Speaker
Description
The low energy neutron treatment in the authentic FLUKA code has been greatly expanded over the last couple of years.
Until recently, the treatment of neutrons below 20 MeV was based on a 260-group library built with NJOY99 (plus ad-hoc extensions) and to various degrees based on ENDF/B-VIR8, ENDF/B-VIIR0-1, and JEFF-3.2. New evaluations are now available which contain more isotopes, more accurate information, and with new formats (eg for unresolved resonances) which are incompatible with older NJOY versions. Consequently, the entire processing chain is now built on NJOY16 (work with NJOY20 is in progress) and a vast majority of the isotope are now processed out of the most recent evaluations, mostly ENDF/B-VIIIR0. At the same time, the library, besides the residual nuclei cross sections, now contains also the probability of producing isomers with cross sections derived from EAF-10
Fully correlated pointwise treatment for some isotopes ($^1$H, $^2$H, $^3$He, $^4$He) was available also in previous FLUKA versions. However no general pointwise treatment was possible, unless in a few exceptional cases (eg $^{40}$Ar, $^{208}$Pb) which were not generally distributed because of several limitations. Now, a novel, pointwise based, alternative is provided in FLUKA for all stable isotopes. This approach, using as much as possible the evaluated data file information, which is strictly inclusive, complemented with physics modelling and some reasonable assumptions, is now able to produce event-by-event, exclusive, fully correlated neutron interactions in the 20 MeV - $10^{-5}$ eV energy range. The pointwise library is based on ENDF/B-VIIIR0 and TENDL-19, and it is built with a combination of PREPRO19, NJOY16 (both somewhat modified) and an extensive in-house developed code.
Examples proving the neutronics performances of the pointwise libraries against the group-wise library will be presented. Applications which otherwise could not be possible without the fully correlated pointwise treatment will also be presented.
During the development of the pointwise libraries, several issues denoting inconsistencies or outright errors in the evaluated data files had emerged. Moreover, some problematic tendencies in the way new evaluated data files are produced were also noted. It is important for our community to be aware of those issues, and possibly exert pressure on the evaluator community to take into account our concerns.