Reanalysis of the gas-cooled fast reactor experiments at the zero power facility Proteus - Spectral indices

PROTEUS is a zero power reactor at the Paul Scherrer Institute which has been employed during the 1970's to study experimentally the physics of the gas-cooled fast reactor. Reaction rate distributions, flux spectrum and reactivity effects have been measured in several configurations featuring PuO₂/UO₂ fuel, absorbers, large iron shields, and thorium oxide and thorium metal fuel either distributed quasi-homogeneously in the reference PuO₂/UO₂ lattice or introduced in the form of radial and axial blanket zones. This papers focuses on the spectral indices - including fission and capture in ²³²Th and ²³⁷Np - measured in the reference PuO₂/UO₂ lattices and their predictions with an MCNPX model specially developed for the PROTEUS-GCFR core. Predictions were obtained with JEFF-3.1 and -3.11, ENDF/B-VII.0 and VII.1, and JENDL-3.3 and -4.0. A general good agreement was demonstrated. The ratio of ²³²Th fission to ²³⁹Pu fission, however, was under-predicted by (8.7±2.1)% and (6.5±2.1)% using ENDF/B-VII.0 and VII.1, respectively. Finally, the capture rates in ²³⁷Np tended to be under-predicted by the JEFF and JENDL libraries, although the new cross section in JEFF-3.1.1 slightly improved the ²³⁷Np capture to ²³⁹Pu fission results (3.4±2.4)%. (authors)