Station blackout (SBO) is a postulated event in a nuclear power plant in which both the offsite power (from electricity grid) and onsite power (from diesel generators) are lost. During an SBO, active cooling systems of the spent fuel pool (SFP) will not be available. The decay heat from spent fuel will cause rise in pool temperature. If not mitigated by adding water to the pool, the water level will decrease to such a level that fuel bundles could be exposed to air causing them to heat up. In the present work, a numerical model has been developed to study the hypothetical extended and unmitigated SBO event in a typical PHWR/CANDU spent fuel pool. The mathematical models required to model this scenario have been described followed by a case study for a typical PHWR/CANDU spent fuel pool. Spent fuel discharged over a period of ten years and one full core unloading are considered which gives the maximum possible decay heat load to the spent fuel pool. The results show the inherent safety in the spent fuel pool and the time available for management of this hypothetical event.