Implementation of a low-activation Au-In-Cd decoupler into the J-PARC 1 MW short pulsed spallation neutron source

Highlights: • Thermal neutron absorber material, called decoupler, is effective to produce narrow pulsed neutron beams with short tails for accelerator based pulsed neutron source. • As the decoupler, an Au-In-Cd alloy, which has high neutron cut off energy of 1 eV, was developed from the viewpoint of reduction of induced radioactivity and irradiation damage in a MW class neutron source. • Au-In-Cd decoupler, for the first time, was implemented into a reflector assembly by investigating the hot isostatic pressing process (HIP) in terms of the surface conditions, sizes, and heat capacities of actual large sized Au-In-Cd alloys. - Abstract: A silver-indium-cadmium (Ag-In-Cd, or AIC) alloy with a 1 eV high neutron cut-off energy was originaly developed as a decoupler to provide a narrow neutron pulse width with a short tail for the J-PARC 1-MW short-pulsed spallation neutron source. As a result of later studies, gold (Au) was chosen as a substitute for Ag for the production of spare decoupled moderators and reflector in order to reduce residual radioactivity thus easing handling and disposal of the spent moderator and reflector. The decoupler material Au-In-Cd (AuIC) was therefore investigated and developed. To implement it into an actual moderator-reflector assembly many critical engineering issues needed to be resolved with regard to large-sized bonding between AuIC and aluminum alloy A5083 by the hot isostatic pressing process (HIP). The HIP process for AuIC and aluminum was investigated in terms of surface condition, size, and heat capacity. Implementation of an AuIC decoupler into a spare reflector assembly was successfuly achieved and will result in a remarkable reduction of radioactivity compared to AIC without sacrificing neutronic performance.