This paper describes the main research lines of the Karlsruhe Institute of Technology (KIT) relevant to Light Water Reactors (LWR) that permit both the safety evaluation of different reactor designs and the prediction of the radiological source term including its impact in the case of hypothetical severe accidents. The numerical tools for core analysis, nuclear power plant analysis of design basis and severe accidents, and the estimation of the radioactive dispersions of fission products after a core meltdown accident are described and selected results are discussed. The analytical investigations at KIT are complemented by experimental investigations at two facilities, namely the COSMOS-H/-L and the QUENCH facilities devoted to safety-relevant Thermal-Hydraulic (TH) phenomena and in-vessel severe accident early-phase for different kinds of fuel rods including Accident Tolerant Fuel (ATF). These data are crucial for the validation of the numerical tools used for safety demonstration. The paper is complemented by a brief description of the in-house tools under development for improved core/plant analysis based on the multi-physics/-scale coupling methodologies, where the in-house tools are combined with external system TH, Monte Carlo, and thermo-mechanic code to increase the prediction accuracy of the core/plant behaviour under stationary and accidental conditions. Selected results are presented and discussed. It can be stated that the long-lasting safety research activities at KIT devoted to LWR led to the development of a computation route combining external with in-house codes that is very much appropriate for safety evaluations and risk assessment; it includes unique multi-physics/-scale coupled tools for improved core and plant analysis of different reactor designs, e.g. PWR, VVER, BWR, SMR, and research reactors.

The improvement of the performance of the modern integral codes for carrying out severe accident analyses in nuclear power plants is one of the key elements of the Nuclear Safety Research program of the Karlsruhe Institute of Technology (KIT). This activity is performed in the frame of the participation of KIT in European projects and international cooperation's on the safety assessment of nuclear power plants and also aims at supporting decision making under high uncertainties for all emergency situations. The use of the Accident Source Term Evaluation Code (ASTEC), developed by the Institut de Radioprotection et de Surete Nucleaire, plays a central role in the KIT strategy on the research activities of severe accidents in LWRs. In the paper examples of the most recent applications of the ASTEC code at KIT in this research field are presented.

In 2019, the research activities of KIT in the field of experimental severe accident research on fast reactors were concentrated on the European ESFR-SMART project. Within this project two large-scale JIMEC (Jet Impingement of Metallic Core-Catcher) experiments have been performed to investigate the thermal ablation kinetics of an internal core catcher material in a SFR (Sodium-Cooled Fast Reactor). Besides this, the planning work for LIVEESFR tests to study the interaction between the corium simulant and the sacrificial simulant of the core catcher started. It has been decided to construct and build a new test vessel with downscaled geometries similar to SFR core catcher design. The actual safety design of a SFR in the case of a postulated severe accident includes removing the corium from the core by corium transfer tubes and collecting the corium in a core catcher in the lower head. One threat scenario of the core-catcher integrity is a high ablation rate by the impingement of a metallic corium jet on the core catcher surface. Experimental data is needed to simulate this ablation behaviour under prototypical conditions including a long impinging duration and high jet temperature. Under such conditions, the insights of a yet insufficiently studied behaviour when a molten pool is created ("pool effect") at the impact point could be obtained. Therefore, the ITES-SAR (Institute for Thermal Energy Technologies and Safety - Severe Accident Research Group) team has adapted the existing MOCKA test facility to perform two JIMEC experiments in the frame of the European ESFR-SMART project. JIMEC-1 and JIMEC-2 tests are two large-scale tests with 1 ton of metallic mass. The objectives of the experiments are to deliver experimental data on the interaction of melt jet parameter and erosion dynamics in prototypical materials and conditions. The melt jet parameters such as jet temperature, jet velocity and jet diameter, and the erosion dynamics including the erosion velocity and the timing of pool effect are obtained. The experimental results will be used for developing new correlations which could be used in codes for simulation of the ablation kinetics for SFR core catcher concepts.

As part of the European Sustainable Nuclear Industrial Initiative European (ESNII), several innovative reactor concepts are being developed, with a roadmap for deployment of demonstrators by 2025. The Karlsruhe Institute of Technology (KIT) participates in the safety assessment of such concepts in the framework of the research program Nuclear Waste Management, Safety and Radiation Research (NUSAFE) and in close collaboration with European partners, e.g. in projects supported by Horizon 2020. This work focuses on experimental investigations on heavy-liquid metal technology performed recently at KIT, relevant for the safety assessment of the MYRRHA and ALFRED reactors, planned for construction in Belgium and Romania, respectively. The experimental facilities available at the heavy-liquid metal laboratories of KIT are suitable for prototypical tests using molten lead or lead alloys at operating conditions (temperature, flow and power) representative of those expected in the reactors. An overview of selected recent results is presented for two main disciplines, namely thermal-hydraulics and materials sciences. Moreover, an outlook on the research topics to be analyzed in the next years is presented.

The paper presents some results of the ISTC (International Science and Technology Center)-financed project 'Investigation of Corium Melt Interaction with NPP Reactor Vessel Steel' (METCOR). In the METCOR experiments the metallic phase of a two-liquid system was produced by the interaction between hot suboxidized corium and cooled VVER vessel steel, with the steel being corroded. Models of corrosion mechanisms in the considered conditions are used to systematize data on the limiting temperature of corrosion/(dissolution) of the vessel steel. A considerable influence of thermal gradient conditions is shown, which has to be taken into account in the analysis of molten pool behaviour.

Abstract The reactor safety research of the Helmholtz Association is an integral part of the national provident research. It focuses on the safety of nuclear power plants in Germany and abroad as well as on safety aspects of internationally developed innovative reactor concepts. The research in the three involved Helmholtz centers Forschungszentrum Jülich, Helmholtz-Zentrum Dresden-Rossendorf and Karlsruhe Institute of Technology covers important areas of design basis and beyond design basis accidents. A unique combination of code and model development supported by own large-scale experiments ensures the active preservation of the reactor safety competences. The research that is embedded in a strong international co-operation will be continued after the completion of the national phase-out from the use of nuclear energy for electricity production in 2022.

Karlsruhe Institute of Technology (KIT) is a resear ch university within the Helmholtz Association (HGF) with about 9.500 employees, and 2 5.000 students. It bundles the missions of a university of the state of Baden-Wuerttemberg and of a large-scale research institution of the Helmholtz Association. Within these missions, K IT is operating along the three strategic fields of action of research, teaching, and innovat i n, including energy. KIT is devoted to top research and excellent academic education as well a s to being a prominent location of academic life, life-long learning, comprehensive ad vanced training, exchange of know-how, and sustainable innovation culture. In the energy b ranch, the Nuclear Waste Management, Safety and Radiation Research program (NUSAFE) has a long tradition, is widely recognized and represents an integral part of national provide nt r search providing core competences on the internationally highest level of science and te chnology regarding nuclear safety and waste management research. This paper will present and di scuss the activities devoted to preserve nuclear knowledge and to foster the education in ke y-ar as important to solve the challenges in nuclear waste management, reactor safety, and ra diation protection and decommissioning of nuclear power plants. In addition, the courses o ffered by the AREVA nuclear professional school at KIT and the activities and KIT-infrastruc t re consisting of unique experimental facilities will be discussed to explore possible co -operations with Latin America. Examples of developed courses (e.g. online courses) in the fram e of the KIT-participation on European projects devoted to E&T e.g. GENTLE, ANNETTE will b e given e.g. under development. Finally, possibilities for co-operations of KIT wit h Latin America universities will be outlined.

This paper describes the KIT numerical simulation tools under extension and validation for the analysis of design and beyond design basis accidents (DBA) of Light Water Reactors (LWR). The description of the complex thermal hydraulic, neutron kinetics and chemo-physical phenomena going on during off-normal conditions requires the development of multi-physics and multi-scale simulations tools which are fostered by the rapid increase in computer power nowadays. The KIT numerical tools for DBA and beyond DBA are validated using experimental data of KIT or from abroad. The developments, extensions, coupling approaches and validation work performed at KIT are shortly outlined and discussed in this paper.

The research activities in the light water reactor (LWR) severe accidents domain at Karlsruhe Institute of Technology (KIT) are concentrated on the inand ex-vessel core melt behavior. The overall objective is to investigate the core melt scenarios from the beginning of core degradation to melt formation and relocation in the vessel, possible melt dispersion to the reactor cavity and to the containment, corium concrete interaction and corium coolability in the reactor cavity, and hydrogen behaviour in reactor systems. The results of the experiments contribute to a better understanding of the core melt sequences and thus improve safety of existing and, in the long-term, of future reactors by severe accident mitigation measures and by safety installations where required. This overview paper describes the experimental facilities used at KIT for severe accident research and gives an overview of the main directions and objectives of the R&D work.

The Fukushima incident in March 2011 caused worldwide a change in the perception of nuclear energy generation. Independent from the decision made by individual nations regarding the future use of nuclear energy for electricity generation, the number of nuclear power plants (NPP) operated worldwide has hardly changed. Essential reasons are mainly rising feedstock prices, increased energy demands and the simultaneous aspiration to reduce substantially the CO2emission by fossil fuels. Especially emerging Asian economies are forced to an aggressive exploitation of all electricity generating technologies including nuclear to match their societal and economic demands. Nevertheless, the Fukushima accident initiated worldwide a new quality in the safety assessment and safety culture by considering additional man made or natural disasters. This process is reflected in enhanced bilateral or international co-operations. One of the most striking consequences is that a safe NPP operation demands a continuous retrofitting and evaluation of the plant behavior based on the current state of science and technology, which is part of the German safety practice since the Three-Mile-Island (TMI) incident. Within this article different new nuclear plant developments with enhanced safety features are presented. Although these concepts as well as their deployment options diverge considerably in design and operational strategy the major nuclear protection goals in terms of confinement, coolability and reactivity control, which have to be met by any plant design, remain the same. Regarding the operational safety increased computational capabilities allow by means of coupled multiphysics and multi-scale method to identify design weaknesses down to the pin scale of a fuel assembly both for steady state and also for plant transients. To master severe accidents the different plant concepts, however, yield to a considerably larger diversity of technical solutions, nearly all of which are based on passive systems that exploit the physical natural laws. A sustainable use of nuclear fuel avoiding large scale deep underground repositories inherently implies a closed fuel cycle and the deployment of fast spectrum reactors, so-called Generation –IV reactors, for which similar nuclear postulations in terms of safety on all levels have to be demonstrated. Within the article for both operational safety and severe accident measures examples are presented to illustrate the main functionality and operational principle. 1 Present status of nuclear electricity generation – observations worldwide and in Europe At present 435 commercial nuclear reactors (NPP) are operating and almost 2/3 ́s of the 72 plants under construction are erected in Asia [1]. More than 75% of the existing reactor fleet is light water reactors and about 85% of the new built belong to the class of pressurized water reactors (PWR). All commercially operated NPP ́s produced in 2013 nearly 11.5% of the global electricity production, which is only slightly less than in the previous years. These commercial plants are complemented by approximately 240 research reactors operated in 56 countries and currently nearly 180 civil nuclear powered ships. Remarkable is that the countries engaged in new built or strongly envisaging the use of nuclear power as a “nuclear newcomer” belongs either to Eastern Europe or to Asia and the motivation to use nuclear power is mainly triggered by their societal decision to rely to a large quantity on industrial production as one major pillar of economic developmentor simply as source of future wealth. The specific reasons of those societies range from vast economic development and rapidly rising electricity consumption, grid independence, fuel independence

Experimental, theoretical and numerical studies of oxidation kinetics of an open surface corium pool have been reported. The experiments have been carried out within OECD MASCA program and ISTC METCOR, METCOR-P and EVAN projects. It has been shown that the melt oxidation is controlled by an oxidant supply to the melt free surface from the atmosphere, not by the reducer supply from the melt. The project experiments have not detected any input of the zirconium oxidation kinetics into the process chemistry. The completed analysis puts forward a simple analytical model, which gives an explanation of the main features of melt oxidation process. The numerical modeling results are in good agreement with experimental data and theoretical considerations. (C) 2013 Elsevier B.V. All rights reserved.

Forty-three organisations from 22 countries network their capacities of research in SARNET (Severe Accident Research NETwork of excellence) to resolve the most important remaining uncertainties and safety issues on severe accidents in existing and future water-cooled nuclear power plants (NPP). After a first project in the 6th Framework Programme (FP6) of the European Commission, the SARNET2 project, coordinated by IRSN, started in April 2009 for 4 years in the FP7 frame. After 2,5 years, some main outcomes of joint research (modelling and experiments) by the network members on the highest priority issues are presented: in-vessel degraded core coolability, molten-corium-concrete-interaction, containment phenomena (water spray, hydrogen combustion…), source term issues (mainly iodine behaviour). The ASTEC integral computer code, jointly developed by IRSN and GRS to predict the NPP SA behaviour, capitalizes in terms of models the knowledge produced in the network: a few validation results are presented. For dissemination of knowledge, an educational 1-week course was organized for young researchers or students in January 2011, and a two-day course is planned mid-2012 for senior staff. Mobility of young researchers or students between the European partners is being promoted. The ERMSAR conference is becoming the major worldwide conference on SA research.

The motivation of the work performed within the work package "Corium and Debris Coolability" of the Severe Accident Research Network of Excellence (SARNET) is to reduce or possibly solve the remaining uncertainties on the efficiency of cooling reactor core structures and materials during severe accidents, either in the core, in the vessel lower head or in the reactor cavity, so as to limit the progression of the accident. This can be achieved either by ensuring corium retention within the reactor pressure vessel or at least by limiting the corium progression and the rate of corium release into the cavity. These issues are to be covered within the scope of accident management for existing reactors and within the scope of design and safety evaluation of future reactors. The specific objectives are to create and enhance the database on debris formation, debris coolability and corium behavior in the lower head, to develop and validate the models and computer codes for simulation of in-vessel debris bed and melt pool behavior, to perform reactor scale analysis for in-vessel corium coolability and to assess the influence of severe accident management measures on in-vessel coolability. The work being performed within this work package comprises experimental and modeling activities with strong cross coupling between the tasks. Substantial knowledge and understanding of governing phenomena concerning coolability of intact rod-like reactor core geometry was obtained in previous projects. Hence the main thrust of experimental and modeling efforts concentrates mainly on the study of formation and cooling of debris beds in order to demonstrate effective cooling modes, cooling rates and coolability limits. Modeling efforts have been aimed at assessing and validating the models in system-level and detailed codes for core degradation, oxidation and debris behavior. The paper describes the work performed up to now and summarizes the main results achieved so far.

After the events at the Japanese nuclear power plant of Fukushima Daiichi, the German federal government decided that Germany will give up electricity generation from nuclear power within a decade. The last reactor will be disconnected from the power grid in 2022. Helping to make this opt-out as safe as possible is one of the duties of the Helmholtz Association with its Nuclear Safety Research Program within the Energy Research Area. Also the demolition of nuclear power plants and the repository problem will keep society, and thus also research, busy for a number of decades to come. Giving up electricity production from nuclear power thus must not mean giving up the required nuclear technology competences. In the fields of reactor safety, demolition, final storage, radiation protection, and crisis management, in critical support of international developments, and for competent evaluation of nuclear facilities around Germany, these competences will be in demand far beyond the German opt-out. This is the reason why the final report by the Ethics Committee on "Safe Energy Supply" emphasizes the importance of nuclear technology research. Close cooperation on national, European and international levels is indispensable in this effort. Also nuclear safety research in the Helmholtz Association is aligned with the challenges posed by the opt-out of the use of nuclear power. It is important that the high competences in the areas of plant safety and demolition, handling of radioactive waste, and safe final storage as well as radiation protection be preserved. The Nuclear Safety Research Program within the Energy Research Area of the Helmholtz Association therefore will continue studying scientific and technical aspects of the safety of nuclear reactors and the safety of nuclear waste management. These research activities are provident research conducted for society and must be preserved for a long period of time. The work is closely harmonized with the activities of the partners in the Nuclear Competence Association. As of January 2011, the Dresden-Rossendorf Helmholtz Center (HZDR), with its 2 Institutes of Safety Research and for Radiochemistry, is an integral part of the Nuclear Safety Research Program within the Energy Research Area. Both institutes work on topics of safety research for nuclear reactors and safety research for nuclear waste management. In this way, the 2 institutes represent very welcome added value as well as a supplement to the Nuclear Safety Research Program.

Research and development activities in the fields of safety of nuclear reactors and safety of nuclear waste management in Germany are carried out by research centers and some 32 universities. In addition, there is industrial research by vendors and research for operational safety and plant safety of nuclear power plants in operation, and by technical-scientific research and expert consultant organizations. The Nuclear Safety Research Program within the Energy Research Area of the Helmholtz Association studies scientific and technical aspects of the safety of nuclear reactors and safety in nuclear waste management. These provident research activities are conducted for society and, for this reason, are long term by nature. The work is closely harmonized with the activities of partners in the Nuclear Technology Competence Group. Effective January 2011, also the Dresden-Rossendorf Helmholtz Center (HZDR) with its 2 institutes, the Institute for Safety Research and the Institute for Radiochemistry, has been integrated into the Nuclear Safety Research Program as part of the Energy Research Area. The 2 institutes will handle subjects of safety research for nuclear reactors and safety research for nuclear waste management. In this way, the 2 institutes generate a most welcome added value supplementing the Nuclear Safety Research Program.

The objectives of the SARNET network are to define common research programmes in the field of severe accidents and to develop common computer tools and methodologies for safety assessment in this field. To reach these objectives, one of the work packages, named "Severe Accident Research Priorities" (SARP), aimed at reviewing and reassessing the priorities of research issues as a basis to harmonize and to re-orient research programmes, to define new ones, and to close - if possible - resolved issues on a common basis. The work was performed in close collaboration with 8 participating institutions, led by GRS, representing technical safety organisations, industry and utilities (IRSN, CEA, EDF, FZK, GRS, KTH, TUS, VTT). This action made use notably of (1) the outcomes of the EURSAFE project in the 5th Framework Programme, i.e. the Phenomena Identification and Ranking Tables (PIRT) on severe accidents, (2) the results of the validation and benchmarking activities on ASTEC, (3) the results of reactor calculations carried out in the other SARNET tasks, and (4) the outcome of the research performed in the three thematic sub-domains of SARNET (corium, containment and source term).The main outcome of EURSAFE was a list of 21 topics which included recommendations for experimental programmes and code developments. This list formed the basis of the work in SARP. Also the methodology applied in EURSAFE to consider both the risk potential and the severe accident issues where large uncertainties still subsist was adopted. The analyses of the progress of research and development activities considered whether (1) any research issue was resolved due to reduction of uncertainties or gain of scientific insights, (2) any new issue had to be added to the list of needed research, (3) any new process or phenomenon had to be included in the general PIRT list taking into account the safety relevance and the lack of knowledge, and (4) any new accident management program has to be developed to cope with unresolved problems. Furthermore a strategy plan was elaborated to ensure a wide consensus with the end-user requirements and the objectives of SARNET research activities. (C) 2009 Published by Elsevier Ltd.