Session: 03-02: Major facilities experience in handling accidents and D/D&D

Paper Number: 112652

112652 - Mps Method Simulation for Estimating Fuel Debris Distributions Under the Damaged Reactor Pressure Vessel of 1f Unit-2 

The muon image of TEPCO's Fukushima Daiichi Nuclear Power Station (1F) Unit 2 indicates massive high-density materials distributed not only inside but also around the bottom of the reactor pressure vessel (RPV). In the meantime, below the RPV, no significant damages have been confirmed, but core material drainage-like traces haven been observed extensively across the structures. Furthermore, an unmolten fuel assembly upper tie plate has been found on the pedestal floor near the pedestal wall. These observations indicate possibility of multiple RPV lower head breaches with core material discharges and relatively large breach(es) near the vessel periphery, while massive fuel debris are still retained within and around the RPV lower head. Such current status may be explained when the fuel debris interactions with the horizontal thermal insulation plate below the RPV lower head is considered. Namely, the fuel debris may be largely held-up by the insulation plate, while some local melt-through of the plate may allow relocation of large core structures, such as the upper tie-plate at the time of the vessel failure. The purpose of this study is to show such possibility with mechanistic simulation of the fuel debris-insulation plate thermal interactions using Moving Particle Semi-implicit (MPS) method.

The MPS method is a Lagrangian based particle method for incompressible flow. In this study, the solid fuel debris were regarded as rigid bodies each of which with the size comparable to that of a fuel pellet. Each rigid body was represented by some calculation points (particles) with fixed relative coordinates so that debris relocation induced by melting of the vicinity or itself could be considered. The analysis domain consisted of the unit section, which consisted of the horizontal insulation plate, the central control rod drive (CRD) housing, the adjacent four in-core monitor (ICM) tubes, and the adiabatic dummy walls to define the four vertical sides of the domain boundaries. The decay heat of the fuel debris, heat conductions among debris and the structures, melting of the structures and convection of the melt were directly calculated in the MPS method while heat losses by radiation and gas convection were modeled as heat removal correlations from the free surface particles.

The results show that whether the insulation plate fails or not depends on the initial enthalpy and temperature distribution of the relocated fuel debris. Due to the limited calculation resolution, the heat conduction within the insulation plate could not be accurately calculated. The estimated initial fuel debris temperature to cause local damage of the plate (but prevent global melting of the plate) ranged from about 2100 to 2300 K, which is well below melting temperature of oxidic fuel debris. Such results support the current interpretation that the muon image shows massive fuel debris retained inside and around the lower head of the RPV of 1F Unit-2. It also explains that local breach of the insulation plate to allow relocation of the upper tie-plate is more likely near the periphery of the RPV, because there is more space for the fuel debris to pileup, which leads to larger thermal load on the insulation plate to melt. These newly gained understandings are currently being considered to update the 3D estimation diagram of 1F Unit-2.

Acknowledgements:

This study has been performed by the funds from Japanese Ministry of Economy, Trade and Industry as The Subsidy Program "Project of Decommissioning, Contaminated Water and Treated Water Management (Development of Analysis and Estimation Technologies for Characterization of Fuel Debris) (Development of Estimation Technologies of RPV Damaged Condition, etc.)"

Keywords: severe accident, decommissioning of Fukushima Daiichi (1F) reactors, MPS method

Presenting Author: Yamato Bando Waseda University

Presenting Author Biography: 1st year Master Course student, Cooperative Major in Nuclear Energy, Graduate School of Advanced Science and Engineering, Waseda University.
Current resarch: Estimation of the current status of Fukushima Daiichi Nuclear Power Plant Unit-2 with Moving Particle Semi-implicit method.