Takaaki Ohsawa

JENDL-4.0 : A New Library for Nuclear Science and Engineering

The fourth version of the Japanese Evaluated Nuclear Data Library has been produced in cooperation with the Japanese Nuclear Data Committee. In the new library, much emphasis is placed on the improvements of fission product and minor actinoid data. Two nuclear model codes were developed in order to evaluate the cross sections of fission products and minor actinoids. Coupled-channel optical model parameters, which can be applied to wide mass and energy regions, were obtained for nuclear model calculations. Thermal cross sections of actinoids were carefully examined by considering experimental data or by the systematics of neighboring nuclei. Most of the fission cross sections were derived from experimental data. A simultaneous evaluation was performed for the fission cross sections of important uranium and plutonium isotopes above 10 keV. New evaluations were performed for the thirty fissionproduct nuclides that had not been contained in the previous library JENDL-3.3. The data for light elements and structural materials were partly reevaluated. Moreover, covariances were estimated mainly for actinoids. The new library was released as JENDL-4.0, and the data can be retrieved from the Web site of the JAEA Nuclear Data Center.

Multimodal analysis of prompt neutron spectra for 237Np(n,f)

Abstract An attempt was made to analyzed the systematic variation of the prompt neutron spectra of 237 Np(n,f) on the basis of the multimodal analysis of mass and kinetic energy distributions. The spectra of neutrons emitted from fragments of each mode (standard I, II, III, superlong) were calculated independently and the total spectra were synthesized. It was found that the partial spectrum for standard I mode is the softest, getting harder in ascending order for standard II, standard III and superlong modes. The calculated total spectra well represented the experimental data and were found to be more sensitive to the variation of the incident energy than the conventional treatment of the Madland-Nix model.

Simultaneous Evaluation of Fission Cross Sections of Uranium and Plutonium Isotopes for JENDL-3.3

A simultaneous evaluation of the fission cross sections of 233U, 235U, 238U, 239Pu, 240Pu and 241Pu was carried out for the evaluated nuclear data library JENDL-3.3. A least-squares method was applied to selected absolute and relative measurements on the fission cross sections. Covariance matrices of the experimental data were constructed from the uncertainty information reported in the references. The fission cross sections obtained were compared with the JENDL-3.2 and ENDF/B-VI evaluations. It was found from the comparison that the present results are not so different from those in JENDL-3.2, except for the fission cross sections of 233U and the cross sections above 15MeV, and give smaller X2 value than the JENDL-3.2 cross sections. The averaged fission cross sections of 233U, 238U, and 239Pu relative to that of 235U were calculated for a neutron spectrum produced by 9Be (d,xn) reaction. It was confirmed that the calculated cross-section ratios are in good agreement with the experimental data.

JENDL Actinoid File 2008

JENDL Actinoid File 2008 (JENDL/AC-2008) was released in March 2008. It includes nuclear data for neutron-induced reactions for 79 nuclides from Ac (Z = 89) to Fm (Z = 100). The neutron energy range is 10−5 eV to 20 MeV. Almost alldata for 62 actinoids in JENDL-3.3 were revised. New evaluations were performed for 17 nuclides, which have half-lives longer than one day. A new comprehensive theoretical model code CCONE was widely used for the evaluation of cross sections and neutron emission spectra. Thermal cross sections for many nuclides were revised based on experimental data. Resonance parameters were readjusted to reproduce them. Simultaneous evaluations of fission cross sections were performed for six important nuclei. The least-squares fitting code GMA was used for the evaluation of fission cross sections for minor actinoids. In this paper, we present the evaluation methods and results of the JENDL/AC-2008.

Multimodal analysis of prompt neutron spectra for (sf), (sf), (sf) and (n,f)

Abstract The prompt neutron spectra for 238 Pu (sf), 240 Pu (sf), 242 Pu (sf) and 239 Pu (n th ,f) were calculated with the modified Madland–Nix model with consideration to the multimodal nature of the fission process. The spectra of neutrons for each mode (Standard-I, -II, -III) were calculated independently and the total spectra were synthesized. The partial spectra for the three modes were found to be considerably different from each other. Different mode branching ratios resulted in different total spectra for the three even-mass spontaneously fissioning isotopes. Comparison of spectra for 240 Pu (sf) and 239 Pu (n th ,f), for which the fissioning nucleus is the same, revealed that the total spectra were harder for 239 Pu (n th ,f) due to the higher excitation energy of the fissioning nucleus. The calculated total spectra well represented the experimental data.

Determination of Subcritical Reactivity of a Thermal Accelerator-Driven System from Beam Trip and Restart Experiment

An experimental technique based on an accelerator-beam trip or restart operation is proposed to determine the subcritical reactivity of an accelerator-driven system (ADS). Applying the least-squares inverse kinetics method to the data analysis, the subcriticality can be inferred from time-sequence neutron count data after these operations. A series ofbeam trip and restart experiments with 14 MeV neutrons were carried out in a thermal ADS of Kyoto University Critical Assembly (KUCA), to demonstrate the applicability of the proposed technique. The subcriticalities evaluated using neutroncounters far from the DT target were consistent with those obtained in a previous pulsed neutron experiment. However, a counter placed close to the target significantly overestimated the subcriticality. The present technique is expected to be available for subcriritcality measurement at startup and shutdown of various ADSs.

Determination of Lambda-Mode Eigenvalue Separation of a Thermal Accelerator-Driven System from Pulsed Neutron Experiment

Basic research on the Accelerator-Driven System (ADS) with thermal neutron spectrum has been promoted by the Kyoto University Research Reactor Institute. At the Kyoto University Critical Assembly (KUCA), various experiments on thermal ADS with a pulsed spallation source are planned. In such an ADS, neutron flux distribution may be sensitive to the injection of neutrons, and the high sensitivity results in various spatial effects. In this study, a pulsed neutron experiment with 14MeV neutrons was carried out in a thermal ADS of KUCA, to determine the λ-mode eigenvalue separation, which is a quantitative indication of spatial effects. An original data-processing technique was applied to infer prompt-neutron decay constants of fundamental and higher modes from neutron count decay data, and then the eigenvalue separation around 13%Δk/k was obtained from these decay constants.

A practical formula for inferring eigenvalue separation from flux tilt measurements in nuclear reactors

Abstract A practical formula for inferring the λ-mode eigenvalue separation from flux tilt measurements is derived. The advantages of the present formula consist in both the generality and the simplicity of data-analysis, even under a higher-order model, compared with the previous expressions. The applicability of the method, based on the first-order version of the formula, is confirmed by the demonstration in the UTR-KINKI reactor. The results indicate that the flux distortions induced by the asymmetric pattern of control rods are not spatially local but global, which is responsible for the dominant excitation of the first harmonics, consequently the analysis is successful in the extraction of the first-mode eigenvalue separation. The effects of higher-harmonics past the first on the first-mode eigenvalue separation, which is neglected in the previous formula, are also evaluated.

Derivation of consistent reactivity worth and eigenvalue separation from space-dependent rod worths on the basis of modal approach

Abstract A method is proposed to infer the consistent reactivity worth and the λ-mode eigenvalue separation from space-dependent control rod worths in loosely-coupled reactors. Using the modal expansion approximation for a transient flux, we derive a two-mode version of the formula for an integral-count rod drop measurement. The formula is very simple and there is no need for the theoretical correction factor. The experimental procedures are convenient as well as standard rod drop measurements. Furthermore, the present formula is more general, compared with the previous two-point formula, and is anticipated to be applicable even to large single-core reactors. The applicability of the formula is confirmed by a demonstration in the UTR-KINKI reactor, a light-water moderated and graphite-reflected reactor.

Feynman-α Analysis for a Thermal Subcritical Reactor System Driven by an Unstable 14MeV Neutron Source

In a series of Feynman-α correlation measurements for a thermal Accelerator-Driven System (ADS) with 14MeV neutrons at the Kyoto University Critical Assembly (KUCA), an unstable accelerator condition such as a drift of beam current has been frequently observed. Neutron source instability caused by such unavoidable beam-current instability resulted in a divergent variance-to-mean ratio and, consequently, the correlation analysis failed. Nevertheless, we attempted to apply a difference-filtering technique to the correlation analysis to reduce the influence of the above instability. The present attempt resulted in consistent prompt-neutron decay constants with those obtained in a previous pulsed neutron experiment. The application of the filtering is expected to enhance the robustness of Feynman-α analysis against various instabilities of accelerator operation in actual ADS.