The spent subassemblies (SA) from nuclear reactors are cooled in spent fuel pools for several years before they become suitable for reprocessing or transfer to other storage locations with air cooling. As part of safety analysis, hypothetical extended and un-mitigated station blackout (SBO) is numerically studied using an inhouse developed code called Severe Accident Analysis Program (SAAP). The present paper describes the scenario, followed by the model for thermal transients in the subassemblies, pool water and building atmosphere and the results obtained for SBO in a typical Pressurized Water Reactor spent fuel pool.