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.
During Clinton`s possible visit here, the two sides are expected to discuss how the US can help Japan rebuild and deal with the troubled Fukushima nuclear plant, the report said.
French President Nicolas Sarkozy came to Japan after the earthquake, and other leaders are also planning visits.
The Japanese government is considering allowing people evacuated from areas around the crippled nuclear plant to return home for a short period of time to collect their valuables.
"Yes, it is true that we are considering it," Chief Cabinet Secretary Yukio Edano told reporters in response to a question.
Edano said the government and nuclear experts are currently studying how to provide them safety during their possible hometown visits.
The government has ordered residents within a 20-km radius of the nuclear power station to evacuate and those in the 20-30 km zone to stay indoors.
The Nuclear Safety Commission of Japan said yesterday that it has advised the government to issue an evacuation order if there is a possibility of residents receiving a dose of 20 millisieverts of radiation during the course of a year, up from the current limit of 1 millisievert per year.
To prevent further contamination of the sea from radiation leakage, TEPCO would install iron sheets as well as "silt fence" barriers close to the No.2 reactor water intake and other areas near the plant. The utility would also put 100 tonnes of sandbags at a breakwater, Kyodo said.
A seawater sample taken near the No.2 reactor on Saturday last showed a radioactive iodine-131 concentration of about 7.5 million times the maximum level permitted under law.
But TEPCO said the iodine-131 density has sharply dropped to around 140,000 times the maximum level in a sample taken yesterday after the leakage of highly toxic water stopped.
A TEPCO employee, who is around among 700 workers dealing with Japan`s worst nuclear crisis inside the Fukushima plant for nearly a month, refused to reveal how much radiation he has been exposed to, during a telephonic interview, Kyodo said.
"I can`t tell you. It`s private information as well," 44-year-old Yasuki Murata was quoted as saying.
Murata has been staying inside a two-story antiseismic building in the plant whose few windows are covered with lead plates to prevent radiation penetration and he goes out only when trucks arrive at the building with food supplies, the report said.
A total of 21 workers at the plant have so far been exposed to radiation exceeding 100 millisieverts, the usual limit for exposure in an emergency, which has been raised to 250 millisieverts for the ongoing crisis.
Meanwhile, Japan`s Emperor Akihito and Empress Michiko would meet nuclear crisis evacuees and may also visit quake-hit areas, Kyodo reported.