Analytical simulation using best-estimate codes were suggested to be extended for the elaboration and improvement of SAMG in current PWRs. This work is about to perform an assessment work using MELCOR for the effectiveness of safety injection to achieve the PBF strategy in SAM following a BDBA scenario, that is LOCA with concurrent SBO. In the simulations, the safety injection is assumed to be retrieved with the postulated power recovery at different timing during core relocation. The simulation results illustrates that the grace period of preventing vessel failure varies with LOCA break size and locations. The safety injection implemented in grace period is capable of retarding or ceasing the core relocation, sequentially avoiding the massive core relocation into lower plenum, mitigating the hydrogen generation and fission product release from core. Meanwhile, the injection later than grace period would be failed to prevent RPV failure, and it negatively affects hydrogen generation in some scenarios. The results also indicate that the smallest injection capacity of HPSI system in Swedish PWR is sufficient to achieve the effective mitigation.