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

Water activated by fast neutrons emits both high energy gammas from ¹⁶N and delayed neutrons from ¹⁷N. The radioisotopes ${}^{\mathrm{16,17}}$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 (${\text{T}}_{\mathrm{1∕2}}=7.13$ $\pm$2 s) ${\to }^{16}$O + $\gamma$ (67% @ 6.13 MeV, 5% @ 7.12 MeV) and ¹⁷N (${\text{T}}_{\mathrm{1∕2}}=4.173$ $\pm$4 s) ${\to }^{17}$O${\to }^{16}$O + n (37.7% @ 0.386 MeV, 0.6% @ 0.886 MeV, 49.8% @ 1.16 MeV, 6.9% @ 1.69 MeV), $\gamma$ (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 ${}^{\mathrm{16,17}}$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 $\pm$60% uncertainty. In the attempt to measure and validate the cross-section data for ${}^{\mathrm{16,17}}$N production, the water activation experiment was repeated at FNG so to measure directly the ${}^{\mathrm{16,17}}$O(n,p)${}^{\mathrm{16,17}}$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.