TOKYO - Tokyo Electric Power Co (TEPCO) began pumping nitrogen gas into a stricken reactor at the troubled Fukushima Daiichi nuclear power plant on Thursday to prevent the risk of a hydrogen explosion.

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Workers injected nitrogen gas into the containment vessel of the plant's No 1 reactor in process that could take several days, according to the embattled utility firm.

But following the firm managing to stem the flow of radioactive water from flowing freely into the Pacific Ocean yesterday, the latest move was described by Japan's nuclear safety agency as "preventative."

"It is necessary to inject nitrogen gas into the containment vessel and eliminate the potential for a hydrogen explosion," Hidehiko Nishiyama, Deputy-General of Japan's Nuclear and Industrial Safety Agency told a press briefing.

Nishiyama added that the risk of further hydrogen explosions that previously blew apart reactor buildings No 1 and No 3, causing dangerous levels of radioactive substances to be released into the atmosphere, was "extremely low."

That said, due to fears that the outside casing of the reactor vessel has been damaged, while an explosion remains unlikely, the scenario has not been completely ruled out, the nuclear safety agency said.

"Under these conditions, if we continue cooling the reactors with water, the hydrogen leaking from the reactor vessel to the containment vessel could accumulate and could reach a point where it could explode," a TEPCO official was quoted as saying Thursday.

A total of 6,000 cubic meters of the inert gas will be pumped into the No. 1 reactor and TEPCO will repeat the process for reactors No.2 and No.3, in a bid to make the six-reactor complex more stable, officials said.

TEPCO have said, to the condemnation of neighboring countries like South Korea, that they intend to pump a further 10,000 tonnes of radioactive water into the Pacific Ocean by this weekend, to free up yet more storage space at the complex to shift more highly-contaminated water into the facility's containment units.

Hence domestic and international fears remain rife as the utility firm has confirmed that 70 percent of the No 1 reactor's fuel rods, 30 percent of the No 2 reactor's fuel rods and 25 percent of the nuclear fuel rods at the No 3 reactor have been damaged.

Previous hydrogen explosions were caused as a result of high levels of hydrogen accumulating following melted nuclear fuel rods reacting with steam produced from the reactors' coolant water.

Huge amounts of water have been poured into the reactors and their spent nuclear fuel pools as an emergency means to cool them down and avert a full meltdown, but the excess water, as well as seawater from the March 11 tsunami coming into contact with exposed fuel rods has caused contaminated water at levels ranging from hundreds to many million times over the legal limit being detected in and around the plant, including in seawater samples taken from the Pacific.

A seawater sample taken near the No 2 reactor water intake on Saturday showed a radioactive iodine-131 concentration of 7.5 million times the maximum level permitted under law.

As well as neighboring countries voicing concern, local fishing and marine industry associations have lambasted the government for permitted radioactive water to be released into the sea.

The National Federation of Fisheries Cooperatives Associations called the move an "outrage," as radioactive cesium has been found at levels above safety limits in tiny kounago fish in waters off Ibaraki Prefecture, south of Fukushima, and in young launce in the sea near the northern part of Ibaraki Prefecture.