裂变产额的Zp模型评价方法及燃耗不确定度分析

裂变产额在核科学技术和核工程中有着重要的应用，发展可靠、高效的产额评价方法和相应燃耗计算不确定度分析方法，对于建立高质量的产额数据库具有重要的意义。本文根据裂变产物核衰变模式和衰变分支比，建立独立产额与累积产额的转换矩阵，用于Zp模型的扩展，使之适用于独立产额和累积产额的统一描述，并以此建立了用于产额统一评价的拟合程序ZpFit。把ZpFit程序应用于中子诱发235U裂变产物产额评价，获得了自洽的独立产额、累积产额和相应的协方差数据，并建立ENDF格式的中子诱发235U裂变的产额数据库。在此基础上，计算了UAM燃耗基准题的TMI 1栅元的kinf、重要核素原子核密度的不确定度，比对了本工作评价的产额数据和ENDF/B Ⅷ0评价库中产额数据传递给响应量的相对不确定度，结果基本一致，差异不大。

Fission Yield Evaluation Method Based upon Zp Model and Uncertainty Analysis in Burnup Calculation

Nuclear fission product yield (FPY) is an important data base for the research of nuclear energy and nuclear devices. The development of reliable and efficient yield evaluation methods is of great significance for the establishment of high quality yield database. Previously, to obtain a FPY for a specific product, the related experimental data would be collected and analyzed to obtain the weighted average value as the recommended value. This method did not consider the physical relation among the yields of the precursor and daughter nuclides on the same mass chain. So a physical model is needed to describe the isobaric (with same mass) Z distribution, and could be used to evaluate the independent and cumulative yields simultaneously. Zp model is a better method to describe the isobaric charge distribution since its physic mechanic is more certain. And to connect with the cumulative yields, the conversion matrix is deduced from the decay data, with which the cumulative yields could be calculated from independent yields on the same chain. Based upon the above idea, a fitting program named ZpFit was established for the unified evaluation of isobaric yields, and was applied to evaluate the yields of n+235U fission. The related independent yield, cumulative yield and corresponding covariance data were obtained self consistently and formed a yield database in ENDF format. For examples, the evaluations for yields of A=125, 140 and 149 of nth+235U fission were expressed in this paper. For A=125, the cumulative yields in literature have discrepancy, roughly could be grouped with higher and lower parts with the help of ZpFit codes. An experiment was carried out to clear this discrepancy by the ream in CIAE, the yield for 125Sn is around 0025, about 20% and 10% lower than the data in ENDF /B Ⅷ0 and JEFF 33, respectively, agrees with the lower group of the literature values. So finally, we got the fitted yields giving more weighting to the lower part. For A=140, the result shows that with the help of ZpFit, we could decide which yields are not reasonable and should be excluded. The uncertainties of nuclear density of kinf and some important nuclides were calculated for TMI 1 cell in UAM burn up benchmark. Compared with those based upon ENDF/B Ⅷ0, the results are roughly in agreement. The burnup relative uncertainty transferred from independent fission yields of 149Nd vs other all product’s from nth+235U fission of this work were also calculated, the density uncertainties are mostly contributed from the uncertainty of 149Nd for nuclides 149Sm, 150Sm, 151Sm, 152Sm, and 151Eu, these nuclides have important impact for the reactor reactivity. In summary, ZpFit based upon Zp model was developed to evaluate the independent and cumulative yields on the same mass chain. The burnup uncertainties of kinf and some important FP were studied based upon the yield database of present work and other libraries.