A spokesman for the Tokyo Electric Power Co (TEPCO) said on Tuesday that 300 tons of highly radioactive water has leaked from a stainless steel tank on the territory of the Fukushima nuclear power plant.

Japan’s Nuclear Regulation Authority has classified the leak as a Level One incident, the second lowest on the International Nuclear Event Scale (INES) for radiological releases, a spokesman informed Reuters.

The incident is the first Level One since the actual catastrophe at Fukushima in March 2011 when Japanese authorities issued an INES rating.

Level One means that the incident is classified as anomalous situation that exceeds limits of safe functioning of an installation. The most dangerous, Level Seven, has only been applied twice: for the Chernobyl catastrophe in 1986 and the meltdown of three reactors at Fukushima plant itself.