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.
Highlights
The water coolant in ITER components such as those inside the first wall, blanket modules, divertor cassettes and vacuum vessel will become activated by neutrons during DT plasma operations
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 measured using a large CsI scintillator detector
Summary
The water coolant in ITER components such as those inside the first wall, blanket modules, divertor cassettes and vacuum vessel will become activated by neutrons during DT plasma operations. The uncertainty in the calculation of radiation maps due to activated water is evaluated to be very large [1], the main sources of uncertainty being due to modelling and nuclear data, and safety factors between 8.2 and 4.7 are applied. The motivation for this new experiment is to accurately measure the. A high flux-prevalence pump (a model type that is frequently used in artesian wells), immersed in a tank of about 100 litres, sent the water through a plastic tube of internal diameter 2.8 cm to the FW panel mock-up. The expansion tank material was aluminium with a maximum wall thickness of 0.5 cm to support the water pressure
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