Hiroki Iwamoto

Sensitivity and uncertainty analysis for an accelerator-driven system with JENDL-4.0

A sensitivity and uncertainty analysis was performed for the accelerator-driven system (ADS) proposed by the Japan Atomic Energy Agency (JAEA) with the latest version of the Japanese Evaluated Nuclear Data Library (JENDL-4.0). Significant discrepancies have been found between the reactor physics parameters of JENDL-4.0 and those of JENDL-3.3. An analysis with the sensitivity coefficients showed that the major contributors to these discrepancies are the differences in the inelastic scattering cross sections of 206Pb and 207Pb, and the capture and inelastic scattering cross sections and ν value of 241Am. The uncertainty analysis with the JENDL-4.0 covariance data found that the covariances of the fission neutron spectrum of minor actinides (MAs) have a considerable impact on the uncertainties of the reactor physics parameters.

Validation of Pb nuclear data by Monte Carlo analyses of sample reactivity experiments at Kyoto University Critical Assembly

Sample reactivity experiments on the uncertainty analyses of Pb nuclear data are carried out by substituting Al plates for Pb ones at the Kyoto University Critical Assembly, as part of basic research on Pb–Bi for the coolant. Numerical simulations of sample reactivity experiments are performed with the Monte Carlo calculation code MCNP6.1 together with four nuclear data libraries JENDL-3.3, JENDL-4.0, ENDF/B-VII.0 and JEFF-3.1, to examine the accuracy of cross-section uncertainties of Pb isotopes by comparing measured and calculated sample reactivities. A library update from JENDL-3.3 to JENDL-4.0 is demonstrated by the fact that the difference between Pb isotopes of the two JENDL libraries is dominant in the comparative study, through the experimental analyses of sample reactivity by the MCNP approach. In addition, JENDL-4.0 reveals a slight difference from ENDF/B-VII.0 in all Pb isotopes and 27Al, and from JEFF-3.1 in 238U and 27Al. Based on these results, further experiments are needed to investigate the uncertainties of Bi isotopes with the use of the Pb–Bi and Bi plates.

Calculation of displacement cross-sections for structural materials in accelerators using PHITS event generator and its applications to radiation damage

The displacement cross-sections, implemented in the particle and heavy ion transport code system (PHITS), have been calculated to estimate the radiation damage in structural materials used at accelerator facilities. The event generator in PHITS was used to calculate displacement cross-sections for 14 elemental targets and two practical alloys irradiated with neutrons at energies from 10−10 MeV to 3 GeV, and protons and deuterons at energies from several keV to 3 GeV. These displacement cross-sections were used to estimate the displacement per atom (DPA) in the target assembly at accelerator driven system (ADS) Target Test Facility (TEF-T) using 400-MeV protons, and in the Type 316 stainless steel target using 40-MeV deuterons. For the target assembly at TEF-T using 400-MeV protons, the DPA value for the proton component was approximately twice higher than that for the neutron component. For the Type 316 target using 40-MeV deuterons, radiation damage took place near the surface of Type 316 because of the Coulomb scattering effect between the incident deuterons and the material.

Impact of PHITS spallation models on the neutronics design of an accelerator-driven system

ABSTRACT The impact of different spallation models and parametrisation of nucleon–nucleus interactions in the particle transport code PHITS on the nuclear characteristics of an accelerator-driven system (ADS) is investigated. Cut-off neutrons below 20 MeV calculated using the default option of the current spallation model (i.e. Liège intranuclear cascade (INC) model version 4.6, INCL4.6) are found to be 14% less than those calculated by the old spallation model (i.e. Bertini INC model). This decrease increases the proton beam current that drives the 800-MW thermal power and impacts various ADS parameters, including material damage, nuclear heating of the proton beam window and the inventory of spallation products. To validate these options based on the ADS neutronics design, we conduct benchmark calculations of the total and non-elastic cross sections, thick target neutron yields and activation reaction rate distributions. The results suggest that Pearlstein–Niita systematics, which is a default option of the nucleon–nucleus interaction parametrisation, would be the best option and that Bertini INC is better suited for cut-off neutrons than INCL4.6. However, because of the difficulty in making a definite conclusion on the spallation models, we conclude that relatively large uncertainty in the cut-off neutrons, which is the difference between the two spallation models (i.e. 14%), should be considered.

Development of three-dimensional reactor analysis code system for accelerator-driven system, ADS3D and its application with subcriticality adjustment mechanism

ABSTRACT In order to perform the parametric survey for an accelerator-driven system (ADS) core with the subcriticality adjustment mechanism, a new calculation code system, ADS3D, was developed on MARBLE which is a comprehensive and versatile framework for reactor analysis. The application of ADS3D was also demonstrated on the neutronics design of ADS operated by control rod (CR) movement. Through the neutronics calculation, it was shown that the maximum proton beam current was decreased from 20.5 to 11.6 mA due to the switch from beam-operated to CR-operated core.

On-line subcriticality measurement using a pulsed spallation neutron source

ABSTRACT To investigate the applicability of the pulsed neutron source method using a pulsed spallation neutron source for an on-line subcriticality monitoring system of an accelerator-driven system, a subcritical experiment is conducted using the Kyoto University Critical Assembly in combination with the fixed-field alternating gradient accelerator. Reactivity values obtained from different traditional techniques, the area-ratio method and the α-fitting method, are discussed with respect to the applicability to on-line subcriticality monitoring. The results show that the area-ratio method robustly and accurately monitors subcriticality in shallow subcritical states with negative reactivity of up to a few dollars; however, this method faces problems with temporal fluctuations, spatial dispersion, and sensitivity to the proton-beam current with increasing depth of subcriticality. On the other hand, it is shown that the α-fitting method alleviates such problems in deep subcritical states. Moreover, a proposed fitting technique using the maximum-likelihood estimation method based on the Poisson distribution is robust enough to be applicable to on-line subcriticality monitoring for negative reactivity levels of up to roughly nine dollars.

Development of a calculation system for the estimation of decontamination effects

A calculation system for the estimation of decontamination effects (CDE) is developed in the present work to aid in the effective planning of decontamination procedures. This system calculates dose rate distribution before and after decontamination according to the distribution of radioactivity and the decontamination factor. A dose rate reduction factor is also used to estimate decontamination effects. Results obtained from CDE were compared with measurements and particle and heavy-ion transport code system (PHITS) simulations. The CDE successfully reproduced the measured and calculated dose rate distributions, requiring less than several seconds of calculation time.

Sensitivity and Uncertainty Analyses of Lead Sample Reactivity Experiments at Kyoto University Critical Assembly

Abstract Sensitivity and uncertainty analyses of lead (Pb) isotope cross sections are conducted with the use of sample reactivity experiments at the Kyoto University Critical Assembly (KUCA). With the combined use of the SRAC2006 and MARBLE code systems, attempts are made to precisely examine the contributions of the reactions and energy regions of Pb isotope cross sections to reactivity based on the covariance data of JENDL-4.0. Moreover, the effect of decreasing uncertainty is discussed in terms of the accuracy of sample reactivity by applying the cross-section adjustment method to the uncertainty analyses. From the results of the sensitivity and uncertainty analyses, the reliability of Pb isotope cross sections, such as the Pb isotope covariance data of JENDL-4.0, is compared with the JENDL-3.3, ENDF/B-VII.0, and JEFF-3.1 libraries. Additionally, the numerical results reveal the applicability of the sensitivity and uncertainty analyses to the thermal neutron spectrum cores, such as the KUCA core, and demonstrate the improvement in the calculation results generated by the cross-section adjustment.

Lead Void Reactivity Worth in Two Critical Assembly Cores with Differing Uranium Enrichments

Abstract To validate lead (Pb) nuclear cross sections, a series of integral experiments to measure lead void reactivity worths was conducted in a high-enriched uranium (HEU)/Pb system and a low-enriched uranium (LEU)/Pb system using the Comet Critical Assembly at the National Criticality Experiments Research Center. There is a follow-on experiment to measure the lead void reactivity worths in a plutonium/Pb system that is currently under investigation. The critical experiments in the two uranium systems were designed to provide complementary data sets having different sensitivities to scattering cross sections of lead. The larger amount of the 238U present in the LEU/Pb core increases the neutron importance above 1 MeV compared with the HEU/Pb core. Since removal of lead from the core shifts the neutron spectrum to the higher energy region, positive lead void reactivity worths were observed in the LEU/Pb core while negative values were observed in the HEU/Pb core. This technical note is a preliminarily report of the experimental analysis results for the lead void reactivity worths with the Monte Carlo calculation code MCNP® version 6.1 together with nuclear data libraries JENDL-4.0 and ENDF/B-VII.1. The calculation values were found to overestimate the negative reactivity worths for the HEU/Pb core while being consistent for the LEU/Pb core.

Reaction rate analyses of accelerator-driven system experiments with 100 MeV protons at Kyoto University Critical Assembly

ABSTRACT At the Kyoto University Critical Assembly, a series of reaction rate experiments is conducted on the accelerator-driven system (ADS) with spallation neutrons generated by the combined use of 100 MeV protons and a lead–bismuth target in the subcritical state. The reaction rates are measured by the foil activation method to obtain neutron spectrum information on ADS. Numerical calculations are performed with MCNP6.1 and JENDL/HE-2007 for high-energy protons and spallation process, JENDL-4.0 for transport and JENDL/D-99 for reaction rates. That the reaction rates depend on subcriticality is revealed by the accuracy of the C/E (calculation/experiment) values. Nonetheless, the accuracy of the reaction rates at high-energy thresholds remains an important issue in the fixed-source calculations. From reaction rate analyses, the indium ratio is newly defined as another spectrum index with the combined use of 115In(n, γ)116mIn and 115In(n, n′)115mIn reaction rates, and considered useful in examining the neutron spectrum information on ADS with spallation neutrons.