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Title: Limiting Factors for External Reactor Vessel Cooling

Abstract

The method of external reactor vessel cooling (ERVC) that involves flooding of the reactor cavity during a severe accident has been considered a viable means for in-vessel retention (IVR). For high-power reactors, however, there are some limiting factors that might adversely affect the feasibility of using ERVC as a means for IVR. In this paper, the key limiting factors for ERVC have been identified and critically discussed. These factors include the choking limit for steam venting (CLSV) through the bottleneck of the vessel/insulation structure, the critical heat flux (CHF) for downward-facing boiling on the vessel outer surface, and the two-phase flow instabilities in the natural circulation loop within the flooded cavity. To enhance ERVC, it is necessary to eliminate or relax these limiting factors. Accordingly, methods to enhance ERVC and thus improve margins for IVR have been proposed and demonstrated, using the APR1400 as an example. The strategy is based on using two distinctly different methods to enhance ERVC. One involves the use of an enhanced vessel/insulation design to facilitate steam venting through the bottleneck of the annular channel. The other involves the use of an appropriate vessel coating to promote downward-facing boiling. It is found that the use ofmore » an enhanced vessel/insulation design with bottleneck enlargement could greatly facilitate the process of steam venting through the bottleneck region as well as streamline the resulting two-phase motions in the annular channel. By selecting a suitable enhanced vessel/insulation design, not only the CLSV but also the CHF limits could be significantly increased. In addition, the problem associated with two-phase flow instabilities and flow-induced mechanical vibration could be minimized. It is also found that the use of vessel coatings made of microporous metallic layers could greatly facilitate downward-facing boiling on the vessel outer surface. With vessel coatings, the local CHF limits at different angular locations of the vessel outer surface could be enhanced by {approx}1.2 to 2 times the CHF compared with a plain vessel without coatings. The CHF enhancement could be attributed to the structure of the porous coating itself and the capillary action it induced. The matrix of cavities and voids within the coating effectively trap vapor, which serve as active nucleation sites. These sites in turn are fed with liquid flowing through the interconnected channels. The pores on the surface of the porous coating serve as flow inlets for liquid supply to the heating surface, leading to appreciable enhancement in downward-facing boiling heat transfer and the local CHF limits. Results of the present study suggest that by utilizing an enhanced vessel/insulation design with vessel coating, it is possible to significantly enhance the CLSV and the CHF limits as well as minimize the two-phase flow instability problems, thus substantially increasing the margin for IVR.« less

Authors:
 [1]
  1. Pennsylvania State University (United States)
Publication Date:
OSTI Identifier:
20840317
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nuclear Technology; Journal Volume: 152; Journal Issue: 2; Other Information: Copyright (c) 2006 American Nuclear Society (ANS), United States, All rights reserved. http://epubs.ans.org/; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS; BOILING; COOLING; CRITICAL HEAT FLUX; DESIGN; INSTABILITY; LIQUIDS; NATURAL CONVECTION; POROUS MATERIALS; POWER REACTORS; REACTOR VESSELS; STEAM; SURFACES; TWO-PHASE FLOW; VAPORS

Citation Formats

Cheung, F.B.. Limiting Factors for External Reactor Vessel Cooling. United States: N. p., 2005. Web.
Cheung, F.B.. Limiting Factors for External Reactor Vessel Cooling. United States.
Cheung, F.B.. Tue . "Limiting Factors for External Reactor Vessel Cooling". United States. doi:.
@article{osti_20840317,
title = {Limiting Factors for External Reactor Vessel Cooling},
author = {Cheung, F.B.},
abstractNote = {The method of external reactor vessel cooling (ERVC) that involves flooding of the reactor cavity during a severe accident has been considered a viable means for in-vessel retention (IVR). For high-power reactors, however, there are some limiting factors that might adversely affect the feasibility of using ERVC as a means for IVR. In this paper, the key limiting factors for ERVC have been identified and critically discussed. These factors include the choking limit for steam venting (CLSV) through the bottleneck of the vessel/insulation structure, the critical heat flux (CHF) for downward-facing boiling on the vessel outer surface, and the two-phase flow instabilities in the natural circulation loop within the flooded cavity. To enhance ERVC, it is necessary to eliminate or relax these limiting factors. Accordingly, methods to enhance ERVC and thus improve margins for IVR have been proposed and demonstrated, using the APR1400 as an example. The strategy is based on using two distinctly different methods to enhance ERVC. One involves the use of an enhanced vessel/insulation design to facilitate steam venting through the bottleneck of the annular channel. The other involves the use of an appropriate vessel coating to promote downward-facing boiling. It is found that the use of an enhanced vessel/insulation design with bottleneck enlargement could greatly facilitate the process of steam venting through the bottleneck region as well as streamline the resulting two-phase motions in the annular channel. By selecting a suitable enhanced vessel/insulation design, not only the CLSV but also the CHF limits could be significantly increased. In addition, the problem associated with two-phase flow instabilities and flow-induced mechanical vibration could be minimized. It is also found that the use of vessel coatings made of microporous metallic layers could greatly facilitate downward-facing boiling on the vessel outer surface. With vessel coatings, the local CHF limits at different angular locations of the vessel outer surface could be enhanced by {approx}1.2 to 2 times the CHF compared with a plain vessel without coatings. The CHF enhancement could be attributed to the structure of the porous coating itself and the capillary action it induced. The matrix of cavities and voids within the coating effectively trap vapor, which serve as active nucleation sites. These sites in turn are fed with liquid flowing through the interconnected channels. The pores on the surface of the porous coating serve as flow inlets for liquid supply to the heating surface, leading to appreciable enhancement in downward-facing boiling heat transfer and the local CHF limits. Results of the present study suggest that by utilizing an enhanced vessel/insulation design with vessel coating, it is possible to significantly enhance the CLSV and the CHF limits as well as minimize the two-phase flow instability problems, thus substantially increasing the margin for IVR.},
doi = {},
journal = {Nuclear Technology},
number = 2,
volume = 152,
place = {United States},
year = {Tue Nov 15 00:00:00 EST 2005},
month = {Tue Nov 15 00:00:00 EST 2005}
}
  • If there were inadequate cooling during a reactor accident, a significant amount of core material could become molten and relocate to the lower head of the reactor vessel, as happened in the Three Mile Island Unit 2 (TMI-2) accident. If it is possible to ensure that the vessel lower head remains intact so that relocated core materials are retained within the vessel, the enhanced safety associated with these plants can reduce concerns about containment failure and associated risk. For example, the enhanced safety of the Westinghouse Advanced 600 MWe Pressurized Water Reactor (AP600), which relied upon External Reactor Vessel Coolingmore » (ERVC) for in-vessel retention (IVR), resulted in the United States Nuclear Regulatory Commission (US NRC) approving the design without requiring certain conventional features common to existing Light Water Reactors (LWRs). Accordingly, IVR of core melt is a key severe accident management strategy adopted by some operating nuclear power plants and proposed for some advanced light water reactors. However, it is not clear that currently-proposed methods to achieve ERVC will provide sufficient heat removal for higher power reactors. A US–Korean International Nuclear Energy Research Initiative (INERI) project has been initiated in which the Idaho National Engineering and Environmental Laboratory (INEEL), Seoul National University (SNU), Pennsylvania State University (PSU), and the Korea Atomic Energy Research Institute (KAERI) will determine if IVR is feasible for reactors up to 1500 MWe. This paper summarizes results from the first year of this 3-year project.« less
  • Experimental and analytical studies on the penetration integrity of the reactor vessel have been performed to investigate the potential for reactor vessel failure during a severe accident in the Advanced Power Reactor 1400. Six tests have been performed to analyze the effects of the annulus water between the in-core instrumentation nozzle and the thimble tube, external vessel cooling, in-vessel pressure, melt mass, and melt flow for the maintenance of penetration integrity using alumina (Al{sub 2}O{sub 3}) melt as a simulant. The experimental results have been evaluated using the Lower head IntegraL Analysis computer Code (LILAC) and the Modified Bulk Freezingmore » (MBF) model. The test results have shown that the water inside the annulus is very effective in the maintenance of the reactor vessel's penetration integrity because the water prevents the melt from ejection through penetration. The penetration in the no external vessel cooling case has more damage than that in the external vessel cooling case. An increase in in-vessel pressure from 1.0 to 1.5 MPa did not create penetration damage, but an increase in melt mass from 40 to 60 kg and melt flow due to the vessel geometry significantly increased the amount of penetration damage. The analytical results using the LILAC computer code and the MBF model are very similar to the experimental results for the ablation depth of the weld and the melt penetration distance through the annulus, respectively.« less
  • Experimental and analytical studies on the thermal behavior of reactor vessel penetration have been performed under external vessel cooling during a severe accident in the Korean next-generation reactor APR1400. Two types of tests, SUS-EXT and SUS-DRY with and without external vessel cooling, respectively, have been performed using sustained heating by an induction heater. Three tests have been carried out varying the cooling conditions at the vessel outer surface in the SUS-EXT tests. The experimental results have been thermally estimated using the LILAC computer code. The experimental results indicate that the inner surface of the vessel was ablated by the 45-mmmore » thickness in the SUS-DRY test. Despite the total ablation of the welding material, the penetration was not ejected outside the vessel, which could be attributed to the thermal expansion of the penetration. Unlike the SUS-DRY test, the thickness of the ablation was {approx}15 to 20 mm at most, so the welding was preserved in the SUS-EXT tests. It is concluded from the experimental results that the external vessel cooling highly affected the ablation configuration and the thermal behaviors of the vessel and the penetration. An increase in coolant mass flow rate from 0.047 to 0.152 kg/s had effects on the thermal behavior of the lower head vessel and penetration in the SUS-EXT tests. The LILAC analytical results on temperature distribution and ablation depth in the lower head vessel and penetration were very similar to the experimental results.« less
  • Flooding of the drywell of a boiling water reactor (BWR) is one of many accident management strategies being proposed to manage severe accidents in light water reactors. The effect of external cooling on the thermal behavior of the BWR vessel lower head containing molten core material has been numerically investigated using a two-dimensional implicit finite difference scheme. Results have been obtained for the vessel shell temperature, the molten pool temperature, and the crust thickness for steady-state conditions. For each equilibrium state, the thermal behavior of the vessel lower head has been investigated by parametrically changing the emissivity of the poolmore » free surface, the vessel wall, the baffle plate, the core shroud, and the upper structure; and the temperature of the upper structure. For a certain set of parameters, nucleate boiling on the outer surface of the vessel wall is found to be effective in lowering the temperature of the inner wall of the vessel below the melting temperature of the steel. For most cases, failure of the BWR baffle plate would occur.« less
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