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

Abstract

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.

Keywords

• fission yield evaluation
• zp model
• n+235u
• uncertainty analysis