Phenomena of the ex-vessel source term

The release of radionuclides and the production of aerosols from core debris outside the vessel in a severe reactor accident are discussed. This ex-vessel source term is caused by expulsion of core debris from a pressurized vessel, core debris interactions with water and core debris interactions with concrete. The models and data available for the ex-vessel source term are described and compared with the models used in the Reactor Safety Study (WASH-1400). The release associated with core debris interactions with concrete has been investigated extensively. Current perceptions concerning the release during these interactions are quantitatively and qualitatively different than perceptions arising from the Reactor Safety Study model. Source terms arising from other ex-vessel phenomena have not been studied extensively, yet these source terms may be important. There is a great need for experimental data for all aspects of the ex-vessel source term. (author)