To assess the source term during a severe accident involving a 300 MWt small pressurized water reactor, this paper employs the MELCOR code to establish a severe accident model about a large break loss of coolant with a double-ended shear fracture of a hot leg. The accident progression, thermal-hydraulic response, and behaviors of radioactive materials are examined in detail. The results show that the design of emergency mitigation strategies should give priority to containment since radioactive materials are primarily present in containment. For radioactive gases, isolation and decontamination should focus on the lower space of containment, while for radioactive aerosols, as the accident progresses, the focus should shift from the lower space of containment to the pit. This research offers valuable insights into the prioritization of isolation and purification strategies about containment, the design of radioactive materials control and removal systems, and the establishment of accident mitigation measures.