Shungo Iijima

Analysis of Uncertainties in Summation Calculations of Decay Heat Using JNDC FP Nuclear Data Library.

Uncertainties of decay heat summation calculations are derived for the thermal fission of 235U and 239Pu and the fast fission of 238U 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 235U, 3.2%” for 239Pu and 4.2% for 238U in the range of cooling time between 1 and 109s, and in the case of infinite irradiation, the corresponding level of maximum uncertainty is below 1.6% for all three fissioning nuclides.

Systematics and Determination of Level Density Parameters of Fission Product Nuclei

Nuclear level density parameters of fission product nuclei were determined in a consistent way with experimental neutron resonance data and level scheme data. The composite level density formula of Gilbert & Cameron was used with a revised expression of spin cut-off factor. The systematic trends in the determined parameters were investigated in detail. In addition to the parameter systematics used hitherto, two new phenomenological systematics were proposed and discussed. These systematics were fully used to estimate the level densities of nuclei whose experimental data are lacking. Parameters of about 130 nuclei were determined to form an extensive data base for calculation of neutron cross sections of fission products.

Fission Product Model for BWR Lattice Calculation Code

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 burn up 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 σ2,200 = 2.6b and.RI = 10.6b, combined with the pseudo fission yield values of 1.3898, 1.3233, 1.3675 and 1.2773 for fissions from 235U, 238U, 239Pu and 241Pu, respectively. The present results are believed as equally applicable to PWR lattice calculation.

Activation of Structural Materials due to Recoil Protons in Light Water Reactor

The long-lived radioactivities of structural materials induced by recoil protons in BWR were estimated for land disposal of low level waste after reactor decommissioning. Reaction products of interest are 53Mn, 91Nb, 94Nb, 97, Tc, 125Sb, 173Lu and 174Lu. A method of calculating the proton spectrum in materials was presented. The program PEGASUS-P was developed by modifying the PEGASUS, a preequilibrium and multistep evaporation theory code, to calculate proton reaction cross sections. The proton-induced activities in stainless steel, Inconel and Zircaloy-2 were calculated under typical irradiation conditions in an operating BWR. It was shown that even the most dominant activity due to 91Nb from Zircaloy-2 did not exceed 1/1,000 of that of a typical neutron induced activity of 63Ni for cooling up to 1,000 yr after the irradiation of 40 yr. The results are believed to hold for the case of PWR as well.

Fission product model for lattice calculation of high conversion boiling water reactor

A high precision fission product model for boiling water reactor (BWR) lattice calculation was developed, which consists of 45 nuclides to be treated explicitly and one nonsaturating pseudo nuclide. This model is applied to a high conversion BWR lattice calculation code. From a study based on a three-energy-group calculation of fission product poisoning due to full fission products and explicitly treated nuclides, the multigroup capture cross sections and the effective fission yields of the pseudo nuclide are determined, which do not depend on fuel types or reactor operating conditions for a good approximation. Apart from nuclear data uncertainties, the model and the derived pseudo nuclide constants would predict the fission product reactivity within an error of 0.1% ..delta..k at high burnup.

JENDL-3 FP Nuclear Data Library

Evaluation of neutron nuclear data for 172 nuclides from As to Tb has been made for JENDL-3. Evaluated quantities are the total, elastic and inelastic scattering, capture and threshold reaction cross sections, the angular and energy distributions of secondary neutrons in the incident neutron energy range from 10-5 eV to 20 MeV. The evaluation was made on the basis of theoretical calculations with optical, statistical, preequilibrium and multi-step evaporation models as well as available experimental data. Below 100 keV, resolved and unresolved resonance parameters were evaluated. The benchmark tests performed by comparing with experimental data at STEK, EBR-II and CFRMF have confirmed the reliability of the present evaluation.

Fission Product Decay Power—AESJ Recommendation

A recommendation regarding the FP decay power was issued from the Research Committee on Standardization of the Decay Heat Power in Nuclear Reactors of Atomic Energy Society of Japan (AESJ). The recommendation is primarily based on the results of summation calculations, obtained through using the JNDC FP Nuclear Data Library, Version 2. It consists of decay power values, their estimated accuracy, and their β- and γ-ray components for five fissionable nuclides; U-235, -238, Pu-239, -240 and -241. A set of 33-term exponential polynomials given therein help the users in calculating the decay-power curve for any irradiation history and cooling time. The time evolution for the γ-ray component energy spectra is also given.