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Past, recent and ongoing R&D programmes (e.g. Phebus FP, ISTP, ARTIST, OECD/BIP and THAI Projects) are mainly focussed on the reduction of uncertainties on the evaluation of the potential source term to the environment in case of a light water reactor (LWR) severe accident. This effort is sustained within the EU/SARNET Network of Excellence.
However, it has been recognised that known phenomenological uncertainties remain for some complex phenomena with a significant impact on source term. For public acceptance of lifetime extension of existing reactors, it must be demonstrated that their safety level continues to be enhanced.
Therefore, a new NEA project named STEM (Source Term Evaluation and Mitigation) was initiated in 2011 to improve the general evaluation of the source term. In addition, the reduction of known phenomenological uncertainties on certain phenomena is expected to help in
•providing better information and tools to emergency teams in order to help them making a more robust diagnosis and prognosis of the progression of an accident and a better evaluation of potential release of radioactive materials;
•investigating phenomena involved in possible complementary mitigation measures, natural or engineered, so as to minimise releases to the environment.
The NEA STEM Project deals with three main issues:
•Radioactive iodine release in mid and long term – In complement to previous programmes, it is proposed to perform experiments to study the stability of aerosol particles under radiation and the long term gas/deposits equilibrium in a containment.
•Interactions between iodine and paints – No experiments are planned but a literature survey especially focussed on the effect of paint ageing that is likely to lead to the definition of experiments in a possible follow-up project.
•Ruthenium chemistry – In complement to previous programmes, it is proposed to perform experiments to study the Ruthenium transport in pipes.
Within the frame of the NEA STEM Project, the first series of experiments (iodine behaviour under radiation) are dedicated to the analysis of radiation effects and will thus be realized in benches built on the EPICUR (Experimental Programme of Iodine Chemistry Under Radiation) facility. The second series of experiments (ruthenium transport) will aim to analyse the chemistry of ruthenium in pipes including the reactor coolant system and in filters. They will be performed in dedicated benches allowing the injection of different chemical compounds followed by their transport through high temperature gradient tubes up to aerosols filters and bubblers for gas trapping
Participating countries: Canada, Czech Republic, Finland, France, Germany, Korea and the United States of America.
Project Period: July 2011 to September 2015
The distribution of this package is restricted and subject to prior approval.
For more information see http://www.oecd-nea.org/jointproj/stem.html
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Keywords: LWR reactors, iodine, light-water reactors, mitigation, severe accident, source term evaluation.