The uncertainties and covariance matrices of the fission yield are important in the uncertainty analysis of the decay heat. At present, there are no covariance matrices of fission yield given in the evaluated nuclear data library, although they provide uncertainties with good estimates. In this study, the generalized least squares (GLS) updating approach was adopted to evaluate the fission yield covariances with constraints from the basic physical conservation equation and chain yield data, using the nuclear data files from ENDF/B-VIII.0, JENDL-5, and JEFF-3.3. Based on the original and updated data, summation calculations were performed for the fission pulse decay heat of thermal neutron-induced fission of 235U. The uncertainties of the decay heat were obtained using the generalized perturbation theory, including the uncertainties propagated from the fission yield, decay energy, decay constant, and branching ratio. The original uncorrelated yield data contributed to a <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\sim 4 \%$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>∼</mo> <mn>4</mn> <mo>%</mo> </mrow> </math></EquationSource> </InlineEquation> uncertainty at all times, and dominated the decay heat uncertainty at cooling times longer than 100 s. With the generated covariance matrices, the uncertainty of the calculated decay heat was significantly reduced, and the decay energy data generally made a major contribution. The relative uncertainties at cooling time 0.1 s were <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$\sim 10 \%$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>∼</mo> <mn>10</mn> <mo>%</mo> </mrow> </math></EquationSource> </InlineEquation> for ENDF/B-VIII.0, JEFF-3.3, and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$\sim 5 \%$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>∼</mo> <mn>5</mn> <mo>%</mo> </mrow> </math></EquationSource> </InlineEquation> for JENDL-5, and those at cooling time 105 s were approximately 1% for the three libraries. The influence of the GLS updating procedure on the contributions of important fission products to the decay heat and their sensitivity coefficients is also discussed.

Generation of fission yield covariance matrices and its application in uncertainty analysis of decay heat

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2026年3月24日

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Wen-Di Chen,
Tao Ye,
Hai-Rui Guo,
Jia-Hao Chen,
Bo Yang,
Yang-Jun Ying

摘要

The uncertainties and covariance matrices of the fission yield are important in the uncertainty analysis of the decay heat. At present, there are no covariance matrices of fission yield given in the evaluated nuclear data library, although they provide uncertainties with good estimates. In this study, the generalized least squares (GLS) updating approach was adopted to evaluate the fission yield covariances with constraints from the basic physical conservation equation and chain yield data, using the nuclear data files from ENDF/B-VIII.0, JENDL-5, and JEFF-3.3. Based on the original and updated data, summation calculations were performed for the fission pulse decay heat of thermal neutron-induced fission of ²³⁵U. The uncertainties of the decay heat were obtained using the generalized perturbation theory, including the uncertainties propagated from the fission yield, decay energy, decay constant, and branching ratio. The original uncorrelated yield data contributed to a $\sim 4 \%$ $\sim 4 %$ uncertainty at all times, and dominated the decay heat uncertainty at cooling times longer than 100 s. With the generated covariance matrices, the uncertainty of the calculated decay heat was significantly reduced, and the decay energy data generally made a major contribution. The relative uncertainties at cooling time 0.1 s were $\sim 10 \%$ $\sim 10 %$ for ENDF/B-VIII.0, JEFF-3.3, and $\sim 5 \%$ $\sim 5 %$ for JENDL-5, and those at cooling time 10⁵ s were approximately 1% for the three libraries. The influence of the GLS updating procedure on the contributions of important fission products to the decay heat and their sensitivity coefficients is also discussed.