Comparison between measurement and calculations for a 14 MeV neutron water activation experiment

Abstract

The nuclear heat loads due to gamma rays emitted from the decay of ¹⁶N and delayed neutrons from¹⁷N, generated by the activation of water in cooling circuits, are critical for ITER design. The assessment of nuclear heating from activated water is complex; it requires temporal and spatial dependent transport and activation calculations taking into account variation of irradiation, water flow conditions and cooling circuits’ parameters. A water activation experiment has been recently conducted at the14 MeV Frascati Neutron Generator (FNG) in order to validate the methodology for water activation assessment used for ITER and to reduce the safety factors applied to the calculation results, which have a large impact on the schedule, commissioning and licensing. Water circulating inside an ITER First Wall (FW) mock-up was irradiated with 14 MeV neutrons and then measured using a large CsI scintillator detector. The system consists of a closed water loop where the cooling water, transiting through an ITER FW mock-up, is irradiated by FNG. The induced ¹⁶N activity via 14 MeV neutrons interactions with ¹⁶O via the ¹⁶O(n,p)¹⁶N reaction is measured in a dedicated counting station via an expansion volume. The water then passes to a much larger holding delay tank, and after several ¹⁶N half-lives decay time, it is then recirculated and exposed again to neutrons in the ITER First Wall (FW) mock-up. The measured ¹⁶N activity is obtained measuring the emitted characteristic 6.13 and 7.12 MeV gamma-rays. Calculations were performed in an accurate model of the FW mock-up using the MCNP Monte Carlo code and FENDL-3.1 nuclear data library to obtain the predicted flux impinging on the water. The EASY-2007 inventory code was used to predict the ¹⁶N activity. In this work, a comparison between measurements and calculations is reported together with associated uncertainty analysis.