LWR severe accident simulation Iodine behaviour in FPT2 experiment and advances on containment iodine chemistry

International audienceThe Phebus Fission Product (FP) Program studies key phenomena of severe accidents in water-cooled nuclear reactors. In the framework of the Phebus program, five in-pile experiments have been performed that cover fuel rod degradation and behaviour of fission products released via the coolant circuit into the containment vessel. The focus of this paper is on iodine behaviour during the Phebus FPT2 test. FPT2 used a 33 GWd/t uranium dioxide fuel enriched to 4.5%, re-irradiated in situ for 7 days to a burn-up of 130 MWd/t. This test was performed to study the impact of steam-poor conditions and boric acid on the fission product chemistry. For the containment vessel, more specifically, the objective was to study iodine chemistry in an alkaline sump under evaporating conditions. The iodine results of the Phebus FPT2 test confirmed many of the essential features of iodine behaviour in the containment vessel provided by the first two Phebus tests, FPT0 and FPT1. These are the existence of an early gaseous iodine fraction, the persistence of low gaseous iodine concentrations and the importance of the sump in suppressing the iodine partitioning from sump to atmosphere. The main new insights provided by the Phebus FPT2 test were the iodine desorption from stainless steel walls deposits and the role of the evaporating sump in further iodine depletion in the containment atmosphere. The current paper presents an interpretation of the iodine behaviour in the FPT2 containment vessel based on dedicated small-scale analytical experiments and computer codes calculations. Other investigations dealing with primary circuit and sump chemistry are also reported. These could help to scale the results of Phebus-FP tests to reactor accidents. Modelling studies were generally successful when a gaseous iodine injection from the primary circuit was assumed. Indeed, though each of the iodine codes has specific iodine chemistry features that should be further developed and each approach to the modelling is distinct, the overall iodine behaviour in the FPT2 containment is generally well reproduced by the models that predicta low final gaseous iodine concentration in the containment atmosphere,a predominant iodine concentration in the sump and to a lesser extent a significant iodine deposition on containment surfaces. The main code-to-code differences, in the results obtained in gaseous iodine speciation, come from the various treatments of gaseous radiolytic reactions. Calculations that include the radiolytic conversion of volatile iodine into iodine oxide particulate show there is a persistence of both gaseous iodine and iodine oxide particles in the atmosphere. There are also some variations between the predicted organic iodine concentrations that depend mainly on the initial assumptions. A key aspect of the Phebus FPT2 test interpretation is that the long term iodine behaviour in the containment can be explained by exchanges between organic iodide released from painted surfaces and inorganic iodine released from deposited aerosol on the containment walls. Further studies of regulatory significance on sump chemistry showed that the gaseous iodine control that was evident in the Phebus tests through silver release and/or alkaline buffered sump solutions may not be assured. As most of the past iodine aqueous chemistry studies were done with rather pure systems and because of the uncompleted understanding of the gaseous iodine speciation, the results may not be extrapolated easily to conditions of reactor accidents thus necessitating deeper investigations. © 2011 Elsevier B.V. All rights reserved

Conclusions on severe accident research priorities

International audienceThe objectives of the SARNET network of excellence are to define and work on common research programs in the field of severe accidents in Gen. II-III nuclear power plants and to further develop common tools and methodologies for safety assessment in this area. In order to ensure that the research conducted on severe accidents is efficient and well-focused, it is necessary to periodically evaluate and rank the priorities of research. This was done at the end of 2008 by the Severe Accident Research Priority (SARP) group at the end of the SARNET project of the 6th Framework Programme of European Commission (FP6). This group has updated this work in the FP7 SARNET2 project by accounting for the recent experimental results, the remaining safety issues as e.g. highlighted by Level 2 PSA national studies and the results of the recent ASAMPSA2 FP7 project. These evaluation activities were conducted in close relation with the work performed under the auspices of international organizations like OECD or IAEA. The Fukushima-Daiichi severe accidents, which occurred while SARNET2 was running, had some effects on the prioritization and definition of new research topics. Although significant progress has been gained and simulation models (e.g. the ASTEC integral code, jointly developed by IRSN and GRS) were improved, leading to an increased confidence in the predictive capabilities for assessing the success potential of countermeasures and/or mitigation measures, most of the selected research topics in 2008 are still of high priority. But the Fukushima-Daiichi accidents underlined that research efforts had to focus still more to improve severe accident management efficiency. © 2014 Elsevier Ltd. All rights reserved