Open Access System for Information Sharing

Login Library

 

Thesis
Cited 0 time in webofscience Cited 0 time in scopus
Metadata Downloads

Debris Bed Formation during Ex-Vessel Severe Accidents in Light Water Reactors

Title
Debris Bed Formation during Ex-Vessel Severe Accidents in Light Water Reactors
Authors
김은호
Date Issued
2016
Publisher
포항공과대학교
Abstract
During severe accidents of light water reactors (LWRs), the coolability of relocated corium from the reactor vessel is a significant safety issue since the failure in cooling and stabilizing the molten core in the containment vessel threatens the integrity of the containment boundary. With a flooded cavity, where a water pool is prepared in the reactor cavity prior to the accidental drop of molten core to it by the reactor vessel failure, it is expected that a porous debris bed develops on the bottom of the pool due to breakup and fragmentation of the melt jet. For conducting a realistic coolability assessment and devising a reasonable accident management plan, it is required to understand the nature of the debris bed formation and its geometrical configuration. Generally, it is expected that the high density particle mixture sedimentation results in a debris bed as a steep-angled pile, and that the debris bed has stratification with larger particles on the bottom. However, a previous work with computer simulation of debris bed formation considering the decay heat of corium (molten core material) particle bed by Yakush et al. (2009) showed a possibility of rather flattened shape of the debris bed. The influence of such a thermal effect on the internal structure of debris bed has not been studied, yet. The Debris Bed Research Apparatus for Validation of the Bubble-Induced Natural Convection Effect Issue (DAVINCI) experimental facility was constructed to investigate the formation of a debris bed under the influence of two-phase flow induced by steam generation due to the decay heat of the debris bed. Cylindrical shaped stainless steel particles were used as simulant debris particles and dropped from the top of a water pool, while air bubbles simulating the vapor flow were injected from the bottom of the water pool through the particle catcher plate. From stepwise sedimentation tests with consideration of decay heat of the debris bed that increases with increase of the mass, the debris bed growth pattern was examined. The change of vapor generation rate according to the bed growth were simulated by individual air injection rate control for the 32 local sections on the particle catcher plate based on the local debris bed volume data from 3D scanning of the resultant debris bed in each step. The experimental results showed that the bubble-induced two-phase natural convection affects particle settling trajectories, and their arrival location, resulting in the growth of a debris bed with a larger distribution radius while keeping an almost constant slope angle. Based on the experimental results, an analytical model was developed to describe the spreading of the debris bed in terms of two-phase flow characteristics and the debris fall parameters. The model was then applied for the analysis of the debris bed formation at the reactor scale, and a sensitivity analysis was carried out based on key accident parameters, including the quantity of corium melt, cavity flooding water level, volumetric decay heat rate, and the size of the melt jet. For investigating the internal structure of the debris bed, sedimentation test with single size particles and mixtures of different size particles were carried out under various two-phase natural convection flow conditions. Based on the local sampling of particles, the characteristics of internal structure of the debris bed were identified. The particle size distribution, porosity, and permeability were examined in radial and axial directions. The experimental results showed that the center part of the particle bed tended to have larger particles more than the peripheral area. For the axial distribution, the lower layer had higher fraction of larger particles in the short term of sedimentation. However, as the sedimentation progressed, the size distribution in the upper layers, which are formed later, shifts to larger sizes due to higher vapor generation rate and stronger flow intensity. The present series of experiments provided the data on the external and internal geometrical configuration of debris beds under realistic two-phase flow conditions, partially by employing the time sequential approach. Also, the data qualitatively provide validation of the previous simulation works on the debris sedimentation and debris bed formation by Yakush et al (2009) that suggested the influence of two-phase natural convection on the debris bed geometry. Furthermore, a pioneering analytical model was suggested.
URI
http://postech.dcollection.net/jsp/common/DcLoOrgPer.jsp?sItemId=000002226379
https://oasis.postech.ac.kr/handle/2014.oak/93432
Article Type
Thesis
Files in This Item:
There are no files associated with this item.

qr_code

  • mendeley

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Views & Downloads

Browse