Japan's Tokyo Electric Power Co. (TEPCO) has started dumping water tainted with low-level radiation from its stricken Fukushima No. 1 nuclear power plant into the sea to make space for the storage of highly radioactive water.

The amount of water to be released into the Pacific Ocean will total some 11,500 tons - 10,000 tons currently stored at a nuclear waste processing facility and 1,500 tons in drainage pits at the plant's No. 5 and No. 6 reactors, the company said.

It will take some five days for all the water to be released, company officials said. The water contains a total of 170 billion becquerels of radioactive substances such as iodine-131, and the level of radioactivity is 100 to 1,000 times the government-set limit, the officials said.

The plant is at the centre of an unfolding nuclear crisis. The facility was severely damaged by tsunami triggered by the powerful March 11 earthquake that hit the northeastern region of Japan.

The latest step is in line with Japan's nuclear reactor control law, which requires nuclear plant operators to store radioactive waste safely.

Junichi Matsumoto, an official at Tokyo Electric Power, apologised for the release of the contaminated water into the environment but said the water is very unlikely to pose a risk to human health.

Tokyo Electric Power will then shift water tainted with high-level radiation in the basement of the turbine building from the plant's No. 2 reactor to the nuclear waste processing facility. The radiation level of this water is more than 1,000 millisieverts per hour.

TEPCO played down the impact of the low-level radioactive water to be dumped in the sea.

It said an adult who eats fish and seaweed from waters near the plant every day for a year would receive an estimated radiation dose of around 0.6 millisievert, compared with the average natural radiation exposure of 2.4 millisieverts per year.

The company also found that some of the low-level radioactive water in the drainage pits at the No. 5 and No. 6 reactors has entered the reactor buildings, raising concerns that key equipment there including diesel generators may be submerged.

The company is also considering releasing contaminated water in the turbine buildings for the No. 1 and No. 4 reactors and water in underground tunnels, called trenches, that stretch from the basements of the No. 1 and No. 3 reactor buildings toward the seas, because the levels of radioactivity are relatively low.

Seiji Shiroya, an official at the government's Nuclear Safety Commission, said at a news conference that the release of the low-level radioactive water into the sea can be justified to prevent the dumping of the water contaminated with high-level radiation.

Shiroya also said that the commission instructed Tokyo Electric Power to continue monitoring radiation levels of seawater.

- Jiji Press