Conception of delayed neutron detector blocks in a sodium cooled fast reactor

The primary vessel of a sodium-cooled fast neutron reactor (SFR) must remain free of any contaminant released by a cladding failure in a fuel assembly. A comprehensive detection system of cladding failures has then to be considered right from the beginning of the reactor design. One sub-system, called delayed neutron detection (DND), aims to search for delayed neutron precursors (DNP) through an ex-vessel analysis of a sodium sample, which goes to a container, usually named a pot, surrounded by neutron detectors. This paper presents a methodology to optimize the hydraulic part of the design of the DND detection block. From a detection scenario that identifies the physical parameters in play, a computational model is derived, which couples fluid mechanics and neutron transport. Different types of pot designs are considered, for which the sodium flow velocity, DNP concentration and detector activities are assessed. A new type of pot with a coil shape is proposed and shown to have the better performances with respect to the detector response to delayed neutrons.