Consistent Data Assimilation of Actinide Isotopes: 235U and 239Pu

Abstract

iii SUMMARYIn this annual report we illustrate the methodology of the consistent data assimilation that allows to use the information coming from integral experiments for improving the basic nuclear parameters used in cross section evaluation.A series of integral experiments were analyzed using the EMPIRE evaluated files for 235 U, 238 U, and 239 Pu. Inmost cases the results have shown quite large worse results with respect to the corresponding existing evaluations available for ENDF/B-VII.The observed discrepancies between calculated and experimental results were used in conjunction with the computed sensitivity coefficients and covariancematrix for nuclear parameters in a consistent data assimilation. Only the GODIVA and JEZEBEL experimental results were used, in order to exploit informations relative to the isotope of interest that are, in this particular case:235 U and 239 Pu.The results obtained by the consistent data assimilation indicate that with reasonable modifications (mostly within the initial standard deviation) it is possible to eliminate the original large discrepancies on the K eff of the two critical configurations. However, some residual discrepancy remains for a few fssion spectral indices that are, most likely, to be attributed to the detector cross sections.

Use of Covariances in a Consistent Data Assimilation for Improvement of Basic Nuclear Parameters in Nuclear Reactor Applications: From Meters to Femtometers

“…Thus, the analysis of these experiments can be effectively utilized for the sodium cross section improvement. Two experimental campaigns taken from the SINBAD database [6] have been used: the EURACOS campaign, and the JANUS-8 campaign. In these experiments measurements with activation detectors were carried out at various distances from the neutron source.…”

“…[6] that the improvement in C/E is the result of very large compensations after the parameters have been adjusted. The χ 2 test after adjustment provided a perfect value of 0.99 per degree of freedom.…”

“…The application of these priors to integral experiments was carried out at INL, which completes a single cycle of assimilation. The first assimilations performed at BNL and INL were for the structural materials 23 Na and 56 Fe, as described in INL report INL/EXT-10-20094 [6]. While much was learned from the assimilation of these materials, the resulting evaluations were not considered an improvement over the the existing ENDF/B-VII.0 and were not used in ENDF/B-VII.1.…”

“…A summary of the details for these assimilations is provided below. Another second set of assimilations were performed for 235 U and 239 Pu as described in INL report INL/EXT-11-23501 [7]. For both of these materials a first round of assimilation was completed, with limited success, and a second round of empire calculations has been completed using an improved version of empire.…”

“…The emphasis of this paper is on the empire code calculations to prepare the priors, sensitivity matrices and covariances for the model parameters. For completeness, we also include a short summary of the assimilation results produced by INL and reported earlier in annual reports [6], [7], [8]. 4 …”