Development of the evaluation methodology for the material relocation behavior in the core disruptive accident of sodium-cooled fast reactors

• Published in: Journal of nuclear science and technology Vol. 53; no. 5; pp. 698 - 706
• Main Authors: Tobita, Yoshiharu, Kamiyama, Kenji, Tagami, Hirotaka, Matsuba, Ken-ichi, Suzuki, Tohru, Isozaki, Mikio, Yamano, Hidemasa, Morita, Koji, Guo, LianCheng, Zhang, Bin
• Format: Journal Article
• Language: English
• Published: Tokyo Taylor & Francis 03.05.2016; Taylor & Francis Ltd
• Subjects: Computer simulation; core disruptive accident; Debris; Discharge; Experiments; in-vessel retention; Methodology; molten core material; Nuclear reactors; Penetration; Pools; Reactors; safety assessment; Sodium; sodium-cooled fast reactor
• ISSN: 0022-3131; 1881-1248
• DOI: 10.1080/00223131.2016.1143409

The in-vessel retention (IVR) of core disruptive accident (CDA) is of prime importance in enhancing safety characteristics of sodium-cooled fast reactors (SFRs). In the CDA of SFRs, molten core material relocates to the lower plenum of reactor vessel and may impose significant thermal load on the structures, resulting in the melt-through of the reactor vessel. In order to enable the assessment of this relocation process and prove that IVR of core material is the most probable consequence of the CDA in SFRs, a research program to develop the evaluation methodology for the material relocation behavior in the CDA of SFRs has been conducted. This program consists of three developmental studies, namely the development of the analysis method of molten material discharge from the core region, the development of evaluation methodology of molten material penetration into sodium pool, and the development of the simulation tool of debris bed behavior. The analysis method of molten material discharge was developed based on the computer code SIMMER-III since this code is designed to simulate the multi-phase, multi-component fluid dynamics with phase changes involved in the discharge process. Several experiments simulating the molten material discharge through duct using simulant materials were utilized as the basis of validation study of the physical models in this code. It was shown that SIMMER-III with improved physical models could simulate the molten material discharge behavior, including the momentum exchange with duct wall and thermal interaction with coolant. In order to develop an evaluation methodology of molten material penetration into sodium pool, a series of experiments simulating jet penetration behavior into sodium pool in SFR thermal condition were performed. These experiments revealed that the molten jet was fragmented in significantly shorter penetration length than the prediction by existing correlation for light water reactor conditions, due to the direct contact and thermal interaction of molten materials with coolant. The fragmented core materials form a sediment debris bed in the lower plenum. It is necessary to remove decay heat safely from this debris bed to achieve IVR. A simulation code to analyze the behavior of debris bed with decay heat was developed based on SIMMER-III code by implementing physical models, which simulate the interaction among solid particles in the bed. The code was validated by several experiments on the fluidization of particle bed by two-phase flow. These evaluation methodologies will serve as a basis for advanced safety assessment technology of SFRs in the future.