Release of radioactive materials from a degrading core (RADECO): RADECO summary report

The formation of organic iodine on painted surface in water and in N2 was studied with and without radiation. The water pH in the experiment was 9.3 or 7. In neutral solution small amount of volatile organic iodide was detected. In high pH all volatile iodine was very small amount of inorganic iodine. Experiments performed in gas phase show some formation of volatile compounds, but most of these were rapidly adsorbed on other surfaces and only a negligible amount of organic iodine was detected. The severe accidents during shutdown conditions were examined with MELCOR code. Olkiluoto 1 was selected for the reference plant. Two accident scenarios starting during shutdown conditions with pressure vessel upper head opened were studied: a station blackout with initially intact pressure vessel (RPV) and a LOCA starting with a break in the bottom head of the RPV. The results of the calculation suggest that even a small hole in the RPV Bottom Head may allow some air ingress to the core region. If the Lower Head is intact no air ingress to the RPV would take place. Station blackout scenario resulted in high zirconium oxidation fraction, about 60%. The oxidation fraction in the Bottom Head LOCA case was 14%. In the calculated cases Ru release from the core equaled to 8-23% of the initial core inventory. The highest Ru release was obtained in the station blackout scenario.