Scientific motivations for a reassessment of the neutron capture cross sections of erbium isotopes in the high-sensitivity thermal energy range for LWR systems

Research conducted in the last twenty years in the field of burnable absorbers showed that erbium isotopes can be considered as an excellent alternative absorber to gadolinium isotopes for their neutronic and nuclear safety improving features. The development of the Erbium Super High Burnup (Er-SHB) concept demonstrated that erbium could be directly mixed in all fuel pins of a fuel assembly (FA) at the Beginning of Life (BOL). This innovative design allows an improvement of nuclear safety, a better control of the operational and accidental transient phase and an extension of the fuel life with respect to the most used burnable absorber (i.e., gadolinium). Furthermore, the extensive use of an Er-SHB fuel design would allow the production of higher enriched nuclear fuel (i.e., >5 wt%) within the existing manufacturing facilities without any modification of the facility itself and with a general improvement of the nuclear safety of the front-end phase of the nuclear fuel cycle. Nevertheless, reported erbium cross-sections are dated and poorly investigated in the high sensitivity thermal energy region for nuclear technology. In addition, some of them (i.e., Er-166) are reported with an uncertainty that is too high for their use in the future design of the erbia-doped LWR assembly by the industry. On the other hand, evaluated uncertainties by the ENDF/B-VIII.0 library in the thermal/epithermal region for the most sensitive isotopes (i.e., Er-167) seem to be too low with respect to both the experimental data and the analysis of the results provided by some erbia-doped critical systems of the International Critical Safety Benchmark Evaluation Project (ICSBEP). Based on the reanalysis of the ICSBEP outcomes, and a sensitivity-uncertainty analysis (S&U) on an Er-SHB LWR assembly, this article shows that recent evaluations appear inadequate to provide accurate criticality calculations for a system all equipped with erbium fuel pins for neutronic design purpose. Moreover, the S&U results have shown the importance of erbium isotopes to correctly evaluate the uncertainty associated with a Light Water Reactor (LWR) critical system. They confirmed the need for a re-evaluation of their neutron capture cross section by means of a new experimental campaign. A proposal aiming at performing a new capture measurement of erbium isotope cross sections has already been submitted to GELINA facility at Geel (Belgium), which is particularly suitable for neutron capture and transmission measurements in the thermal and epithermal energy regions. On August 2021, U.S. Nuclear Energy Agency (NEA) added the revaluation of Er-167(n, γ) in its High Priority Request List (HPRL) based on the outcomes reported in this work. On January 2022, GELINA Scientific Committee accepted the proposal within the 2021 calls for open access to JRC Research Infrastructures in the research filed of European Research for nuclear reaction, radioactivity, radiation and technology studies in science and application (EUFRAT).