Masatoshi Yamasaki

Resonance calculations using the multiband method and interface currents

To provide accurate effective cross sections for core calculations, the multiband method was applied to light water reactor assembly calculations. The multiband method has been extended to arbitrary geometries by introducing band-dependent currents at the boundaries of a region. The transport of neutron is treated by the angular space-dependent current coupling collision probability method. A fuel assembly is divided into heterogeneous domains where the multiband method is applied directly by using collision probabilities. Several examples of numerical calculations for UO{sub 2} and mixed oxide fuel assemblies are shown. The space dependence of the effective cross section can be expressed accurately by this method, which leads to an accurate prediction of k{sub {infinity}} values.

Development of Erbia-Credit Super High Burnup Fuel: Experiments and Numerical Analyses

Abstract Erbia-credit super high burnup (Er-SHB) fuel offers a means to introduce >5 wt% 235U enrichment fuel; small amounts of erbia added to all the high-enriched UO2 powder can reduce the initial reactivity to <5 wt% enrichment level. By using this erbia credit, the new fuel can be treated as <5 wt% enriched fuel, and most modifications to the existing facilities and equipment can be avoided. One of the key issues for developing the Er-SHB fuel is to validate the criticality safety analysis tools for this fuel based on a series of experiments using fuel with small amounts of erbia in the entire core. For that purpose, a series of critical experiments have been performed at the Kyoto University Critical Assembly (KUCA). Four critical cores were constructed utilizing two different average enrichments, three different erbia contents, and four different H/U ratios. Numerical analyses have also been performed using several different cross-section libraries, and the results were compared with the measurements from the KUCA experiments. These results confirm the validity of the calculations and the cross-section libraries for determining erbia reactivity. This paper outlines the basic concepts of the Er-SHB fuel, the erbia experiments, and the analyses results.

Estimating Temperature Reactivity Coefficients by Experimental Procedures Combined with Isothermal Temperature Coefficient Measurements and Dynamic Identification

A method to evaluate the moderator temperature coefficient (MTC) and the Doppler coefficient through experimental procedures performed during reactor physics tests of PWR power plants is proposed. This method combines isothermal temperature coefficient (ITC) measurement experiments and reactor power transient experiments at low power conditions for dynamic identification. In the dynamic identification, either one of temperature coefficients can be determined in such a way that frequency response characteristics of the reactivity change observed by a digital reactivity meter is reproduced from measured data of neutron count rate and the average coolant temperature. The other unknown coefficient can also be determined by subtracting the coefficient obtained from the dynamic identification from ITC. As the proposed method can directly estimate the Doppler coefficient, the applicability of the conventional core design codes to predict the Doppler coefficient can be verified for new types of fuels such as mixed oxide fuels. The digital simulation study was carried out to show the feasibility of the proposed method. The numerical analysis showed that the MTC and the Doppler coefficient can be estimated accurately and even if there are uncertainties in the parameters of the reactor kinetics model, the accuracies of the estimated values are not seriously impaired.

Development of Erbia-Credit Super-High-Burnup Fuel: Evaluation of Minimum Erbia Content for Criticality Safety Analyses

The use of highly-enriched fuels is an effective method for reducing the number of spent fuel assemblies and improving fuel cycle economics, e.g., with >5 wt% 235U. However, from a criticality safety point of view, such high enrichment levels require a significant investment for the considerable modification of most facilities and equipment. Erbia-credit super-high-burnup fuel offers an effective solution that can solve the problem: Small amounts of erbia added to the entire amount of UO2 powder can reduce the reactivity level to less than that observed at a 5 wt% enrichment level, thus eliminating the need for the modifications mentioned above. A series of criticality safety analyses has been performed to determine the minimum and sufficient content of erbia that can guarantee a suitable erbia credit. As a noteworthy result, the erbia content required was determined for corresponding values of uranium enrichment in a range >5 wt%, as indicated in our ECOS (Erbia COntent for Sub-criticality judgment) diagram. This paper outlines a series of criticality safety analyses and explains how the minimum erbia content can be determined to ensure subcriticality for a >5 wt% enrichment fuel to ensure that the fuel obtained is equivalent to that whose enrichment is <5 wt%.

Sensitivity analysis for multiplication factor change of LWR cell caused by the differences between JENDL-3.2 and JENDL-3.3

A detailed sensitivity analysis based on transport theory was performed for typical LWR cells of LWR to clarify the contribution to infinite multiplication factor (k-inf) change for nuclide, reaction type and energy range. The k-inf change caused by the difference in cross section sets between JENDL-3.2 and JENDL-3.3 was investigated for some UO2 and MOX cells. The k-inf change is −0.50 to −0.74%Δk/k for the UO2 cells and −0.04 to +0.02%Δk/k for the MOX cells. In the UO2 cells, the difference in 235U capture cross section causes the change of −0.36 to −0.70%Δk/k mainly in the resonance energy range, and 235U fission cross section causes the change of −0.22 to −0.16%Δk/k mainly in the thermal energy range. The difference in 238U capture cross section causes a change of about −0.05%Δk/k mainly in the energy range above 1 MeV. In the MOX cells, a small k-inf change results from the cancellation of relatively large contributions of capture reactions of 240Pu and 241 Am. The capture reaction of 240Pu has a k-inf contribution of +0.10 to +0.30%Δk/k mainly in the resonance energy range. This is offset by the capture reaction of 241 Am which has a contribution of −0.13 to −0.22%Δk/k primarily at the resonance peak near 0.6eV.

Development of Erbia-Credit Super-High-Burnup Fuel: Assessment of Fuel Cycle Economics

The use of high-enrichment fuels, e.g., fuels with >5 wt% 235U, is an effective method of reducing the number of spent-fuel assemblies and improving fuel cycle economics. However, from a criticality safety point of view, such high enrichment levels would entail considerable modification of most facilities and equipment, which would require a significant investment. Erbia-credit super-high-burnup fuel offers the potential for an effective solution to this problem. The fuel is based on the concept that small amounts of erbia added to the entire amount of UO2 powder can reduce the reactivity level to less than that observed at a 5 wt% enrichment level, thus eliminating the majority of the modifications mentioned above. In this paper, a feasibility assessment from the viewpoint of fuel cycle economics is performed to confirm the benefits of erbia-credit-fuel implementation. A simple model to consider the erbia penalty is also proposed. The results show that the generation cost can be significantly reduced by using erbia credit, although the fuel cycle cost would not necessarily decrease in any of the cases when the enrichment level is increased. In addition, implementation scenarios of erbia credit are discussed considering the current industrial situation and the reactivity penalty incurred by the usage of erbia fuel. These implementation scenarios are also considered from the viewpoint of energy security.

Analysis of integral experiment on erbia-loaded thermal spectrum cores using Kyoto University Critical Assembly by MCNP code with various cross section libraries

Under the project on high burnup nuclear fuel development using erbium as a burnable poison, a series of experiments were performed at the Kyoto University Critical Assembly. The experimental results have formed the basis for this study which aims to analyze the suitability of various evaluated nuclear data libraries for using them in neutronic calculations under the project. The MCNP code was used for the analysis. Calculation model geometry was fully detailed, and ENDF, JENDL, JEFF, and TENDL libraries were used during calculation. For the cross sections of erbium nuclides, the analysis revealed that calculated results upon all the libraries corresponded with experimental data within the errors. However, in some libraries, significant differences were found in case of carbon and uranium nuclides under certain conditions.

A New Uncertainty Reduction Method for Fuel Fabrication Process with Erbia-Bearing Fuel

A new uncertainty reduction method is proposed to evaluate the prediction accuracy of neutronics characteristics by combining the generalized bias factor method and the cross section adjustment method. The present method is applied to a fuel fabrication process with erbia-bearing fuel. The cross section uncertainty of erbium is improved by means of cross section adjustment using experimental data of the erbia worths. For a blending machine (H/U = 0) used in the fuel fabrication process, the uncertainty reduction, which shows the rate of reduction of uncertainty, of the keff is 0.604 for the present method and 0.555 for the conventional bias factor method. Thus, the prediction uncertainties are reduced by the present method compared with by the bias factor method.

Estimation of the Doppler Coefficient from a Lower Power Transient Observed in a Zero-Power Reactor Physics Test of a PWR (I)—Methodology—

A method for estimating the Doppler coefficient from low power transient data that are measured in a routine experiment conducted in a zero-power reactor physics test of a PWR power plant has been developed. A dynamics identification model was introduced by considering two dynamics components for reactor kinetics and heat removal in the primary coolant loops. The dynamics identification method in the time domain was applied under consideration that the effect of reactivity feedback observed in the experiment was very small and measured transient data contained some intermittent intervals unavailable for dynamics identification. Unknown parameters, including the Doppler coefficient, contained in the dynamics model were determined stepwise through five estimation steps. In these steps, gamma ray noises were removed from the NIS signal, the reactivity feedback component was extracted from reactivity transient, and fuel temperature transient was determined in such way as to reproduce the observed low power transient in the digital simulation. In the final step, the Doppler coefficient was estimated from the extracted reactivity feedback component using the quantities evaluated in the preceding steps. The estimated value agreed well with the value evaluated with a core design code.