Development and Verification of Covariance Data Generation Module in NECP-Atlas

Abstract:   Background : The quantification of uncertainty has become a common requirement in reactor physics analysis, with the covariance data of nuclear data serving as the foundational data for conducting uncertainty quantification.  Purpose : This study aims to develop a covariance data generation module, covar_calc, in the nuclear data processing software NECP-Atlas to produce continuous energy covariance data for Monte Carlo programs and multi-group covariance data for deterministic programs. Methods : Covar_calc module was developed to process all covariance data provided in evaluated nuclear databases, according to the different storage formats of nuclear data and different computational methods. This included the covariance data of various parameters such as average fission neutron multiplicities, cross sections, angular distributions of secondary particles, fission spectrum, resonance parameters, and neutron activation cross-sections. Results : A comparative verification is carried out with the covariance data production module in the nuclear data processing software NJOY21 to demonstrate the accuracy of the multi-group covariance databases produced by NECP-Atlas and the maximum bias is less than 0.1%. Utilizing Monte Carlo calculation code NECP-MCX and uncertainty analysis code NECP-UNICORN, sensitivity coefficients for different benchmarks are calculated. By incorporating both continuous energy covariance databases and multi-group covariance data, and utilizing the Sandwich formula, the final uncertainties are computed. The results indicate that the uncertainties calculated using the multi-group covariance data generated by NECP-Atlas exhibit comparable accuracy to those obtained with NJOY21. Conclusions : The precision in creating both continuous energy covariance and multi-group covariance meets the requirements for usage in Monte Carlo programs and deterministic programs, validating the efficacy of covar_calc module within NECP-Atlas for uncertainty quantification in reactor physics analysis.