Meltdown Scenario Research Articles

Design of core catchers for sodium cooled FBRs – Challenges

The whole core meltdown scenario in sodium cooled fast reactors is considered under design extension criteria and has a very low probability of occurrence. To mitigate such a hypothetical severe accident in fast breeder reactors, a core catcher has been provided to accommodate the core debris within the primary containment boundary. In sodium cooled fast breeder reactor (FBR), an in-vessel core catcher, is provided to receive and disperse the fuel debris arising out of core meltdown during Hypothetical Core Disruptive Accident (HCDA). The core catcher prevents hot debris from reaching and damaging the main vessel. It also enables adequate heat transfer by natural convection to keep debris in stable conditions. The present study focuses on design of core catcher with sacrificial barriers and techniques to improve natural cooling of debris on the core catcher for FBRs. Studies are carried out towards the selection of suitable sacrificial material for core catcher, numerical analysis on multilayer and thermal-hydraulic analysis on multi jets core catcher concepts for selection of best suitable technique for debris bed coolability. The core catcher concept with sacrificial barrier and cooling pipes is found to possess enhanced coolability of debris bed. Experimental studies on compatibility of sacrificial ceramic material with sodium indicate strong dependence on microstructure. It has also been found that the multilayer core catcher with MgO as delay bed of 50 mm thickness is adequate to reduce the bottom temperature of core catcher below the design safety limit. Judicious mix of all these findings is expected to have unique design to accommodate whole core meltdown in future fast reactors.

Modelling guidelines for safety analysis of Station Black Out sequences based on experiments at the PKL test facility

After the Fukushima accident, “stress-test” activities carried out worldwide pointed out the need to study additional accident management measures to deal with prolonged Station Black Out (SBO) scenarios. Without any operator actions, a total loss of the secondary side heat sink leads to core uncovery, to core damage and ultimately to a melt-down scenario. The international NEA/OECD PKL-3 project has addressed the efficiency of possible accident management actions to re-establish core cooling by experiments at the PKL test facility. Since best estimate system codes were mainly developed to simulate LOCA scenarios, their performance and the general guidelines followed to simulate PWR power plants are called into question. In this paper, RELAP5 simulations of three SBO experiments are presented. An assessment of the code for the particular phenomenology in the experiments have been conducted. Specific guidelines on modelling and a list of the most important sources of uncertainties are provided.

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

The aim of this paper is to present a Lagrangian formulation for thermo-coupled fluid–structure interaction (FSI) problems and to show its applicability to the simulation of hypothetical scenarios of a nuclear core melt accident. During this emergency situation, an extremely hot and radioactive lava-like material, the corium, is generated by the melting of the fuel assembly. The corium may induce collapse of the nuclear reactor devices and, in the worst case, breach the reactor containment and escape into the environment. This work shows the capabilities of the proposed formulation to reproduce the structural failure mechanisms induced by the corium that may occur during a meltdown scenario. For this purpose, a monolithic method for FSI problems, the so-called Unified formulation, is here enhanced in order to account for the thermal field and to model phase change phenomena with the Particle Finite Element Method (PFEM). Several numerical examples are presented. First, the convergence of the thermo-coupled method and phase change algorithm is shown for two academic problems. Then, two complex simulations of hypothetical nuclear meltdown situations are studied in 2D as in 3D.

Integrated CFD investigation of heat transfer enhancement using multi-tray core catcher in SFR

To render future SFR more robust and safe, certain BDBE have been considered in the recent years. A Core Disruptive Accident leading to a whole core meltdown scenario has gained the interest of researchers. Various design concepts and safety measures have been suggested and incorporated in design to address such a low probability scenario. A core catcher concept, in particular, has proved to be inevitable as an in-vessel core retention device in SFR for safe retention of core debris arising out after the severe accident. This study aims to analyse the cooling capability of the innovative design concept of core catcher to remove decay heat of degraded core after the accident. First, the capability of single collection tray is established and then the study is extended to two and three collection trays with different design concepts. Transient forms of governing equations of mass, momentum and energy conservations along with k-ε turbulence model are solved by finite volume based CFD solver. Boussinesq approximation is invoked to model buoyancy in sodium. The study shows that a single collection tray is capable of removing up to 20MW decay heat load in a typical 500 MWe pool type SFR. Further, studies are carried out to improve the natural circulation of sodium around the source, in the lower plenum and to distribute core debris of the whole core to multiple collection trays. It is found that the double and triple collection trays can accommodate decay loads up to 29MW. Provision of openings in the collection trays has proved to be effective in improving the heat transfer and sodium flow as well as in distributing the core debris to the successive plates. Detailed analysis of isotherms and velocity field in the multiple collection trays with openings reveals improved natural circulation in and around the collection trays in the lower plenum. A three tray core catcher device with multiple openings can accommodate the decay heat of whole core meltdown, without exceeding the safe temperature limits of structural material. Special attention has been paid to the space constraint in the lower plenum while incorporating the three tray core catcher device.

Investigation on multilayer failure mechanism of RPV with a high temperature gradient from core meltdown scenario

The Fukushima accident shows that in-vessel retention (IVR) of molten core debris has not been appropriately assessed, and a certain pressure (up to 8.0MPa) still exists inside the reactor pressure vessel (RPV). In the traditional concept of IVR, the pressure is supposed to successfully be released, and the temperature distributed among the wall thickness is assumed to be uniform. However, this concept is seriously challenged by reality of Fukushima accident with regard to the existence of both internal pressure and high temperature gradient. Therefore, in order to make the IVR mitigation strategy succeed, the numerical investigation of the lower head behavior and its failure has been performed for several internal pressures under high temperature gradient. According to some requirements in severe accident (SA) management of RPV, it should be ensured that the IVR mitigation takes effect in preventing the failure of the structure within a period of 72h. Subsequently, the failure time and location have to be predicted under the critical heat flux (CHF) loading condition for lower head, since the CHF is limit thermal boundary before the melt-through of RPV. In illustrating the so called ‘multilayer failure mechanism’, the structural behaviors of RPV are analyzed in terms of the stress, creep strain, deformation, damage on selected paths.

Creep deformation and damage behavior of reactor pressure vessel under core meltdown scenario

The Fukushima accident shows that In-Vessel Retention (IVR) of molten core debris has not been appropriately assessed, and a certain pressure (upto 8.0 MPa) still exists inside the reactor pressure vessel (RPV). Generally, the pressure was supposed to be successfully released, and the externally cooled lower head wall mainly experienced the temperature difference which may be more than 1000 °C. Therefore, in order to make the IVR succeed, it is necessary to investigate the creep behavior and damage distribution of the RPV under complex thermal-mechanical loadings. Accordingly, considering the unlikely core meltdown scenario as a severe accident condition, the failure site and time have to be predicted for a safety assessment purpose. Due to heavy thick wall of the RPV, the high temperature gradient inevitably results in various failure modes, i.e., plastic failure and creep failure. In disclosing it, the finite element model (FEM) has been developed for simulating the failure processes and the visco-plastic behavior of vessel wall. In accounting for the most dangerous situation, the critical heat flux (CHF) loading is applied on the RPV as a limit boundary. Subsequently, some failure modes has been found as local melting, plastic damage and creep damage by FEM. Furthermore, it is found that the RPV failure varies with the internal pressures. For the situation of low pressure and high temperature, the failure dominated by creep takes place at the hot focus site. With the further increase of pressure, the thinnest wall thickness is weakest for maintaining the RPV integrity, the damage of which initiates at the outside of RPV by plasticity. Besides, the multiaxial state of stress is found to accelerate the damage evolution on the thinnest site, and the uniaxial failure criterion underestimate the damage accumulation.

Numerical simulation of passive heat removal under severe core meltdown scenario in a sodium cooled fast reactor

A sequence of highly unlikely events leading to significant meltdown of the Sodium cooled Fast Reactor (SFR) core can cause the failure of reactor vessel if the molten fuel debris settles at the bottom of the reactor main vessel. To prevent this, pool type SFRs are usually provided with an in-vessel core catcher above the bottom wall of the main vessel. The core catcher should collect, retain and passively cool these debris by facilitating decay heat removal by natural convection. In the present work, the heat removal capability of the existing single tray core catcher design has been evaluated numerically by analyzing the transient development of natural convection loops inside SFR pool. A 1-D heat diffusion model and a simplified 2-D axi-symmetric CFD model are developed for the same. Maximum temperature of the core catcher plate evaluated for different core meltdown scenarios using these models showed that there is much higher heat removal potential for single tray in-vessel SFR core catcher compared to the design basis case of melting of 7 subassemblies under total instantaneous blockage of a subassembly. The study also revealed that the side pool of cold sodium plays a significant role in decay heat removal. The maximum debris bed temperature attained during the initial hours of PAHR does not depend much on when the Decay Heat Exchanger (DHX) gets operational, and it substantiates the inherent safety of the system. The present study paves the way for better understanding of the thermal hydraulics of Post Accident Heat Removal (PAHR) towards extending the functionality of core catcher for handling Whole Core Meltdown Accident (WCA) scenarios in future SFRs.

Development of a phenomena identification ranking table for simulating a station blackout transient of a pressurized water reactor with a thermal-hydraulic integral effect test facility

KAERI (Korea Atomic Energy Research Institute) will launch an OECD/NEA project by utilizing a thermal-hydraulic integral effect test facility, ATLAS (Advanced Thermal-hydraulic Test Loop for Accident Simulation) from April, 2014. In the OECD-ATLAS project, design extension conditions (DECs) such as a station blackout (SBO) and a total loss of feed water (TLOFW) will be experimentally investigated to identify the major thermal-hydraulic phenomena of high risk multiple failure accidents considering magnified safety concerns after the Fukushima accident. A SBO is one of the most important DECs in that without any proper operator actions, a total loss of heat sink leads to core uncover, to core damage, and ultimately a core melt-down scenario under high pressure. Due to this safety importance, a SBO is considered to be a base test item of the OECD-ATLAS project.In this study, a phenomena identification ranking table (PIRT) has been developed for identifying the major parameters affecting the thermal-hydraulic phenomena in SBO transients. Development of a PIRT for the SBO transients is expected to contribute to making strategy for performing a thermal-hydraulic integral effect test with ATLAS which includes determination of proper test conditions and improvement of measurements. The primary safety criterion (PSC) was determined to be the core mixture level from the expert panel discussion. As for the major components of the nuclear power plant, the rank of the importance and the knowledge level were summarized for each of the three temporal phases. Taking into account the important phenomena from the PIRT result, it can be confirmed that ATLAS has a capability of simulating the major thermal-hydraulic phenomena during a SBO transient as realistically as possible. The present PIRT will be directly used to make strategy for performing a thermal-hydraulic integral effect test with ATLAS in the framework of the OECD-ATLAS project.

Feeding ecology and ecological impact of an alien ‘warm-water’ omnivore in cold lakes

The present study attempted to investigate the feeding ecology and ecological impact of Procambarus clarkii , the world's worst invasive crayfish and a recent invader in colder climates, by linking stomach-content analysis with an in situ enclosure experiment in lakes in southern Germany. The stomach-content analysis showed that P. clarkii is a polytrophic omnivore that feeds on macrophytes, detritus and macroinvertebrates. The trophic diversity of its diet was highest in mid-summer and in smaller crayfish. Chironomidae larvae and Dreissena polymorpha were the most preferred prey, whereas sediment-dwelling taxa were rarely consumed. The number of consumed small and agile prey negatively correlated with the crayfish size, suggesting an ontogenetic shift in diet. A five-week enclosure experiment was used to determine the impact of P. clarkii on the basal levels of a typical littoral food web of cold lakes at different crayfish densities (0, 2.5, and 5 crayfish m −2 ). The abundance of aquatic snails sharply decreased with increasing crayfish density and conditioned leaf breakdown was up to five times higher in the presence of crayfish than in the control treatment without crayfish. Crayfish also had a negative effect on macrophyte biomass, resulting from both consumption and uprooting. However, the impact mechanisms and outcomes differed among macrophyte species. In the crayfish treatments, the final biomass of the indigenous Myriophyllum spicatum and Chara sp. was significantly reduced relative to the initially stocked biomass, whereas the alien Elodea nuttallii was able to gain biomass. This finding is consistent with an invasional meltdown scenario, in that P. clarkii indirectly facilitated a dominance of E. nuttallii . Overall, the results concordantly suggest that P. clarkii is a keystone species that can profoundly alter recipient communities via direct trophic links and non-consumptive destruction, and may indirectly facilitate other invasive alien species.

Highbrow culture in Finland: Knowledge, taste and participation

This article explores the social distribution of involvement in highbrow culture in light of three issues being discussed in cultural sociology. One is that highbrow cultural orientation is an indicator of cultural capital or of social status. A second, the ‘meltdown scenario’, suggests that not only the popularity of highbrow activities, but also their distinctiveness, has decreased among younger cohorts in comparison to older cohorts. A third deals with the ‘feminization’ of highbrow culture. These issues are empirically addressed in contemporary Finland using nationally representative survey data. Highbrow culture is measured in three dimensions of cultural practices – knowledge, taste and participation – covering four different fields: music, literature, cinema and the visual arts. The results support all three arguments: First, education and occupational class are important social determinants of involvement in highbrow culture in Finland. Second, younger age cohorts show less interest in highbrow culture than do older Finns. Third, women tend to be more involved in highbrow culture than men. The results indicate considerable stability across the measures of highbrow culture and cultural fields. Social determinants of knowledge and cinema, however, are different from those in other dimensions and fields.

European buckthorn and Asian soybean aphid as components of an extensive invasional meltdown in North America

We consider the possibility of an extensive invasional meltdown occurring in central North America involving eleven Eurasian species. The scenario begins with the potential co-facilitation between the European earthworm Lumbricus terrestris and European buckthorn, Rhamnus cathartica. Once introduced, European buckthorn has served as the overwintering host for two important invasive crop pests, oat crown rust, Puccinea coronata and the soybean aphid, Aphis glycines. The spread of R. cathartica itself may have been aided by seed dispersal by the European starling, Sturnus vulgaris, and the presence of L. terrestris has likely facilitated the invasion of Bipalium adventitium, an Asian predatory flatworm that specializes on earthworms. Beyond this, the soybean aphid is consumed by a number of introduced species, including the lady beetle Harmonia axyridis, the ground beetle Agonum muelleri and the parasitoid Aphelinus certus. We hypothesize that the presence of soybean aphid increases regional abundances of these species. We discuss both the evidence for this multi-species invasional meltdown scenario and potential implications of meltdown dynamics for invasive species management. The particular management issues that we discuss are: (1) opportunities for managing multiple invasive species simultaneously by targeting facilitator species, and (2) implications of meltdown dynamics for biological control introductions against the soybean aphid.

Arts participation as cultural capital in the United States, 1982–2002: Signs of decline?

We analyzed Surveys of Public Participation in the Arts for 1982, 1992, and 2002 to see if trends in U.S. arts attendance are consistent with the perception of many sociologists of culture that the role of the arts as cultural capital is in decline. From a Bourdieuian perspective, a dramatic deflation in the value of the arts as cultural capital (the “meltdown scenario”) would manifest itself in (1) large declines in high-culture arts participation rates (2) especially among the youngest cohorts that (3) are not evident in participation in middlebrow activities and (4) are concentrated among groups for whom cultural capital is most important, i.e. (4a) highly educated people and (4b) women.) Results are mixed. Trend data are not consistent with the meltdown scenario, but do suggest change in the position of different arts genres within cultural capital and ongoing attrition in the audience for many of the arts. Consistent with the decline perspective, younger cohorts’ attendance rates have fallen for most high-culture performing-arts attendance activities. (Because college attendance increased in the 1960s, the decline is not visible in the middle cohorts until one disaggregates by education level.) In contrast to the decline perspective, however, declines are as bad or worse for several middlebrow cultural activities; attendance rates for art museums and jazz concerts have increased; and rates have declined more slowly for women and college graduates than for others. Two changes are evident: first, greater elite and general interest in the visual arts and jazz and less in classical music, ballet, and theatre (trends consistent with Peterson's “omnivore theory,” aspects of postmodern theory, and the notion that the content of cultural capital evolves over time); and, second, gradual decline among almost all age/gender/education groups in rates of attendance at live cultural events broadly defined, probably reflecting greater competition from at-home entertainment options and changes in population composition and family structure.