This week about 7 tons of highly radioactive water have leaked from a desalination facility at the severely damaged Fukushima Daiichi nuclear power plant but did not reach the sea.

Workers accidentally disconnected a pipe to the facility and caused the leak that continued for more than an hour in the morningadding that six of the 11 workers were covered with toxic water but unlikely to have suffered internal exposure.

Nuclear Regulation Authority chairman, Shunichi Tanaka, told a press conference that the exposure of the workers would likely be small as long as they wore full-face masks and boots, but added the frequent troubles at the plant caused by what appears to be carelessness is “serious.”

The water leaked from about 9:35 a.m. and the liquid contained some 34 million becquerels per litre of radioactive substances that emit beta rays such as strontium-90. Water containing more than 30 becquerels of strontium-90 per litre is not allowed to be released into the environment.

The site continues to face difficulties in managing the massive quantity of toxic water created as a result of continuing water injections into the three reactors that experienced meltdowns during the nuclear crisis that erupted in 2011.

Water used to cool the reactors passes through facilities that reduce radioactive cesium and remove salt before it is stored in tanks. The water that leaked had undergone the cesium-reduction process but still contained about 1 690 becquerels of cesium per litre, and was due to be desalinated.

It took more than an hour until the leakage stopped because workers experienced difficulties reconnecting the pipe.