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[en] Highlights: • We simulate the possibility of corium material in the lower head with CFD code. • The thermal hydraulics in the oxidic and metal phase is modeled and validated. • The integrity of crust may be maintained to separate oxidic and metal materials. • Material separation indicates 2-layer corium pool in the lower head of AP1000. - Abstract: MASCA experiment indicated the possibility of oxidic and metallic material interaction and the following analysis produced a possible 3-layer configuration in the lower head. However, 3-layer corium pool is valid only under the circumstance that materials are well mixed. This assumption has not been studied thoroughly in IVR analysis. In this paper, a CFD code is used to predict the possibility of material mixing based on the MAAP4 results and conservative assumptions. The analysis indicates that the validated LES model and a grid of y+ < 10 can well predict thermal hydraulics in the oxide and metal phase, thus producing boundary conditions for the crust integrity analysis. The stress in the crust is calculated by ANSYS code to determine the integrity of crust by comparison with MACE experiments. The results show that the crust is likely to maintain its integrity and keep the oxidic and metallic materials separated, thus restricting the material interaction. This study may support a 2-layer corium pool configuration in the lower head
[en] PROCOR stands for PROpagation of CORium: its objective is to simulate the transient of severe accidents phenomena. – Only propagation of corium and its interactions are modeled (no Fission Products or H2 release); – A software platform composed by a kernel, a library of models, and different reactor applications; – A PROCOR application is an assembly of models of the library that use the services of the kernel; – PROCOR is composed of two parts: → a physical part (models and applications): to simulate corium propagation (deterministic part); → a statistical part (based on URANIE (CEA), Monte Carlo method): to perform studies on the physical part; – PROCOR has an associated work cycle in order to enhance the modelling
[en] In-vessel retention of molten core debris (IVR) through the Extemal Reactor Vessel Cooling (ERVC) is an important severe accident management feature employed in the 300 MW PWR plant. In the previous IVR analysis, it is usually assumed that there is a two-layer configuration (metal-over oxide) of molten core material in the RPV lower head, however, an alternate three-layer molten pool configuration with a dense bottom metal layer below the oxide layer can be formed due to molten material interactions in the lower plenum, and it may bring a greater threat to the RPV integrity. Based on the established model, IVR analysis is performed for the assumed three-layer molten pool configuration for 300 MW PWR plant, and the results show that the dense bottom metal layer will not threaten the RPV integrity, but the thin top metal layer has a small margin-to-failure, which may threaten the RPV integrity. (authors)
[en] Background: In Loss of Coolant Accidents (LOCAs), jet impact of fluid from a high energy pipe could damage insulation materials, etc. near the break into debris, which would be transported along with fluid in containment and finally deposit on sump screen to form a debris bed, impeding normal operation of Emergency Core Cooling System (ECCS). A part of the debris may bypass the screen and flow into the reactor pressure vessel, resulting in a series of effects. The issue is referred as GSI-191 (Generic Safety Issue 191) problem. Purpose: In order to solve this issue, quantity of debris generated near the break should be determined firstly. Methods: One calculation tool for jet impact Zone of Influence (ZOI) was developed independently based on ANSI/ANS 58.2-1988 standard and equivalent volume sphere model. Results: The jet outline and isobar of the Nuclear Energy Institute (NEI) test case obtained by the tool agreed well with the results of U.S NRC (United States Nuclear Regulatory Commission) and thus the duplication of the NEI test case was realized successfully. Furthermore, sensitivity analysis under different stagnation conditions was performed. Analysis results showed that, under the same stagnation pressure, increase of fluid temperature will result in the decrease of the ZOI destruction radius and debris quantity. Conclusion: So when jet impact test is performed to obtain the ZOI destruction radius, double ended guillotine break of cold leg should be selected conservatively as the limiting condition to maximum the debris quantity generated during jet impact process. (authors)
[en] There are many uncertainties in severe accident phenomena which affect the effectiveness of External Reactor Vessel Cooling (ERVC) measurement. The uncertainties of In-Vessel Retention (IVR) issue are analyzed based on Risk-Oriented Accident Analysis method, and the probability distribution of ERVC effectiveness is obtained. The calculation results using VTA sampling program show that if the melt pool in lower head achieves final bounding state, the success probability of core debris IVR is over 99% for 8 kinds of severe accident sequences, supposing a postulated severe accident occurs in Chashma-2 Nuclear power plant and ERVC is performed successfully. (authors)
[en] Conclusion: • IVR strategy is used to contain the core debris because it is highly compatiblewith CAP1400 design features. • Some uncertainties, such as complex physico-chemical phenomena still exist. However, the design features and measures taken for CAP1400 to prevent large heat flux to RPV make IVR strategy robust. • IVR efficiency is demonstrated by decomposition event tree supported bydetailed analyses. • IVR concept is extended as retaining the molten debris in the RPV byexternally cooling water in the cavity and/or in-vessel injection.
[en] When the molten core debris relocates to the lower plenum of the Reactor Pressurized Vessel (RPV) during a severe accident, penetrations may fail due to the thermal attack from the molten core debris, then the RPV loses its integrity, and molten core debris releases into the reactor cavity, resulting in the failure of In Vessel Retention (IVR). Based on the two kinds of penetration failure modes, i.e. penetration ejection and debris entering the penetration channel, the ablation of weld and friction force due to thermal expansion are calculated by VTA code, and flowing distance of debris along the penetration channel are analyzed with MBF model respectively. The results demonstrate that 300 MW PWR plant RPV lower head will not lose its integrity due to penetration failure, and the molten core debris will not relocate to the reactor cavity through the failed penetrations if the Extemal Reactor Vessel Cooling (ERVC) is performed. (authors)
[en] Conclusion: IVR is one of important severe accident management strategies of CAP1400. The purpose of IVR-ERVC experiments is to obtain CHF at RPV lower head and research its relevant mechanism. IVR-ERVC experiment facility was designed and built with a series of improvements. Insights achieved in IVR-ERVC experiments contribute to IVR evaluation, design improvement and safety review of CAP1400.
[en] In vessel retention (IVR) is an important severe accident mitigation strategy. Currently some study is based on two-layer melt pool configuration assumption. However, MASCA experiment indicates that in some case heavier metallic layer containing dissolved uranium may be formed beneath traditional two-layer configuration. For three-layer configuration, heat flux distribution might be changed. Thus three-layer configuration needs to be analyzed. However, some of current code is not capable to simulate this three-layer configuration. While others proposed three-layer models are different from each other. In order to perform IVR analysis based on three-layer configuration, a new modular IVR analysis code is developed and verified based on C++. Results show that the developed code has a good agreement with the data from literature. Thus the developed code is applied to perform IVR analysis based on three-layer configuration. Based on simulation results, compared to two-layer configuration, heat flux at the bottom of the vessel and light metallic layer are notably increased in three-layer configuration. According to sensitivity analysis, within the scope of this study, CHF ratio remains below unity in all the cases. Besides uranium oxidation rate and total mass of stainless steel have larger influence on heat flux distribution and maximum CHF ratio. (author)
[en] Highlights: • The paper presents a summary of effort made for the employment of IVR to CAP1400. • Some design improvements are carried out upon the results of IVR-relevant phenomenology. • A method of Decomposition Event Tree (DET) is developed to evaluate and quantify IVR strategy in CAP1400. - Abstract: In-Vessel Retention (IVR), which arrests relocated molten core materials in the vessel during severe accident, is an appealing accident management approach to many newly-designed reactors. It is implemented in CAP1400 because it’s highly compatible with CAP1400 design philosophy. Extensive studies of relevant phenomena are carried out to investigate the possible effect on IVR strategy, which include core melting and relocation, in-vessel steam explosion, corium material interaction, heat flux to RPV (Reactor Pressure Vessel) wall under the assumption of different corium pool configuration, ex-vessel CHF (Critical Heat Flux) test, RPV structural analysis and so on. For those that may have negative impact on IVR success, the corresponding design improvements are conducted to aid and facilitate the employment of IVR, such as lowering core support plate, increasing the mass of core internals, optimizing ex-vessel insulator and so on. The benefit of taking decay heat from in to vessel cooling is also achieved by reflooding breaks or injecting RCS (Reactor Coolant System) under the instructions of SAMG (Severe Accident Mitigation Guideline). All the IVR-related phenomena and operator actions are reflected and linked together by IVR decomposition event tree (DET) to evaluate CAP1400 IVR strategy in a comprehensive manner. There is reasonable assurance that the IVR strategy implemented in CAP1400 is successful. About 93% of core damage sequences can be terminated by retaining corium in the vessel.