An investigation by the Hungarian Atomic Energy Agency concluded that the cause of the incident was inadequate cooling of the fuel elements, which were heated due to the radioactive decay of short-lived fission products. These were kept cool by water circulated by a submerged water pump. However the cooling was inadequate, leading to the damage to some elements through a build-up of steam around them, depriving them of most of their cooling. The investigation proposed that the severe damage probably occurred when the lid was released, causing thermal shock to cladding because of the sudden entry of cool water into the system, and explosive steam production.