Most likely the leak occurred because the level of water in the container was higher than in the area where the pipes penetrate, TEPCO officials said.

In January, TEPCO discovered that water was pouring into a drain on the first floor of the building of Fukushima Reactor Number 3.

TEPCO continues to grapple with the problem of contaminated water storage, with about 450,000 tons of highly-radioactive water currently being stored in Fukushima’s underground facilities and tanks. Experts say some 15,000 tons is also being held in a service tunnel. According to recent estimates, up to 400 tons of contaminated water from the damaged plant is seeping into the Pacific Ocean every day.

In an effort to prevent further irradiation, TEPCO has adopted a plan to draw off groundwater from the plant. The water is later to be sent for analysis that will determine whether its level of radioactivity is low enough to be safely disposed of by dumping into the ocean.

The dumping will allow the operator to reduce the accumulation of radioactive water at the plant by 100 tons a day.

Last August saw the worst radioactive water leak at the crippled Fukushima plant since the 2011 disaster, after 300 tons of water with strontium levels equaling 80 million becquerels per liter leaked from a storage reservoir into the Pacific Ocean. The leak was then classified as a level three incident on the International Nuclear Event Scale (INES).

In March 2011, Japan was hit by a massive magnitude 9.0 earthquake and subsequent tsunami, claiming more than 15,000 lives and causing a number of explosions at the Fukushima plant.