Release of radioactivity and formation of aerosols during LWR core meltdown

At the SASCHA facility, melting experiments were carried out to determine fractional release rate coefficients for all relevant elements in the core at two temperatures, 2200 and 24000C. By use of a simplified time dependence of the average core temperature, integral amounts of the released nuclide activities and of the aerosol masses were calculated. The results are applicable to the reference reactor Biblis B of the German Risk Study for which a large pipe break in the primary system and a medium burnup of 21 GWd/t was assumed. The main findings are: 1. The integral activity release from the core is 16 +- 2% of the radioactive inventory two hours after the reactor shutdown. The integral aerosol mass emerging from the primary system is about 1560 kg with an uncertainty factor of two. 2. About 99% of the released activity results form the nuclides of only 6 elements: Kr, Xe, I, Cs, Te, and Sb. The highest contribution to the aerosol mass is due to the element Ag, the main component of the absorber material. 3. The contributions of Te, Ba (Sr), and Ag depend strongly on the degree of the Zircaloy oxidation and on the time/temperature function assumed for the core. A potential overestimation of the release of Te and Ag prior to the RPV failure will be compensated by the fact that these elements are also strongly released during the melt/concrete interaction. (orig.)