Uncertainties of decay heat summation calculations are derived for the thermal fission of 235 U and 239 Pu and the fast fission of 238 U through sensitivity analyses using data given in the JNDC FP nuclear data library. The uncertainties analyzed are those relevant to decay energies, fission yields and decay constants among the nuclear data contained in the summation calculation. For nuclides lacking complete experimental data, the uncertainties of the decay energies are theoretically calculated. Thus analyzed, the maximum uncertainties of burst fission are 2.8% for 235 U, 3.2% for 239 Pu and 4.2% for 238 U in the range of cooling time between 1 and 10 9 s, and in the case of infinite irradiation, the corresponding level of maximum uncertainty is below 1.6% for all three fissioning nuclides.
Results of evaluation of neutron cross sections are presented for 27 fission product nuclides selected as being most important for fast reactor calculation. The cross sections considered are total, elastic scattering, inelastic scattering and capture cross sections in the energy range from thermal to 15 MeV. Thermal and resonance cross sections were calculated from resonance parameters. The calculated thermal capture cross section was adjusted by the measured value by adding a background cross section of 1/v form. A modified multi-level Breit.Wigner formula was developed to avoid the well-known occurrence of negative values in elastic scattering cross section. Smooth cross sections above resonance region were calculated with the spherical optical model and the statistical theory, taking account of neutron width fluctuation and level interference. The calculation was adjusted by capture data when available. The joining between the resonance and smooth cross sections was performed with the aid of statistical examination using Monte Carlo method. Present results are discussed in comparison with other evaluated data sets. Numerical results are stored on magnetic tape in the ENDF/B format.
A benchmark calculation of full fission product was performed for thermal reactor application using an isotope transmutation code DCHAIN based on 185 nuclides with revised nuclear data library. The fission product model for BWR lattice calculation was studied and tested with the benchmark results, and a model containing 45 explicit nuclides and one pseudo nuclide was selected as a reasonably best model to predict the burnup reactivity with high precision for practically all types of fuel and reactor operating conditions. The evaluated thermal cross section and resonance integral for the pseudo nuclide are o2.2oo=2.6 b and RI = 10.6 b, combined with the pseudo fission yield values of 1.3898, 1.3233, 1.3675 and 1.2773 for fissions from 286 U, 288 U, 239 Pu and 241 Pu, respectively. The present results are believed as equally applicable to PWR lattice calculation.
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