After the earthquake and tsunami disaster in Japan, INES-7 scaled accident had happened in Fukushima Daiichi Nuclear Power Plant. When the disaster occurred, Unit 4 of Fukushima Nuclear Power Plant is under the refuelling and maintenance stage. All the fuel assemblies of the core had been taken from the reactor vessel and transferred to the spent fuel pool. Approximately four days after the station black out, there was an explosion observed at Unit 4, then all focused on the spent fuel storage pool. This study focuses on the loss of coolant flow accident in spent fuel storage pool which is modelled by using RELAP5/SCDAP code to observe the coolant level reduction and fuel uncovery because of decay heat generation of the fuel in the spent fuel pool.