Shinya Mizokami

Cosmic-ray muon radiography of UO2 fuel assembly

A technical demonstration of cosmic-ray muon radiography of a UO2 fuel assembly was performed at Toshiba Nuclear Critical Assembly (NCA). The fuel assembly in the NCA was imaged through obstacles such as steel and concrete. The result suggested that the method can be applicable to assess the damage to the reactors at the Fukushima Daiichi nuclear power plant. Here, both scattering and displacement methods are presented, and the results are shown to agree with Monte Carlo simulations. In addition, detailed Monte Carlo simulations of the Fukushima Daiichi reactor were performed, which showed capability of muon radiography to locate the fuel in the damaged reactors.

Update of the First TEPCO MAAP Accident Analysis of Units 1, 2, and 3 at Fukushima Daiichi Nuclear Power Station

Abstract Because of the Great East Japan Earthquake, and the resulting tsunami, which occurred on March 11, 2011, a serious accident occurred in Units 1, 2, and 3 of the Fukushima Daiichi nuclear power station. Since the accidents, data from interviews with operators and on-site surveys have been continuously compiled. Based on the data, a plant-state analysis has been conducted using the severe accident analysis code MAAP (Modular Accident Analysis Program). Parallel to the MAAP analysis, the responses of the plant to site operations, such as water injection, are analyzed, and core conditions are comprehensively evaluated. According to the evaluation, in Unit 1, it is presumed that almost no fuel was left at the original position; it was molten and moved downward. The fuel likely damaged the reactor pressure vessel (RPV), and it is assumed that most of it had dropped to the primary containment vessel (PCV) pedestal. In Units 2 and 3, it is presumed that some of the fuel was left at the original position and the rest dropped to the bottom of the RPV or to the PCV pedestal. In the MAAP analysis, the behavior of the plants before core melt is reproduced. However, RPV damage of Units 2 and 3 does not occur in the MAAP analysis, which is contrary to the observed facts. This shows that the analysis capability of the current MAAP code is limited. Therefore, by developing severe accident analysis codes to achieve higher levels of accuracy and by evaluating the plant responses to site operation, we will continue to obtain a clear picture of the states inside the reactor so that fuel debris can be removed.

Analysis of muon radiography of the Toshiba nuclear critical assembly reactor

A 1.2 × 1.2 m2 muon tracker was moved from Los Alamos to the Toshiba facility at Kawasaki, Japan, where it was used to take ∼4 weeks of data radiographing the Toshiba Critical Assembly Reactor with cosmic ray muons. In this paper, we describe the analysis procedure, show results of this experiment, and compare the results to Monte Carlo predictions. The results validate the concept of using cosmic rays to image the damaged cores of the Fukushima Daiichi reactors.

Unsolved issues related to thermal-hydraulics in the suppression chamber during Fukushima Daiichi accident progressions

ABSTRACT On 11 March 2011, the Great East Japan Earthquake and Tsunami hit the Fukushima Daiichi Nuclear Power Station. The Fukushima Daiichi Units 1–3 lost all DC and AC power supplies, which set in motion a chain of events that led to releases of radioactivity to the environment. Since then, TEPCO has made many efforts to investigate the accident progressions and the status of the reactors and containment vessels. However, there still exist several tens of unsolved issues to be investigated for the fully understanding of the accident. In this paper, we introduce the unsolved issues related to thermal-hydraulics in the suppression chamber during the Fukushima Daiichi accident progressions. Especially, in Units 2 and 3, there are possibilities that thermal stratification inside their suppression chambers played an important role. It is important that these phenomena are addressed following both theoretical and experimental approaches as support to severe accident simulations.

Validation of MAAP model enhancement for Fukushima Dai-ichi accident analysis with Phenomena Identification and Ranking Table (PIRT)

The Modular Accident Analysis Program (MAAP) model enhancement items to improve the simulation capability for molten corium behavior in the accidents at the Fukushima Dai-ichi nuclear power plants were validated with the Phenomena Identification and Ranking Table (PIRT). The importance ranks of the identified phenomena were evaluated for each time phase through brainstorming and discussion with the experts in the Atomic Energy Society of Japan and the members of the MAAP model enhancement project. When the current MAAP evaluation models were reviewed with the PIRT, it is found that 95 of the 386 high-ranked phenomena were not considered in MAAP 5.0.1. While 62 of these phenomena will have been addressed in the MAAP enhancement project and 25 others are not suitable to be analyzed by MAAP, 8 important phenomena should be considered in post-MAAP enhancement project with additional experiments or fundamental studies.

BWR Stability Issues in Japan

The present paper reviews activates relevant to the boiling water reactor (BWR) stability phenomenon, which has a coupled neutronic and thermal-hydraulic nature, from the viewpoint of model and code developments and their applications to the BWR stability solution methodology in Japan.

Phenomena Identification Ranking Table (PIRT) for the MAAP Enhancement Project

A phenomena identification ranking table (PIRT) was constructed to identify important phenomena that should be considered in simulating the accident progression for the Fukushima Dai-ichi nuclear power station and predicting the molten core and debris distribution in the reactor system with the MAAP code. The accident scenario, starting from reactor scram to PCV failure, was divided into four time phases in which fuel rod degradation, molten core relocation, and reactor vessel failure were selected as the phase-changing events. Phenomena that would have occurred in the accident were identified in the plant system including the reactor vessel and the containment that consist of 16 sub-components, consistent with the MAAP nodalization for the BWR system. The importance of the identified phenomena was evaluated for each time phase in cooperation with the experts in the Atomic Energy Society of Japan. Eventually total of 1047 phenomena were identified, of which 386 were ranked as ‘highly’ important for the analysis code to evaluate the behavior of molten fuel rods and reactor internal materials. These important phenomena are compared with those considered in the current MAAP analysis models to make sure what should be enhanced to improve the analysis capability for the accident progression.Copyright

Event Sequence of the Fukushima Daiichi Accident

On March 11, 2011, the Great East Japan Earthquake and subsequent tsunami hit Fukushima Daiichi Nuclear Power Station. Flooding by the tsunami induced loss of AC and/or DC power for reactor cooling, hence the reactor water level decreased and fuel was exposed. Water reacting with high temperature fuel metal covering resulted in hydrogen generation and hydrogen explosion of reactor buildings. This accident caused radioactive release to the environment. In this chapter, an attempt has been made to understand in detail the mechanism of the accident progression for Units 1–3 that were in operation by utilizing results of computer simulations. It should be noted that, due to limited information and capability of the state-of-the-art severe-accident simulation tools, there are still unanswered questions, which should be tackled by academic research for improving and enhancing safety for the nuclear industry now and in the future.

Cosmic-Ray Muon Imaging of Fukushima Daiichi

Reactor imaging using scattering of cosmic-ray muon is proposed to assess the damages to the reactors at Fukushima Daiichi. Simulation studies showed feasibility of the reactor imaging with muons, and the technique has been demonstrated at a research reactor, Toshiba Nuclear Critical Assembly, where the reactor core was imaged with spatial resolution of 3 cm after 1 month of exposure time.Copyright

Imaging of a reactor with muons

A technical demonstration to image a research reactor, Toshiba Nuclear Critical Assembly, with cosmic-ray muons is presented. The demonstration was performed as a precursor to Fukushima Daiichi muon imaging. We have obtained resolution of 3 cm during 1 month of exposure time. This result is in agreement with previous simulation results conducted on Fukushima Daiichi reactors 1 and 2.