Sustainable integration of EU research in severe accident phenomenology and management

In order to optimise the use of the available means and to constitute sustainable research groups in the European Union, the Severe Accident Research NETwork of Excellence (SARNET) has gathered, between 2004 and 2008, 51 organizations representing most of the actors involved in severe accident (SA) research in Europe plus Canada. This project was co-funded by the European Commission (EC) under the 6th Euratom Framework Programme. Its objective was to resolve the most important pending issues for enhancing, in regard of SA, the safety of existing and future nuclear power plants (NPPs).SARNET tackled the fragmentation that existed between the national R&D programmes, in defining common research programmes and developing common computer codes and methodologies for safety assessment. The Joint Programme of Activities consisted in:

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Implementing an advanced communication tool for accessing all project information, fostering exchange of information, and managing documents;

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Harmonizing and re-orienting the research programmes, and defining new ones;

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Analyzing the experimental results provided by research programmes in order to elaborate a common understanding of relevant phenomena;

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Developing the ASTEC code (integral computer code used to predict the NPP behaviour during a postulated SA) by capitalizing in terms of physical models the knowledge produced within SARNET;

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Developing scientific databases, in which the results of research experimental programmes are stored in a common format;

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Developing a common methodology for probabilistic safety assessment of NPPs;

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Developing short courses and writing a text book on severe accidents for students and researchers;

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Promoting personnel mobility amongst various European organizations.

This paper presents the major achievements after four and a half years of operation of the network, in terms of knowledge gained, of improvements of the ASTEC reference code, of dissemination of results and of integration of the research programmes conducted by the various partners.Most initial objectives were reached but the continuation of the SARNET network, co-funded by EC in the 7th Framework Programme (SARNET2 project that started in April 2009 for 4 years), will consolidate the first assets and focus mainly on the highest priority pending issues as determined during the first period. The objective will be also to make the network evolve towards a complete self-sustainability.     

Views on R&D needs about in-vessel reflooding issues, with a focus on debris coolability

In-vessel reflooding during a severe accident in a PWR was analyzed in 2000–2002 by a group of experts with the aim to prepare a global IRSN R&;D strategy to answer the corresponding pending safety issues. Indeed, water is today systematically injected if available during a severe accident in a PWR. However, knowledge on consequences of such an injection is not complete and answers are necessary for accident management in present PWR as well for design and safety analysis of future PWR: is in-vessel corium retention possible? What is the kinetics of hydrogen production? What is the reactor cooling system (RCS) re-pressurization? Is there a risk of steam explosion (steam explosion is not discussed in this paper)? What is the impact on source term? And more generally, how to optimize water injection? (When? How?) R&;D needs of investigation of these aspects were identified. This should cover separate-effect and integral tests, as well as modeling and code development.The approach consisted first in updating the synthesis of knowledge, based on the multiple reports released in an international frame (OECD, European Commission (EC) Framework Programs (FwP), …), and then in focusing on a detailed re-analysis of the most important experiments CORA, QUENCH, LOFT-LP-FP2 and of TMI-2 accident, the latter two being directly related to debris coolability phenomena. Several out-of-pile experiments on debris bed coolability were also analyzed.A qualitative analysis of different possible core degradation scenarios was performed for French PWR, depending on operator actions or procedures. Simplified and mechanistic models were used to evaluate orders of magnitude of the phenomena in reactor conditions.A Phenomena Identification Ranking Table (PIRT) ranked the elementary phenomena with respect, on one hand, to their importance from the point of view of safety consequences and, on the other hand, to their level of understanding (often based on experts opinion). In particular, coolability of debris either in the core or in the lower plenum was identified as an important issue to be solved: uncertainties were underlined on debris characterization (size, distribution, composition, …) and on multi-D thermal–hydraulics in a debris bed. This ranking is fully consistent with the outcomes of the EURSAFE 5th FwP project. Existing or future experiments that could satisfy the needs were then identified, including for debris coolability POMECO, DEBRIS, STYX, etc. This specific issue will be analyzed in the frame of the Network of Excellence on severe accidents SARNET which has started in 2004 in the 6th FwP. Further model developments are being performed in the IRSN codes ICARE/CATHARE (detailed modeling) and ASTEC (simplified fast-running modeling), the latter being jointly developed with GRS.     

ASTEC and ICARE/CATHARE modelling improvement for VVERs

ASTEC and ICARE/CATHARE computer codes, developed by IRSN (France) (the former with GRS, Germany), are used in RRC KI (Russia) for the analyses of accident transients on VVER-type NPPs. The latest versions of the codes were continuously improved and validated to provide a better understanding of the main processes during hypothetical severe accidents on VVERs.This paper describes modelling improvements for VVERs carried out recently in the ICARE common part of the above codes. These actions concern the important models of fuel rod cladding mechanical behaviour and oxidation in steam at high and very high temperatures. The existing models were improved basing on the experience in the field and latest literature data sources for Zr + 1%Nb material used for manufacture of VVERs fuel rod claddings.Best-fitted correlations for the Zr alloy oxidation through a broad temperature range were established, along with recommendations on model application in clad geometry and starvation conditions. A model for the creep velocity was chosen for the clad mechanical model and some cladding burst criteria were established as a function of temperature.After verification of modelling improvements on Separate Effect Tests, validation was carried out on integral bundle tests such as QUENCH, CODEX-CT, PARAMETER-SF (the application to the CORA-VVER experiments is not described in the present paper) and on the Paks-2 cleaning tank incident. The comparison of updated code results with experimental data demonstrated very good numerical predictions, which increases the level of code applicability to VVER-type materials.     

Progress of IRSN R&D on ITER Safety Assessment

The French “Institut de Radioprotection et de S?reté Nucléaire” (IRSN), in support to the French “Autorité de S?reté Nucléaire”, is analysing the safety of ITER fusion installation on the basis of the ITER operator’s safety file. IRSN set up a multi-year R&D program in 2007 to support this safety assessment process. Priority has been given to four technical issues and the main outcomes of the work done in 2010 and 2011 are summarized in this paper: for simulation of accident scenarios in the vacuum vessel, adaptation of the ASTEC system code; for risk of explosion of gas-dust mixtures in the vacuum vessel, adaptation of the TONUS-CFD code for gas distribution, development of DUST code for dust transport, and preparation of IRSN experiments on gas inerting, dust mobilization, and hydrogen-dust mixtures explosion; for evaluation of the efficiency of the detritiation systems, thermo-chemical calculations of tritium speciation during transport in the gas phase and preparation of future experiments to evaluate the most influent factors on detritiation; for material neutron activation, adaptation of the VESTA Monte Carlo depletion code. The first results of these tasks have been used in 2011 for the analysis of the ITER safety file. In the near future, this R&D global programme may be reoriented to account for the feedback of the latter analysis or for new knowledge.     

European Validation of the Integral Code ASTEC (EVITA): First experience in validation and plant sequence calculations

The main objective of the European Validation of the Integral Code ASTEC (EVITA) project is to distribute the severe accident integral code ASTEC to European partners in order to apply the validation strategy issued from the VASA project (4th EC FWP). Partners evaluate the code capability through validation on reference experiments and plant applications accounting for severe accident management measures, and compare results with reference codes.The basis version V0 of ASTEC (Accident Source Term Evaluation Code)—commonly developed and basically validated by GRS and IRSN—was made available in late 2000 for the EVITA partners on their individual platforms. Users’ training was performed by IRSN and GRS. The code portability on different computers was checked to be correct. A “hot line” assistance was installed continuously available for EVITA code users. The actual version V1 has been released to the EVITA partners end of June 2002. It allows to simulate the front-end phase by two new modules:

• for reactor coolant system 2-phase simplified thermal hydraulics (5-equation approach) during both front-end and core degradation phases,

• for core degradation, based on structure and main models of ICARE2 (IRSN) reference mechanistic code for core degradation and on other simplified models.

Next priorities are clearly identified: code consolidation in order to increase the robustness, extension of all plant applications beyond the vessel lower head failure and coupling with fission product modules, and continuous improvements of users’ tools.As EVITA has very successfully made the first step into the intention to provide end-users (like utilities, vendors and licensing authorities) with a well validated European integral code for the simulation of severe accidents in NPPs, the EVITA partners strongly recommend to continue validation, benchmarking and application of ASTEC. This work will continue in Severe Accident Research Network (SARNET) in the 6th Framework Programme where ASTEC plays a key role as the reference European integral code.     

Core loss during a severe accident (COLOSS)

The COLOSS project was a 3-year shared-cost action, which started in February 2000. The work-programme performed by 19 partners was shaped around complementary activities aimed at improving severe accident codes. Unresolved risk-relevant issues regarding H₂ production, melt generation and the source term were studied through a large number of experiments such as (a) dissolution of fresh and high burn-up UO₂ and MOX by molten Zircaloy, (b) simultaneous dissolution of UO₂ and ZrO₂, (c) oxidation of U–O–Zr mixtures, (d) degradation–oxidation of B₄C control rods.Corresponding models were developed and implemented in severe accident computer codes. Upgraded codes were then used to apply results in plant calculations and evaluate their consequences on key severe accident sequences in different plants involving B₄C control rods and in the TMI-2 accident.Significant results have been produced from separate-effects, semi-global and large-scale tests on COLOSS topics enabling the development and validation of models and the improvement of some severe accident codes. Break-throughs were achieved on some issues for which more data are needed for consolidation of the modelling in particular on burn-up effects on UO₂ and MOX dissolution and oxidation of U–O–Zr and B₄C–metal mixtures. There was experimental evidence that the oxidation of these mixtures can contribute significantly to the large H₂ production observed during the reflooding of degraded cores under severe accident conditions.The plant calculation activity enabled (a) the assessment of codes to calculate core degradation with the identification of main uncertainties and needs for short-term developments and (b) the identification of safety implications of new results.Main results and recommendations for future R&D activities are summarized in this paper.