At about 3:15 p.m. on Dec. 3 2003, personnel in the Expended Core Facility at the Naval Reactors Facility heard a loud, unexpected noise. Upon investigation, personnel found an irradiated material storage container fell from its normal submerged storage location in the water pool to the pool floor, and that the lid had come off.The lid was found about 10 feet away from the container on the bottom of the water pool. No nuclear fuel was involved.According to a press release, a team of technical experts determined the most likely cause was a hydrogen gas explosion within the container. It is believed the hydrogen gas was generated from corrosion of the brass end cap and the separation of water into hydrogen and oxygen due to radiation.Examination of the area showed no damage to the water pool structure, cranes or facility structures. It was concluded the loud noise reported in ECF was caused by the shock wave from the failed container when it encountered submerged equipment.NRF conducted an analysis of all container types used in the Naval Reactors program to determine if other containers are susceptible to this type of event. They found nothing to cause concern and don't believe the event will be repeated. Based on a review of the containers and conditions necessary for this type of event, a similar event with another container is highly unlikely and would not result in personnel injury or facility damage.State officials from the INEEL Oversight program were informed of the event and visited NRF Dec. 8 to be briefed on the investigation.