Development of reference benchmark neutron field in LR-0 reactor

Abstract

A neutron reference field is defined as a permanent reproducible neutron field with well defined neutron fluence rate and neutron energy spectra. The standard reference field has neutron spectra characterized employing neutron spectra measurements by means of time-of-flight measurements. Historically there was only one standard neutron reference field, 252Cf(s.f.). In 2017 into neutron standards, there was also included 235U(n_th, fiss), which becomes secondary neutron standard. Reference neutron benchmark fields are defined as permanent and reproducible neutron fields, less well characterized as a standard field, but still acceptable as a measurement reference by a community of users. This work deals with the development of a reference neutron benchmark field in a special core placed in an LR-0 reactor. In this reference neutron benchmark field, there were precisely measured neutron and gamma spectra using stilbene spectrometry. This is very important in the case of this reactor benchmark field because it was observed that neutron spectra above 6 MeV are nearly identical with 235U PFNS, and also it was confirmed that the gamma has a negligible effect on measured neutronic quantities. Namely, the impact of the photo-nuclear reactions (γ,n) competing with (n,2n) in the production of the same residual nucleus was shown to be negligible. To confirm the spatial distribution of the field, there was also validated the fission density distribution across the driver core. This precise characterization of the core is important and well usable also in other issues. It’s worth noting it is an excellent tool for validation of neutronic properties of materials inserted in the center of the special core, or surrounding it, for example, sand, whose neutronic parameters are important not only in space programs but also in used fuel storage management, because major material of earth crust is SiO2. The most recent also interesting application was validation of neutronic description of stainless steel, which is important in criticality safety issues as stainless steel is the major component of water moderated reactors internals. The essential results, namely criticality, flux distribution, neutron spectra and also some of the measured spectral averaged cross section evaluated as SACS averaged in 235U PFNS were benchmarked in the IRPhEP database. This database is the most reliable one, which is used in improvements of nuclear cross sections. The field became one of the IRDFF-II reference benchmark neutron fields, and the measured data were used for improving the newly developed IRDFF-II neutron dosimetry library.