The plant's operator Tokyo Electric Power Company (TEPCO), which yesterday warned that hydrogen gas build-up at the No.1 reactor could cause another explosion, said its workers injected chemically-stable nitrogen into the unit and that the operation to reduce the risk of a blast has gone smoothly.
Pressure inside the containment vessel of the No.1 reactor has risen slightly and this indicates that the operation has gone well, TEPCO said.
The utility said it plans to pump nearly 6,000 cubic metres of nitrogen, an inert gas, over a 6-day period and will also consider taking measures at the No.2 and No.3 reactors, as it workers struggled to contain Japan''s worst atomic crisis in decades triggered by the March 11 magnitude-9 quake and tsunami that left nearly 30,000 people dead or missing.
Last month''s hydrogen blasts at the No.1 and No.3 reactors following the mega quake destroyed reactor buildings, causing release of radioactive steam from the plant.
TEPCO, which yesterday in a rare progress sealed a 12-inch crack leaking high radiation into sea, also continued to dump low-level radioactive wastewater from the plant into Pacific Ocean to create storage for more highly contaminated water.
Around 6,000 tonnes of water have been discharged so far.
The government''s nuclear safety agency said most of the low-level toxic water will be released soon, amid concerns among neighbouring countries over sea contamination and strong protests from the domestic fishing industry.
After TEPCO successfully stopped leakage of highly radioactive water into the sea from a cracked pit, it detected a temporary rise in the level of tainted water in an underground trench connected to the No. 2 reactor building, from which the toxic liquid is believed to have originated.
Hidehiko Nishiyama, a spokesman for the nuclear regulatory body, said the water level which rose about 4 cm and then returned to the previous level, suggests that highly radioactive water may have begun leaking again from somewhere else, Kyodo reported.
He said that TEPCO was expected to boost monitoring of seawater radiation levels.
Meanwhile, US Secretary of State Hillary Clinton is planning to visit Japan to underline American support for its efforts to overcome the devastation caused by the March 11 twin disaster, national broadcaster NHK reported.
It quoted diplomatic sources as saying that both governments are making final arrangements for her to meet Premier Naoto Kan and Foreign Minister Takeaki Matsumoto while in Japan. Clinton is hoping to arrive around April 17 after attending a NATO Foreign Ministers'' meeting in Berlin.