|Type||H - High priority request|
|Quantity||SIG - Cross section RP - Cross section resonance parameters|
|Incident energy||0.01 eV - 100 eV|
|Subfield||LWR, innovative fuel|
|Status||Work in progress|
|Latest review date||28-Apr-2022|
Dr Antonio GUGLIELMELLI at ENEA-BOLO, ITY
A more precise knowledge of the erbium capture cross section in the most important energy region for LWR technology would reduce the criticality uncertainty margin in the erbia-credit super high burnup (Er-SHB) fuel concept. The Er-SHB concept is a system constituted of LWR fuel assemblies in which all fuel pins are erbia-doped. This system has already been subject to a research phase, and it has been verified that its application on large-scale commercial reactors would allow both to extend the fuel life cycle and to obtain several physical enhancements (i.e., less downgrade of the flux distribution, improving the intrinsic reactor safety parameters, better control of the transient power phase). The adoption of a Er-SHB fuel also allows to manufacture fuel with an enrichment greater than 5 wt% without modifying the current fuel manufacturing facilities that should be handling more enriched (> 5 wt%) uranium fuel, but whose criticality is hold-down by the erbia absorbers. In turn, the improvement of the erbia – especially Er-167(n,g) – capture cross sections, would make possible to assess better the criticality safety of the manufacturing facilities of an erbia-doped highly enriched uranium (Er-HEU) fuel. The erbia cross sections improvement will also allow to significantly reduce the uranium fuel cost because the code used to perform core neutronic design calculations will be able to optimize the uranium mass necessary to reach criticality conditions of an erbia-doped core system with less uncertainty.
< 2% in all the energetic range below 100 eV
The target uncertainty for the Er-167(n,g) cross section is proposed to be less than 2% in order to reach the same level of uncertainty attributable to U-235(n,f) on the integral parameter (criticality coefficient) of a erbia-doped LWR system.
The Sensitivity and Uncertainty (S&U) analysis performed on an erbia-doped LWR system showed that the relevant region should be extended up to 100 eV because of minor but not-negligible contributions beyond 10 eV.
A Sensitivity and Uncertainty study (S&U) has been performed to evaluate the criticality uncertainty contribution of the erbium isotopes on an Er-SHB system. The analysis was performed with the TSUNAMI-2D code of SCALE 6.2.3 tool. The data library used (v7-252) is a 252-energy group library based on ENDF/B-VII.1 evaluation, the covariance data used is a 56-energy group collapsed data based on ENDF/B-VII.1 data. The results showed that Er-167(n,g) is the largest contributor to criticality uncertainty after the U-235 and U-238 cross section reactions. The contribution of Er-167(n,g) to the criticality uncertainty was found equal to 123 pcm, this value has to be considered not negligible for core design purpose and equal to 18% of the total evaluated uncertainty. The uncertainty on the criticality due to the overall erbium isotopes (Er-166, Er-167 and to a lesser extent Er-168 and Er-170) was found equal to 166 pcm. A review of the historical experimental data values at thermal point present in the EXFOR database of the most uncertainty-related important erbium isotopes (i.e., Er-167) showed that the Er-167(n,g) data are inconsistent because they show a standard deviation with respect to the average value equal to 8.4%. An intercomparison between the most recent Er-167(n, g) measures at thermal point also revealed a relative difference roughly equal to 12%. The Er-167(n,g) ENDF/B-VII.1 and B-VIII.0 evaluated uncertainty at thermal energy and in the high-sensitivity region (0.5 - 5 eV) are set to 1.23% and 2.35%, respectively, on the basis of a pragmatic "low-fidelity" approach that would benefit from additional measurements. A data analysis of the experimental and calculated results provided by all criticality facilities of the International Criticality Safety Benchmark Evaluation Project (ICSBEP) that contain erbia in solid form confirmed that the declared evaluated uncertainty data (i.e., 2.35 %) of the ENDF evaluation would be considered an underestimation of the real uncertainty to be associated with the thermal range of the current Er-167(n,g) nuclear cross section data.
Comment from requester
The request is on capture, but complementary transmission measurements would help extract more accurate resonance parameters for both neutron and radiative widths.
Depending on the Er-167 sample enrichment used in the experiments, complementary measurements on other major natural erbium isotopes (Er-166, 168, 170) may be necessary to accurately determine the Er-167 cross section and resonance parameters. In addition, Er-166 capture cross section has a slight neutronic impact because of its relatively large evaluated uncertainty (about 8% in the energy range of interest for ENDF/B-VIII.0).
The request is well justified, but the target accuracy of less than 2% is very demanding even if focusing the efforts on the thermal region and the low energy resonances. In order to achieve such an accuracy the experimental results should be provided in the form of capture yields for optimal assimilation of the data in the evaluation process, and complementary information from integral measurements might be necessary.
Work in progress (as of SG-C review of May 2022)
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