Hiroki Takezawa

New Approach to Space-Dependent Kinetic Analysis by the Integral Kinetic Model

Abstract This work aims to show the possibility of using the integral kinetic model, which is applicable to any geometry, for general space-dependent kinetic analysis. A space-dependent kinetic analysis methodology and code were developed based on the integral kinetic model. The developed kinetic analysis code was verified by comparing results from the developed code with the one-point model in the Godiva reactor geometry. It is possible to explain discrepancies between the two kinetic models using error introduced into Cij(τ) in the fitting process of original Monte Carlo data Cij(kΔτ). This is because the fitting error changes the mean generation time of a system. The verification concluded that it is important to always monitor the fitting error introduced to Cij(τ) in order to understand the calculation results of the developed code. The space-dependent kinetic analysis code was also demonstrated in a fast-thermal coupled reactor geometry including feedback effects. The demonstration results showed a time difference in kinetic behaviors between a fast region and a thermal region that was theoretically expected to appear. In conclusion, this work shows a new approach to solving general space-dependent kinetic problems by using the integral kinetic model including feedback effects.

Calculation of the Integral Kinetic Model Transient Parameters by the Monte Carlo Method for Space-Dependent Kinetic Analysis

Abstract The integral kinetic model is applicable to space-dependent kinetic analysis for any weakly coupled system because of its applicability to any geometry. Transient parameters that describe the time distribution of neutron transport between regions in a system are essential for this model. This paper presents a formula for calculating the parameters based on the nonanalog Monte Carlo neutron transport simulation technique. A continuous-energy Monte Carlo code MVP2.0 was modified to calculate the parameters, and the modification was verified using the static coupled reactor theory. The parameters were calculated in a simple fast-thermal coupled reactor. The results showed a difference in fission starting times between a fast region and a thermal region, which can cause a time lag in the transient behavior between the two regions. The results also revealed the time distribution of neutron energy groups that trigger fissions in each region. A space-dependent kinetic analysis code based on the integral kinetic model is under development, and these parameters can be used in the integral kinetic model to perform space-dependent kinetic analysis for weakly coupled systems.

Improved Approach to Multiregion Supercritical Transient Analysis Based on the Integral Kinetic Model and Monte Carlo Method

Abstract An approach to multiregion supercritical transient analysis based on the integral kinetic model (IKM) and Monte Carlo method is further developed with new features. The IKM describes the region-dependent fission rate during the transient in a system of arbitrary geometry using a secondary fission probability density function, which takes the explicit neutron transport time between successive fissions across the regions into account. The new features of the improved approach include treatment of the multiregion transient using repeated multidimensional linear interpolation between pre-obtained kinetic functions (i.e., secondary probability density function), a new method for calculating the kinetic functions using the continuous-energy Monte Carlo code MVP2.0, and utilization of kinetic functions directly in the IKM without the fitting function that introduces a fitting error. The improved approach is verified by applying it to the supercritical transient in simple Godiva systems of different region combinations without feedback. In addition, we attempt to validate the improved approach by applying it to the supercritical transient in a simplified Godiva system with thermal expansion feedback and compare the obtained and experimental results. The verification results indicate the improved approach works well with different combinations of regions while the validation results show promising agreement with the experimental results. This study is part of an ongoing research activity on the development of Multi-region Integral Kinetic (MIK) code for general space- and time-dependent kinetic analyses.

Passive measure for preventing recriticality of fuel debris dust for defueling at Fukushima Daiichi nuclear power station

ABSTRACT The drilling or cutting of resolidified fuel debris required to defuel the Fukushima Daiichi nuclear power station is certain to generate debris dust. This paper focused on drilling resolidified fuel debris in water and conservatively confirmed by criticality calculations that neutron multiplication effect is higher if debris dust is suspended separately from the debris rather than if it is suspended closely around the debris. No use of vacuuming of debris dust, borated water, and active components was assumed in this study. Also, this paper confirmed that the use of a debris dust guide effectively and passively limited the increase in neutron multiplication by debris dust because the guide distributes dust particles so flatly that sufficient neutron leakage limits neutron multiplication even if the volume fraction of the particles in water reaches the optimum condition. In the actual defueling operation at the nuclear power station, the use of a flatter debris dust guide will be more effective to prevent local recriticality concurrently with the careful control of the mass of debris dust. The physics and ideas in this paper should be applicable to other defueling technologies such as laser cutting as long as debris dust is generated and suspended in water.

Possibility of nuclear pumped laser experiment using low enriched uranium

Possibility to perform experiments for nuclear pumped laser oscillation by using low enriched uranium is investigated. Kinetic analyses are performed for two types of reactor design, one is using highly enriched uranium and the other is using low enriched uranium. The reactor design is based on the experiment reactor in IPPE. The results show the oscillation of nuclear pumped laser in the case of low enriched uranium reactor is also possible. The use of low enriched uranium in the experiment will make experiment easier.