PWR Medium Break LOCA source term analysis using ASTEC code

Highlights: • A MBLOCA in a generic PWR 1000 MWe with SBO has been analysed with ASTEC code. • The accumulator availability extends the duration before RV failure occurs. • The Mo/Cs(Rb) molar ratio affects the form of Iodine reaching the containment. • The different location of the break affects the chemical speciation of the FPs. • Different pathways influence the total retention of FPs/SMs in the circuit. - Abstract: During a hypothetical severe accident in a nuclear reactor most of the radioactivity would be released from the damaged core and the broken primary circuit in the form of aerosols and chemical vapours and gases. However the effective radio-toxicity per unit release from the fuel depends both on the released mass and on the chemical form as well. The fission products release and transport in the circuit has been analysed for a Medium Break cold leg Loss Of Coolant Accident (MBLOCA) combined with the total loss of electric power supply (Station Black Out) for a reference PWR 1000 reactor using the severe accident integral code ASTECV1.3 rev2. A detailed description of the reactor model is given. Sensitivity analyses were performed in terms of break location and accumulator availability. The break location has an impact on the carrier gas temperature and in turn on the chemical speciation of the fission products. Also retention is influenced by the different flow pathways establishing in the circuit. The accumulator availability extends the duration of the core degradation process affecting the total production of hydrogen and the cumulative released mass of low, semi and highly volatile fission products. In agreement with the ASTEC equilibrium chemistry model, the Mo/Cs (Rb) molar ratio has the most influence on the fraction of iodine which can reach the containment in the gaseous or highly volatile form. Due to the surplus of Cs, the predicted dominant iodine species to the containment are CsI and its dimers. A 3–4% of iodine which reaches the break exists in gaseous (HI) and highly volatile metal-iodides (I₂MoO₂, SnI₂, SnI₄) species.