Source Term Assessment: Method and Tool in the Acute Phase

Abstract

Early in a nuclear power plant accident there is a need to distinguish a potentially large release of Class 4 or higher on the International Nuclear Event Scale (INES) from smaller ones. The main goal is to plan off-site measures ahead of the events. A large release requires prompt off-site emergency actions while a small release may be handled with a slower response. The method for achieving this distinction with source term predictions is described in the SKI Source Term Handbook. Appended to the handbook are or will be plant specific data for each reactor type in and around Sweden, including BWR, PWR and RBMK. The method promotes a closer integration between the source term prediction tools at the plants and at the regulatory authority in order to ease and speed up communications and improve early emergency management. The source term is the radioactive release to the atmosphere, like noble gases, iodine and caesium, that can occur as a result of an accident at a nuclear power plant. The possible scenarios are mapped out, focussing on reactor core and containment states as these most strongly affect releases to the atmosphere. A rule of thumb formalism is used by the SKI emergency staff in order to provide an order of magnitude prediction of an imminent source term in an acute phase of an emergency situation. This early prediction will later be modified and substantiated when monitor observations become available.