Speaker
Description
The development of bias estimation methodologies has been a great tool for data uncertainty assessments. In particular, keff data and their uncertainties are becoming paramount for criticality-safety and reactor physics assessment. Moreover, the adjustment of cross sections implemented to match experimental keff within their uncertainty margins is important to produce the requested bias and associated uncertainty. A computation error on keff can strongly influence the calculated bias and its associated uncertainty. Therefore, it is highly recommended to have a high confidence level on the calculated keff values. Many Monte Carlo and deterministic codes and various nuclear data libraries are available for assessing keff. Since the bias due to the Monte Carlo simulation is relatively small, the biases in a calculation are mostly in connection to the nuclear data libraries.
There had been initially in 2018 a motivation under the US/DOE NCSP1 for promoting inter-comparison exercise. The focus was mainly on keff values produced by various Monte Carlo codes for a three year period.
The aim of the keff inter-comparison exercise was:
• To check the consistency between results from different codes using the same nuclear data libraries and therefore test the “independent modelling effect”; this task allowed providing a rigorous basis for quality experiments with a view to validate nuclear data,
• If lack of agreement exists, to find out the source of the discrepancy between the Monte Carlo model, interpretation of benchmark (detailed/simplified), revision number differing, processing of nuclear data libraries and correct the input decks so that all validation databases involve input decks describing the same benchmark model,
• To provide feedback on the nuclear data libraries (cross sections and thermal scattering) through the comparison of results using different nuclear data libraries and through trending analyses.
A progressive analysis was done from 2019 dealing with the keff comparison of critical benchmarks involving Pu, HEU (2019), LEU, IEU (2020), MIX, U233 (2021) fissile media. Quite a few mistakes were highlighted on input decks explaining some calculated discrepancies. Issues on nuclear data processing were also outlined, such as the processing by AMPX2 of the incoherent elastic scattering of hydrogen cross sections being a good example.
The main conclusions of this inter-comparison exercise will be presented in the final paper along with some analysis on keff sensitivity values. It should also be noted that this work is complementary to the work done in the framework of the VaNDaL3 whose aim is to ensure that existing validation databases are under quality assurance and will also help improve the quality of the ICSBEP/DICE4 database.
Finally, quite recently the need for comparison on reactor physics kinetics parameters and shielding benchmarks values has arisen and it was proposed and accepted to extend the exercise until FY 2024.
[1] https://ncsp.llnl.gov
[2] Alex Lang, Alex Shaw, Chris Chapman, and William Marshall, “Discovery of AMPX Thermal Scattering Law Processing Issue for Solid Moderators”, ANS transactions, Volume 124, Number 1, June 2021, pages 368-371.
[3] https://www.oecd-nea.org/jcms/pl_23388/wpec-subgroup-45-sg45-validation-of-nuclear-data-libraries-vandal-project
[4] International Handbook of Evaluated Criticality Safety Benchmark Experiments, NEA/NSC/DOC(95)03/I, Volume I, 2020 edition