ATR Al–B neutron filter design for fusion material experiment

Ferritic steels, vanadium alloys, and ceramics are being developed for potential use in fusion reactors. Properties of these materials after irradiation are required for evaluation of their acceptability. The Irradiation Test Vehicle (ITV) was installed in the Advanced Test Reactor (ATR) center flux trap position for irradiation of materials. The ratio of fast to thermal neutron flux is low when compared to a fusion reactor. In this study, one of the chosen materials for potential testing is a vanadium alloy. The test assembly design goals are vanadium (VLi bond) specimen displacement per atom (dpa) greater than 10 dpa per year, and 51V transmutation at 30 dpa less than 0.5 atom%. The dpa is proportional to and dominated by the fast neutron fluence (E>0.1 MeV). The 51V transmutation rate is proportional to and dominated by the thermal neutron fluence (E<0.414 eV). To prevent excessive transmutation of the vanadium, the thermal neutron fluence needs to be limited while attaining the required dpa. A neutron filter to harden the neutron spectrum is required such that the design goals can be met. The computer codes MCNP and ORIGEN-2 were coupled by MCWO, and were utilized to design and evaluate a thermal neutron filter in the ATR ITV that provides an acceptable environment for vanadium alloy testing.