Post-Test Analyses of the CMMR-4 Test

Takuya Yamashita,Hiroshi Madokoro,Ikken Sato

doi:10.1115/1.4051443

Takuya Yamashita, Hiroshi Madokoro + Show 1 more

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https://doi.org/10.1115/1.4051443

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Abstract

Abstract Understanding the final distribution of core materials and their characteristics is important for decommissioning the Fukushima Daiichi Nuclear Power Station (1F). Such characteristics depend on the accident progression in each unit. However, boiling water reactor (BWR) accident progression involves great uncertainty. This uncertainty, which was clarified by MAAP-MELCOR crosswalk, cannot be resolved with existing knowledge and was thus addressed in this work through core material melting and relocation (CMMR) tests. For the test bundle, ZrO2 pellets were installed instead of UO2 pellets. A plasma heating system was used for the tests. In the CMMR-4 test, useful information was obtained on the core state just before slumping. The presence of macroscopic gas permeability of the core approaching ceramic fuel melting was confirmed, and the fuel columns remained standing, suggesting that the collapse of fuel columns, which is likely in the reactor condition, would not allow effective relocation of the hottest fuel away from the bottom of the core. This information will help us comprehend core degradation in boiling water reactors, similar to those in 1F. In addition, useful information on abrasive water suspension jet (AWSJ) cutting for debris-containing boride was obtained in the process of dismantling the test bundle. When the mixing debris that contains oxide, metal, and boride material is cut, AWSJ may be repelled by the boride in the debris, which may cut unexpected parts, thus generating a large amount of waste in cutting the boride part in the targeted debris. This information will help the decommissioning of 1F.

Highlights

Core degradation in a boiling water reactor (BWR) like the Fukushima Daiichi Nuclear Power Station (1F) has not been comprehensively understood
The core material melting and relocation (CMMR)-4 test was conducted to comprehend core material relocation (CMR) in a BWR, like the 1F2, and address questions related to CMR (Q1: What about the gas permeability of the high-temperature core? Q2: What about the downward relocation of hot unmelted fuel and its heating of the structure?)
(2) The hot fuel tends to remain as columns; effective fuel relocation that removes the hottest fuel from the middle of the core and effectively heats the support structure, is unlikely

Summary

Introduction

Core degradation in a boiling water reactor (BWR) like the Fukushima Daiichi Nuclear Power Station (1F) has not been comprehensively understood. The XR2-1 test [1,2], CORA test [3,4], and Phebus FP test series [4,5,6] can be cited as representative studies that simulate the core damage in a BWR system Most of these tests simulated the behavior within the core region, and only the XR2-1 test, CORA17 test [7], and CORA-28 test [8] focused on the relocation of material to the lower plenum. These tests confirmed that initial core damage in a BWR system was caused by eutectic melting due to contact between the control blade and channel box. Even in this test, the temperature was limited basically to the melting point of relocating molten metal, and uncertainty remained about the CMR behavior in the high-temperature domain near the ceramic-fuel melting

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