Masao Yamanaka

Neutron Characteristics of Solid Targets in Accelerator-Driven System with 100-MeV Protons at Kyoto University Critical Assembly

Abstract At the Kyoto University Critical Assembly, a series of reactor physics experiments on the accelerator-driven system (ADS) coupled with the fixed-field alternating gradient (FFAG) accelerator are carried out, and the spallation neutrons generated by 100-MeV protons from the FFAG accelerator are successfully injected into the cores. In the ADS experiments, the neutron characteristics of the solid target are investigated through static and kinetic analyses, when the external neutron source of the neutron spectrum (the W, W-Be, or Pb-Bi target) is varied. The results demonstrate that the neutron yield is large with the W target, but a discrepancy is observed between the experiments and the calculations, because the experimental uncertainty of proton monitoring is attributable to defocusing of proton beams. With the use of reaction rate distribution in the core region, the static parameters are estimated fairly well in the analyses of the neutron multiplication and subcritical multiplication factor. In the kinetic experiments, the variation of the solid target used is clearly evident in the prompt neutron decay constant and the subcriticality.

Accuracy of Reactor Physics Parameters in Thorium-Loaded Accelerator-Driven System Experiments at Kyoto University Critical Assembly

Abstract Experiments on the thorium-loaded accelerator-driven system (ADS) were carried out at the Kyoto University Critical Assembly to reevaluate the accuracy of reactor physics parameters, including prompt neutron decay constants reaction rate distributions, subcriticality, and subcritical multiplication factor, and to reveal the dependency of these parameters on the external neutron source by varying the external neutron source of 14-MeV neutrons and spallation neutrons generated by 100-MeV protons. In preparation for thorium-loaded ADS experiments, renewed irradiation experiments are conducted with small and thin foils of thorium in the critical state to reevaluate the accuracy of the experimental analyses. In the ADS experiments, reactor physics parameters are found to be different in the same core when the external neutron source is injected separately with 14-MeV neutrons and spallation neutrons. By comparing with the calculated results, the significant impact of external neutron sources on the neutron characteristics of ADS is obtained in both the static and the kinetic experimental analyses.

Monte Carlo Approach of Effective Delayed Neutron Fraction by k-Ratio Method with External Neutron Source

Abstract In an accelerator-driven system (ADS), the effective delayed neutron fraction βeff is required for subcriticality measurements in order to convert measured reactivity in dollar units into pcm units. Here, an alternative calculation methodology with source-dependent reaction rates is proposed to estimate the subcriticality (pcm units) with an external neutron source for the newly defined parameter. In the proposed methodology, the parameter is obtained by the k-ratio method with reaction rates considering the external neutron source (fixed-source calculation) and is compared with that obtained by the eigenvalue calculations (MCNP6.1). To investigate its applicability to the estimation of subcriticality, subcriticality measurements by the extrapolated area ratio method are carried out in the experiments in ADS at the Kyoto University Critical Assembly, with the variation of subcriticality (pcm units) and the spectrum of the external neutron source. Then, is estimated by the proposed methodology with MCNPX-2.5.0. The measured subcriticality with the use of obtained by the fixed-source calculations shows good agreement with that obtained by the different measurement methodology and MCNPX-2.5.0. Through the subcriticality estimation, the applicability of the proposed methodology is confirmed for different subcriticality levels and the spectrum of the external neutron source.

A batch size selection strategy by checking stability of fission source distribution for Monte Carlo eigenvalue calculations

ABSTRACT The power iteration method is generally used for Monte Carlo eigenvalue calculations. In the process, the fission sources at each cycle are not independently sampled by the iteration scheme. This dependency of the sampled fission source can cause underestimation of response uncertainty. To solve these issues, previous studies have recommended the selection of large-size batches, which nonetheless, would require large calculation cost. This study proposes a method for selecting a reference criterion of batch size avoiding significant biases in the local responses, and mainly emphasizing the maintenance of the stability of fission source distribution. To determine the criterion for the diagnostics of stability, real-to-apparent standard deviations are analyzed with the stability of fission source distribution. The results show that the batch sizes determined by the proposed method allow the evaluation of local responses without large biases. The proposed method is expected to contribute to improving the reliability of the Monte Carlo criticality calculations.

Experimental analysis and uncertainty quantification using random sampling technique for ADS experiments at KUCA

ABSTRACT Nuclear data-induced uncertainties of neutronics parameters (neutron multiplication factor keff, one-point kinetics parameters and prompt neutron decay constant α) are quantified for lead-bismuth zoned accelerator-driven system experiments at the Kyoto University Critical Assembly, in order to contribute validation for subcritical core analysis. The random sampling technique using SCALE6.2.1/Sampler/NEWT/PARTISN is utilized for the validation and the uncertainty quantification, because the random sampling technique is applicable for a problem which is not easy to apply the perturbation theory. Consequently, it is confirmed that the numerical results of α reasonably agree with the experimental ones, compared with the nuclear data-induced uncertainties. In addition, it is clarified that the nuclear data-induced correlations between α and keff and between α and neutron generation time Λ are strongly negative and positive, respectively. This fact implies that the numerical predictions of keff and Λ can be improved by the data assimilation technique using subcritical experimental results of α, which can be directly measured even for a deep subcritical system.

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.

Improvement of fission source distribution by correlated sampling method in Monte Carlo perturbation calculations

ABSTRACTTo increase the accuracy of Monte Carlo perturbation calculations, the main issue is to estimate the variations of fission source distribution in perturbed systems. For estimating the difference between effective multiplication factors in unperturbed and perturbed systems, this study proposes combining the fission matrix method and the correlated sampling method, and applying the weight window technique for stabilizing the weight fluctuation of fission sources in perturbed systems. By applying the proposed method to the Smart and User-frIendly Monte Carlo Particle Transport Code (SUIT) Monte Carlo code, perturbation calculations are carried out for GODIVA and for simplified STACY problems. The results thus estimated show good agreement compared with those of reference calculations, demonstrating that the proposed method can effectively estimate variations of fission source distribution in perturbed systems for improving the accuracy of Monte Carlo correlated sampling method, especially for large v...

Uncertainty quantification of criticality in solid-moderated and -reflected cores at Kyoto University Critical Assembly

ABSTRACT Uncertainty quantification is conducted for the criticality of excess reactivity and control rod worth obtained at the Kyoto University Critical Assembly (KUCA). By combining SRAC2006 and MARBLE code systems, the sensitivity coefficients of the cross sections for aluminum-27 (27Al) comprising mainly of core components are large in the solid-moderated and -reflected cores (A cores) at KUCA. Also, the uncertainty is dominant in the uranium-235 isotope by the covariance data of JENDL-4.0, and a quantitative value is about 150 pcm induced by the JENDL-4.0 data library in the KUCA A cores, whereas the covariance data of 27Al are not prepared in JENDL-4.0. Moreover, the effect of decreasing uncertainty is obtained by applying the cross-sectional adjustment method to the uncertainty analyses. From the results, a series of uncertainty quantifications is expected to clarify the uncertainty of sub-criticality in accelerator-driven system experiments with spallation neutrons in the KUCA A cores.

Experimental analyses of bismuth sample reactivity worth at Kyoto University Critical Assembly

ABSTRACT Sample reactivity worth experiments are carried out by substituting aluminum (Al) plates for bismuth (Bi) ones at the Kyoto University Critical Assembly. At the beginning, uncertainty quantification of bismuth isotope is conducted by deterministic calculations with nuclear data library JENDL-4.0, with the use of experimental results of sample reactivity worth. Then, with the combined use of current (Bi) and previous (Pb) experimental results that demonstrate the comparative difference in the sensitivity and uncertainty of Bi and Pb isotopes, experimental results of cross-section uncertainties of Bi isotope are available for examination of neutron characteristics of Pb–Bi coolant material in the accelerator-driven system. From the experimental analyses, further uncertainty analyses by neutron transport calculations are needed for several reactions of Bi isotope, especially with the use of the covariance data of capture, elastic scattering and inelastic scattering reactions in another nuclear data library.

Effective delayed neutron fraction by Rossi-α method in accelerator-driven system experiments with 100 MeV protons at kyoto university critical assembly

ABSTRACT In the accelerator-driven system (ADS), the effective delayed neutron fraction (βeff) is a requisite for converting the subcriticality from dollar units to pcm units. To evaluate the accurate βeff value in ADS, measurement of βeff complements its calculation methodology and the nuclear data on delayed neutrons. Subcriticality measurements are carried out by the pulsed neutron source method in the Kyoto University Critical Assembly, and the neutron noise analyses are conducted by the Rossi-α method with the pulsed shape of the spallation neutron source. The value of βeff is deduced with the combined use of measured subcriticality in dollar units and correction parameters by MCNP6.1 together with JENDL-4.0 and JENDL/HE-2007. A comparison between the calculated and the measured βeff represents the acceptable accuracy within the subcriticality range of around keff = 0.93 in the ADS operations. Here, the applicability of the measurement methodology based on the Rossi-α method is demonstrated by varying the subcriticality.