Primary fission fragment yields and prompt-neutron emissions for after-heat analysis Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9s161913x

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  • Independent fission product yields are primary input nuclear data in the evaluation of nuclear reactor after-heat when using a nuclide-decay summation method. The error in yield values is one of the major causes of uncertainty of this calculation. Since only a small fraction of the fission product yields has been measured directly, the adoption of model-predicted unknown yields is inevitable. The uncertainty involved is usually quite large. This dissertation presents a systematic approach to correlating the relevant experimental and calculated ²³⁵U thermal fission data with other nuclear data, so that with application constraints from exact physical laws, a consistent set of primary fission fragment yields can be generated. These are for use in improving the precision of after-heat calculations by constraining yield errors and thereby reducing the uncertainties caused by them. Fission fragment kinetic energy data and nuclear mass table data are used to calculate the total excitation energy of a fragment pair. This energy is partitioned between light and heavy fragments assuming equal temperature of the fragments at the time of scission. The prompt de-excitation of the primary fission fragment via neutron and gamma ray emissions is simulated by nuclear evaporation processes. A FORTRAN IV Monte Carlo Code called EVAPOR was developed for this purpose. Neutron yields and emission probabilities for various numbers of neutrons from over 1,000 possible fission fragments were generated by EVAPOR. The major uncertainties of the emission probabilities calculated by EVAPOR can be simulated by lumping them together as though they were the result of uncertainties in the mean excitation energy of a fission fragment. A sensitivity study of the effect of this uncertainty thus permits an a priori estimate of emission probability error. The resulting emission probabilities and their errors can then be used as an input for inference of primary fragment yields. This inference, a form of probabilistic data unfolding, has been explored for a sample case. Results to date have shown that a consistent set of primary (before neutron emission) and secondary (after neutron emission) yields can be generated using combined data for light and heavy fragments of complementary charge number. However, there are significant discrepancies between the secondary yields obtained by this process and those used as input to it, which stem from the ENDF/B-IV evaluation. The sources of these discrepancies must still be explored.
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