The melted cores must be cooled with about 400 tonnes of water daily. When the water is pumped back to the surface it is extremely radioactive and cannot be released, so it is stored in huge steel tanks. Most of them are welded but about 1/3 are only bolted, with rubber seals along the edges. A radioactive puddle that leaked from one such tank would in one hour give a worker two feet away a dose equivalent to 5 years worth of exposure at the maximum level of radiation dosage permissible for atomic workers.