A defective vaporising valve was named as the reason of the accident. According to the documentation the valve had not been tested since 1972. The reactor was launched back in 1957 and its lifetime can be considered critical at the moment. Earlier the nuclear safety regulators pointed out to the inappropriate storage of the radioactive materials at Chalk River, where cobalt, cesium-137 and mercury was damped to the trenches without decontamination, ITAR-TASS reported.