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Nagaya, Yasunobu
EPJ Nuclear Sciences & Technologies (Internet), 11, p.1_1 - 1_7, 2025/01
Japan Atomic Energy Agency (JAEA) has been developing a general-purpose continuous-energy Monte Carlo code MVP for nuclear reactor core analysis. Recently improvements to MVP have been focused on the development of an advanced neutronics/thermal-hydraulics coupling code. JAEA has also developed a new Monte Carlo solver Solomon for criticality safety analysis. Solomon aims to calculate the criticality of a damaged reactor core including fuel debris. This paper provides an overview of the capabilities and reviews recent applications of MVP and Solomon.
Watanabe, Tomoaki; Ueki, Taro; Suyama, Kenya
Proceedings of International Conference on Physics of Reactors (PHYSOR 2024) (Internet), 10 Pages, 2024/04
Solomon, a Monte Carlo solver being developed by JAEA, can calculate criticality in multi-material randomized systems for criticality evaluation of fuel debris. This study investigates the applicability of Solomon to critical mass calculations of fuel debris. We performed critical mass calculations of fuel randomization systems using Solomon. The fuel randomization systems, where burned fuels with different burnups and water are randomly distributed, were modeled by the incomplete randomized Weierstrass function (IRWF) model or voxel geometry in Solomon. Critical mass calculations of simple homogeneous and heterogeneous systems were also performed, and the critical sizes were compared to fuel randomization systems. The results showed that the fuel randomization causes significant variations in the critical mass. The obtained critical sizes were distributed close to a normal distribution, which made it reasonable to estimate the uncertainty of critical mass as the standard deviation. The critical sizes with uncertainty obtained by Solomon were smaller than those of a simple heterogeneous system. This indicates Solomon would be useful for estimating or evaluating a reasonable safety margin in criticality safety evaluations of fuel debris.
Ueki, Taro
Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10
A Monte Carlo Solver Solomon has been under development as an object-oriented code written in the C++14 standards. It consists of regular capabilities of criticality safety analysis and a special capability of random media criticality. In the latter capability, Solomon is equipped with a class for the random media modeled by the incomplete randomized Weierstrass function (IRWF). By this modeling, the uncertainty of random media criticality can be evaluated by executing criticality calculations over many IRWF-replicas. However, it is impossible to know beforehand how many IRWF-replicas should be computed. To deal with this issue, a bounded amplification (BA) technique has been newly equipped in Solomon. Applying BA to IRWF, it is possible to reduce the number of IRWF-replicas by more than 95% in terms of the upper limit estimation of neutron effective multiplication factor. Solomon is also equipped with a voxel-overlay (VO). This functionality is shown to be valuable for evaluating the resonance self-shielding effect.
Nagaya, Yasunobu; Ueki, Taro; Tonoike, Kotaro
Proceedings of 11th International Conference on Nuclear Criticality Safety (ICNC 2019) (Internet), 9 Pages, 2019/09
A new Monte Carlo solver Solomon has been developed for the application to fuel-debris systems. It is designed not only for usual criticality safety analysis but also for criticality calculations of damaged reactor core including fuel debris. This paper describes the current status of Solomon and demonstrates the applications of the randomized Weierstrass function (RWF) model and the RWF model superimposed voxel geometry.
Gunji, Satoshi; Watanabe, Tomoaki; Tonoike, Kotaro; Araki, Shohei
no journal, ,
The criticality safety research group had been conducted research using deterministic methods to ensure the criticality safety. However, the retrieval work of fuel debris in the Fukushima Dai-ichi Nuclear Power Station cannot be evaluated by the conventional criticality management methods, therefore it is necessary to develop a risk-informed control method. To solve these research issues, we are making a critical risk basic database that covers the possible composition and properties of fuel debris. Its validity will be confirmed by a critical experiment by modified STACY critical assembly. We are also building a database that can manage the risk of the impact of exposure to critical events. We are developing a new calculation model to evaluate fuel debris. And post irradiation examinations have been conducted to accurately measure the burnup.
Nagaya, Yasunobu; Hagura, Hiroyuki*
no journal, ,
In order to build the criticality safety database for fuel debris, a Monte Carlo Solver Solomon has been under development. The probability table method has been implemented into Solomon to treat the self-shielding effect in the unresolved resonance region correctly. The implementation has been verified with the calculation of effective multiplication factors for simple geometry systems.
Yoshikawa, Tomoki; Izawa, Kazuhiko; Araki, Shohei; Watanabe, Tomoaki; Gunji, Satoshi
no journal, ,
We conducted performance tests on five types of cores at the modified STACY as first critical cores for resumption of operation. The performance test consisted of a core with 253 and 277 fuel rods and 253 fuel rods loaded with a sample drive at grid intervals of 1.5 cm, and 213 and 241 fuel rods at grid intervals of 2.54 cm. We analyzed the critical water level in each core using MVP with JENDL-4 as the nuclear data for the configuration. Similar calculations were conducted for the other codes MCNP, Solomon and SERPENT to compare the evaluation results.
Ueki, Taro
no journal, ,
The randomized Weierstrass function (RWF) is a useful tool for the uncertainty evaluation of the criticality of disordered media. This excerpt reports the extension of RWF methodology covering a wide range of power law spectra in order to simulate various disordered mixtures of materials. The extended RWF is then demonstrated using the Monte Carlo Solver Solomon.
Ueki, Taro
no journal, ,
Randomized Weierstrass function (RWF) is a model specifically designed for the criticality of two material systems under uncertain distribution. It is reported in this excerpt that the RWF has been extended to systems of any number of multi-species materials based on the divisions of volume fractions and the combinatorial couplings of different materials. The extended RWF model is implemented in Monte Carlo solver Solomon.
Nagaya, Yasunobu; Hagura, Hiroyuki*
no journal, ,
Careful criticality management must be required for the removal of fuel debris generated at the accident in the Fukushima Daiichi Nuclear Power Station; the uncertainties in fuel debris properties such as amount, composition, location, densities, etc. must be taken into account. For determining the policy of such criticality management, it is important to build the fundamental criticality safety database (criticality maps) for as many fuel debris conditions as possible. In order to contribute the building of the database, the development of a novel Monte Carlo solver has been initiated to perform criticality calculations of fuel debris with flexible randomized models. In this work a model of collision analysis with the ACE formatted nuclear data has been implemented and verified with criticality calculations for simple spherical geometries.
Nagaya, Yasunobu
no journal, ,
In order to build a criticality characteristics database for fuel debris, a Monte Carlo Solver Solomon has been under development. Thermal neutron scattering models have been implemented into Solomon to extend the applicability area to thermal reactor systems. The implementation has been verified with the inter-code comparison of effective multiplication factors for simple geometries.
Tuya, D.; Nagaya, Yasunobu
no journal, ,
Ueki, Taro
no journal, ,
In the power spectrum measurement of natural and engineering phenomena, there are upper and lower limits in the frequency domain variables. Therefore, in this excerpt, we report that the randomized Weierstrass function for the modelling of inverse power law spectrum has been extended so that the range of frequency domain variable can be set arbitrarily. This extension is born out of breaking the relationship with the convergence issue in the fractalness of Weierstrass function and thus named an incomplete randomized Weierstrass function (IRWF). An example of the uncertainty evaluation of neutron effective multiplication factor using IRWF is shown for fuel debris in a sufficiently water-moderated environment.
Ueki, Taro
no journal, ,
The Monte Carlo solver Solomon is a nuclear criticality code that has been under development at JAEA. Solomon has special functionality for the criticality of a disordered system like fuel debris to evaluate the fluctuations of neutron effective multiplication factor (keff) due to uncertainties in the average composition and spatial distribution. However, this functionality lacked a method of representing the macroscopic nuclear cross sections corresponding to measurement data. Therefore, in this presentation, we report a method for generating random spatial variation functions using arbitrary-shaped power spectra as input.
Tuya, D.; Nagaya, Yasunobu
no journal, ,
Ueki, Taro
no journal, ,
A Solomon Monte Carlo solver has been under development. One of its missions is to establish a random media criticality analysis method for fuel debris criticality safety. To this end, random media replicas are generated on which the delta tracking Monte Carlo particle transport is implemented. This excerpt reports the application of a bounded amplification (BA) technique to the random media modeled by the incomplete randomized Weierstrass function (IRWF). The generalized extreme value analysis of effective multiplication factor (keff) over IRWF replicas shows that the extreme value statistics follows the Weibull distribution whose upper limit is effectively bounded by the largest realization of keff over BA-applied IRWF replicas.
