRs. The paper also mentions some open options that will be studied during the next basic design phase