Measurement of the 16,17O(n,p)16,17N cross sections for validating the water activation experiment for ITER at the Frascati neutron generator

Abstract Water activated by fast neutrons emits both high energy gammas from 16N and delayed neutrons from 17N. The radioisotopes 16,17 N are produced through the 16O(n,p)16N and 17O(n,p)17N reactions (thresholds about 10 MeV) and decay through the reactions 16N ( T 1∕2 = 7 . 13 ± 2 s) → 16 O + γ (67% @ 6.13 MeV, 5% @ 7.12 MeV) and 17N ( T 1∕2 = 4 . 173 ± 4 s) → 17 O → 16 O + n (37.7% @ 0.386 MeV, 0.6% @ 0.886 MeV, 49.8% @ 1.16 MeV, 6.9% @ 1.69 MeV), γ (3.7% @ 0.87 MeV), respectively. Activated water, flowing throughout a nuclear plant distributes the radioactivity to many critical components where personnel and/or instrumentation can be located thus depositing additional radiation dose which requires a proper shielding. This source of radiation represents an issue for a D-T fusion device like ITER which uses water as the main cooling fluid for components such as e.g. first wall and divertor. Owing to the scarce experimental information available for the cross sections of the 16O(n,p)16N and 17O(n,p)17N reactions and few benchmark validations, large uncertainties are affecting the calculations of the water activation induced by DT neutrons in a tokamak. In the attempt to improve the available nuclear data Fusion for Energy (F4E) tasked ENEA to carry on a benchmark experiment to validate the water activation calculations presently performed for ITER. This benchmark experiment was carried out in the year 2019 irradiating with the 14 MeV neutrons produced by the Frascati Neutron generator (FNG) a small size ITER First Wall mock-up. Demineralized water was circulating inside the mock-up and downstream of the irradiation zone the 16N gamma-rays decay and the 17N neutrons decay were measured using proper detectors. The experiment was simulated using the MCNP and the FISPACT codes. The obtained C/E values are in a range close to unit for both 16,17 N isotopes. However, due to the large uncertainty on the 16O(n,p)16N and 17O(n,p)17N reactions cross-sections, for some of the used cross-section data base, e.g. ENDF/B-VIII.0, the C/E is affected by large uncertainty. This is especially true for the case of 17N which production cross-section is affected by ± 60% uncertainty. In the attempt to measure and validate the cross-section data for 16,17 N production, the water activation experiment was repeated at FNG so to measure directly the 16,17 O(n,p) 16,17 N reaction cross-sections at almost monochromatic energies. The results of this new experiment are presented in this paper and compared to the data currently used by ITER project for calculating the water activation.