Tokyo Electric Power Company says it has started injecting nitrogen gas into the containment vessel of a reactor at the crippled Fukushima Daiichi nuclear power plant to prevent hydrogen gas from exploding inside the vessel.

The nuclear fuel rods in the Unit 1 reactor are still nearly half exposed as the coolant water inside the reactor has not covered them, despite weeks of pumping first sea water and then fresh water into the reactor.

The cooling systems for the plant's six reactors and spent fuel pools were lost when outside power and emergency power failed after the deadly March 11 earthquake and tsunami.

The company fears that hydrogen and oxygen are being released by the reaction between the cooling water and the zirconium alloy that covers the fuel. Hydrogen also can be released when water molecules are broken apart by radiation.

Injection of nitrogen is intended to displace oxygen inside the containment vessel, thereby reducing a risk of explosion due to the combustible combination of hydrogen and oxygen, according to the International Atomic Energy Agency. Chemically stable nitrogen gas does not react with hydrogen.

Increasing hydrogen density inside the containment vessel heightens the risk of an explosion like those that blasted through Unit 1 on March 12, Unit 3 on March 14, and Unit 4 on March 15.

The nitrogen injections have been approved by the Ministry of Economy, Trade and Industry and are expected to continue for six days.

Work to inject nitrogen gas into the Unit 1 reactor containment vessel began at 10:30 pm Wednesday, and the actual injection began at 1:30 am Thursday, local time.

The company is also considering making similar injections in the Unit 2 and 3 reactors.

The Nuclear and Industrial Safety Agency warns that the nitrogen gas injection could cause gases, including radioactive substances, to leak outside the reactor containment vessel. The agency has directed TEPCO to monitor radioactivity in surrounding areas and to fully disclose its findings.

TEPCO has had success in stopping the leak of highly radioactive water into the Pacific Ocean from the damaged power plant.

On Saturday, the company found that a concrete pit where supply cables are stored outside the Unit 2 reactor was leaking from a crack in the concrete.

On Tuesday, the plant operator drilled a hole into a layer of gravel around the pit, and poured a hardening agent called liquid glass, or sodium silicate, to stop the leak.

On Wednesday morning, TEPCO said the flow was confirmed to have stopped.

"Today at 5:38 am, we have observed stoppage of spilling of water from the crack on the concrete lateral of the pit," TEPCO said in a statement. "For the sake of completeness, we put further reinforcement for the stoppage of leakage and consider countermeasure including continuous injection of coagulant."

The company said the water level in the Unit 2 turbine building and in the pit remains unchanged, indicating no further leakage. Workers are checking for other cracks that could leak radioactive water.

In sampling of seawater in front of a screen near the pit, TEPCO has found three nuclides - iodine 131, cesium 134 and cesium 137 - and is now testing for other radionucleides in accordance with directions from the Nuclear and Industrial Safety Agency.

At the same time, TEPCO is deliberately releasing about 8,000 tons of wastewater contaminated with low-level radiation into the ocean to make room in storage tanks for highly radioactive water. The company says about 6,000 tons of water have been released to date, with government approval and over the objections of the National Federation of Fisheries Cooperative Associations.

The company is spraying a synthetic resin solution on the plant's premises to prevent radioactive dust from becoming airborne. On Wednesday, the resin was sprayed over a 300 square meter (3,230 square foot) area around a spent nuclear fuel pool.

Radioactive debris and dust were spread across the facility by the hydrogen explosions at the Unit 1, 2, and 3 reactors in March.