The Source Term Evaluation and Mitigation Project (STEM) project, hosted by the Nuclear Energy Agency (NEA) and operated by the Institut de Radioprotection et de Sûreté Nucléaire (French Institute of Radiation Protection and Nuclear Safety, IRSN), was launched in mid-2011 in order to improve the evaluation of source term (ST) for a severe accident (SA) at a nuclear power plant (NPP) and to reduce uncertainties concerning specific phenomena related to the chemistry of two major fission products: iodine and ruthenium (Ru). Three main issues were considered for these two radionuclides:
medium-term iodine releases, with specific attention to the stability of iodine aerosol particles under radiation (decomposition induced by radiation producing gaseous iodine species);
short-term and medium-term iodine-paint interactions under radiation;
ruthenium transport chemistry in order to determine the speciation of Ru, in particular the partition between gaseous and condensed forms during its transport through the reactor cooling system (RCS).
The present report summarises the main outcomes of the STEM project. The first chapter deals with the main outcomes concerning the iodine part (EPICUR facility), while the second chapter focuses on the main outcomes related to the ruthenium part (START bench).