300 tonnes of contaminated water have leaked from one of the steel storage tanks at the damaged Fukushima Dai-ichi nuclear power plant.
The Tokyo Electric Power Co (Tepco), the plant’s operator, has admitted the leak was the worst since the 2011 earthquake that damaged the facility. The company has not yet determined the exact cause of the seepage but said a faulty or damaged valve connecting the tank to the surrounding gutter was the most likely explanation.
A concrete barrier, as well as reinforcing sandbags, failed to contain the leak and the radioactive water soaked into the ground after passing the protective structure.
Measurements have shown the level of radioactivity of the contaminated liquid has been about 100 millisieverts per hour – five times the annual exposure limit for plant workers.
According to Masayuki Ono, Tepco’s spokesman, workers have been pumping the remaining water out of the puddle, in order to transfer it into another tank.
The tank, one of hundreds that were built around the plant after the earthquake to store the contaminated water from three melted reactors, has not been the first to experience leakage.
Since last year, four other tanks have been reported to fail – all of them using rubber seams and designed to last for at least five years. Tepco has now decided to build additional welded containers, which it hopes will be more leak-proof.
The contaminated water is probably the most pressing issue hindering the clean-up process, which is anticipated to last for decades. Some of the previous leaks were even said to result in contaminated water flowing into the sea. The company is trying to prevent further spills by building chemical underground walls but have so far achieved limited success.
Apart from the leakage, two workers were found on Monday to have been contaminated by radioactive particles outside the main operations centre. It has been the second incident of this type within a week involving the area where radiation levels are usually low enough to avoid the need for full facemasks.
The two affected workers had to go through additional cleaning procedures before leaving the site.
The alarm was triggered by contamination monitors while a group of workers was waiting for a bus. Full body controls later confirmed there was no internal body contamination of the affected workers.
Last week, a similar incident involved ten workers. Tepco claimed the most probable cause of the incident was the mist sprayer used to cool staff down during the current hot summer. However, the device has been turned off since the first accident.
Earlier this month, Japanese government has announced increasing its involvement in the plant’s clean-up, following the revelation contaminated water has been leaking into the sea.