Multiphysics analysis of reactivity changes due to solution flow in the past criticality accident at Windscale Works in 1970

Fukuda, Kodai; Yamane, Yuichi

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10

This study presents the results of multiphysics analysis, which investigates the change of reactivity caused by the motion of fluids, of Windscale Works criticality accident. The purpose of this study is to confirm previously reported trends of emulsion formation and increase in reactivity by the multi-physics analysis which takes the motion of fluids into account. Continuous energy Monte Carlo code MVP3 was used to calculate reactivity based on the material distribution obtained by CFD calculation using OpenFOAM. An interface program in python was developed to transfer data from OpenFOAM to MVP3. The change of reactivity caused by the motion of solutions was calculated without considering the generation of heat by fissions in a system that simulated the transfer vessel at Windscale Works. As a result, trends of emulsion formation and increase in reactivity were confirmed. The influence of the resolution of the calculation system on the results was also discussed.

Molecular dynamics analysis of reactor graphite for preparing thermal neutron scattering law

Okita, Shoichiro; Goto, Minoru

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 10 Pages, 2023/10

The Nuclear criticality accident in Japan, Revisited

Okuno, Hiroshi; Suyama, Kenya

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 8 Pages, 2023/10

The criticality accident that resulted in the evacuation of residents occurred on September 30, 1999, at JCO's nuclear fuel fabrication plant in Ibaraki prefecture, Japan. This paper presents the outline, technical issues and background of the accident and the situations that followed. The review of this accident was one of the hot issues in the last ICNC2003 organized in Tokai-mura. At the this turn of ICNC in Japan, we would like to revisit the accident to reaffirm and share the idea that the response and preparedness should be strengthened to protect people and the environment from nuclear disasters. The accident occurred in a factory of JCO during the informal and unusual process of preparing a uranium nitrate solution of medium-enriched (18.8 weight% U) uranium from UO using the precipitation tank of 45 cm in diameter, which exceeded the critical diameter (ca. 23 cm) of an infinite cylinder with a full thick water reflector. A 2.2-cm-thick water "jacket" surrounded and enclosed the tank, and the jacket was connected to the cooling tower beside the factory. The jacket not only functioned as the neutron reflector but also prevented the evaporation of the solution, and then the criticality continued for about 20 hours. Because JCO's plant had not anticipated the criticality accident, the response to the accident was confusing. During the accident, the JAERI and JNC, both the predecessors of the Japan Atomic Energy Agency (JAEA), acted to terminate the criticality and reduce the residents' exposure to radiation. After the accident, the JAERI and the National Institute of Radiological Sciences provided the telephone consultation at the village office of Tokai-mura. The JNC did the same things at the prefectural building of Ibaraki to advise the residents. The presentation may include issues of applying the Slide rule, identifying a nuclear criticality accident to occur, and responding to the emergency.

Comparison of neutronic characteristics of BWR burnup fuel between JENDL-4.0 and JENDL-5

Watanabe, Tomoaki; Tada, Kenichi; Endo, Tomohiro*; Yamamoto, Akio*

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 10 Pages, 2023/10

The latest Japanese nuclear data library, JENDL-5, was released in December 2021. In JENDL-5, nuclear reaction cross sections for Gd-155 and Gd-157 were modified in addition to many heavy nuclides such as U-235. Fission yields and decay data, which are essential to characterize burnup fuels, were completely revised. This study investigated the effects of the nuclear data revisions from JENDL-4.0 to JENDL-5 on the neutronic characteristics of burnup fuels to validate JENDL-5. Burnup calculations of the 9x9 STEP-3 BWR fuel assembly based on the OECD/NEA Phase III-C benchmark were performed using JENDL-4.0 and JENDL-5. As a result, the k for JENDL-5 was smaller than that of JENDL-4.0 throughout the burnup, with a large difference of about 600 pcm at 12 GWd/t, around the peak of the k. Above 20 GWd/t, the difference in k increases with increasing burnup value, reaching nearly 600 pcm at 50 GWd/t. In addition, this study investigates which nuclear data contribute significantly to the difference in k by performing burnup calculations with replacing nuclear data of individual nuclides from JENDL-4.0 to JENDL-5.

Study on the basic core analysis of the new STACY

Gunji, Satoshi; Yoshikawa, Tomoki; Araki, Shohei; Izawa, Kazuhiko; Suyama, Kenya

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 8 Pages, 2023/10

Since the compositions and properties of the fuel debris are uncertain, critical experiments are required to validate calculation codes and nuclear data used for the safety evaluation. For this purpose, JAEA has been modifying a critical assembly called "STACY". The first criticality of the new STACY is scheduled for spring 2024. This paper reports the consideration results of the core configurations of the new STACY at the first criticality. We prepared two sets of gird plates with different neutron moderation conditions (their intervals are 1.50 cm and 1.27 cm). However, there is a limitation on the number of available UO fuel rods. In addition, we would like to set the critical water heights for the first criticality at around 95 cm. This is to avoid the reactive effect of the aluminum alloy middle grid plates (Approx. 98 cm high). The core configurations for the first criticality satisfying these conditions were constructed by computational analysis. A square core configuration with the 1.50 cm grid plate that is close to the optimum moderation condition needs 261 fuel rods to reach criticality. As to the 1.27 cm grid plate, we considered two core configurations with 1.80 cm intervals by using a checkerboard arrangement. One of them has two regions core configuration with 1.27 and 1.80 cm intervals, and the other has only 1.80 cm intervals. They need 341 and 201 fuel rods for the criticality, respectively. This paper shows these three core configurations and their calculation models.

Inter-codes and nuclear data comparison under collaboration works between IRSN and JAEA

Gunji, Satoshi; Araki, Shohei; Watanabe, Tomoaki; Fernex, F.*; Leclaire, N.*; Bardelay, A.*; Suyama, Kenya

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10

Institut de radioprotection et de sret nuclaire (IRSN) and Japan Atomic Energy Agency (JAEA) have a long-standing partnership in the field of criticality safety. In this collaboration, IRSN and JAEA are planning a joint experiment using the new STACY critical assembly, modified by JAEA. In order to compare the codes (MVP3, MORET6, etc.) and nuclear data (JENDL and JEFF) used by both institutes in the planning of the STACY experiment, benchmark calculations of the Apparatus B and TCA, which are critical assemblies once owned by both institutes, benchmarks from the ICSBEP handbook and the computational model of the new STACY were performed. Including the new STACY calculation model, the calculations include several different neutron moderation conditions and critical water heights. There were slight systematic differences in the calculation results, which may have originated from the processing and/or format of the nuclear data libraries. However, it was found that the calculated results, including the new codes and the new nuclear data, are in good agreement with the experimental values. Therefore, there are no issues to use them for the design of experiments for the new STACY. Furthermore, the impact of the new TSL data included in JENDL-5 on the effective multiplication factor was investigated. Experimental validation for them will be completed by critical experiments of the new STACY by both institutes.

Dependence of the average total kinetic energy of fission fragments on incident neutron energy studied by a 4D Langevin model

Shimada, Kazuya; Ishizuka, Chikako*; Chiba, Satoshi*

Proceedings of 12th International Conference on Nuclear Criticality Safety (ICNC2023) (Internet), 9 Pages, 2023/10

TKE (total kinetic energy), one of the nuclear data, is the kinetic energy of fission fragments, and influences not only the energy obtained in nuclear fission, but also the criticality safety assessment. Exploring the underlying factors influencing TKE is vital for fundamental research and utilization of nuclear energy. Instinctively, it is expected that as the energy of incident neutrons on the target nucleus increases, thus leading to a corresponding increase in TKE. However, experimental evidence suggests that TKE decreases as the excitation energy increases. In this study, we investigate this phenomenon. The four-dimensional Langevin model captures the overall shape of the nuclear fission fragment as Brownian motion until the nuclear fission. Using this model, we calculated the excitation energy dependence of TKE. Our investigation reveals that the decreasing trend in TKE can be attributed to the deformation of the heavy fission fragments.