Computational evaluation of N-16 measurements for a 14 MeV neutron irradiation of an ITER first wall component with water circuit

During ITER operations the water coolant flowing through components such as the first wall, blanket modules, divertor cassettes and vacuum vessel will become activated by high energy neutrons. Two key neutron-induced reactions will occur with oxygen in the water producing the radioactive isotopes N-16 and N-17, which have relatively short half-lives of a few seconds. These nuclides are transported in coolant loops and, unmitigated, their decay emissions will induce additional nuclear heat in components, potentially including superconducting magnets, and lead to an increase in the occupational dose for workers and sensitive equipment outside the biological shield. Variations in irradiation, water flow rate and cooling circuit parameters make it difficult to predict nuclear heating. A water activation experiment has recently been performed at the 14 MeV Frascati Neutron Generator to accurately measure N-16 and N-17 produced by irradiating an ITER first wall mock-up. This experiment aimed to validate the methodology for water activation assessment used for ITER and to provide scientific justification to reduce safety factors, which have a large impact on ITER component design and qualification. This paper provides a detailed description of neutronics calculations performed together with the GammaFlow code to model the temporal evolution of activated water, along with MCNP6.1 and FISPACT-II to calculate the detector response. The calculated reaction rates associated with nuclear data from ten libraries have been compared with measured data, although as many cross-sections originated from the same library effectively five nuclear data libraries have been compared.