Development of failure evaluation method for BWR Lower head in severe accident; - Creep damage evaluation based on thermal-hydraulics and structural analyses -

Abstract

In existing severe accident codes such as MELCOR and THALES2, rupture of reactor pressure vessel (RPV) by relocated molten core is judged using simple models such as temperature and/or stress criteria. However, it is difficult to assess rupture behavior of the lower head of RPV in boiling-water-type nuclear power plants due to severe accident like Fukushima Daiichi. One reason is that boiling water reactors (BWRs) have geometrically complicated structure with a lot of penetrations. Another one is that BWR lower head is composed of various types of materials of RPV, weld-overlay cladding, control rod guide tubes, stub tubes, welds, etc. Therefore, we have been developing an analysis method to predict time and location of RPV lower head rupture of BWRs considering creep damage mechanisms based on coupled analysis of three-dimensional thermal-hydraulics (TH) and thermal-elastic-plastic-creep analyses. The detailed three-dimensional model of RPV lower head with control rod guide tubes, stub tubes, and welds are constructed. TH analysis is performed to obtain three-dimensional temperature distribution in relocated debris. Using TH analysis results, structural analysis is carried out to evaluate creep damage distributions using four types of damage criterions of "considere", strain, Kachanov, and Larson-Miller-parameter (LMP) criteria. Creep damage evaluation based on Kachanov and LMP models is made by using experimentally determined parameters. From comparison of damage criterions, it is shown that failure regions of BWR lower head are only penetrations under simulated conditions, although there is a large difference in failure time.

Development of failure evaluation method for BWR Lower head in severe accident; high temperature creep test and creep damage model

“…To assess progress of severe accident and to predict rupture time and location of reactors due to severe accident, Katsuyama et al (2016) developed finite element model of RPV lower head including, penetrations, and welds, and four types of creep damage failure evaluation methods of Kachanov and LMP criterions confirmed in this study. Through the analyses, it was shown that failure occurred only near the penetrations under simulated conditions.…”

PFEM formulation for thermo-coupled FSI analysis. Application to nuclear core melt accident

“…Recently, some interesting attempts to simulate numerically specific NSA scenarios have been done [11,47,38,48,70]. In [71] the relocation of molten materials in nuclear reactors was modeled using a thermo-coupled Volume of Fluid (VOF) method [30], while in [10] the flow and solidification of molten core debris was studied with the Moving Particle Semi-implicit (MPS) method [41].…”