The Japanese government has made all efforts to prevent a nuclear disaster on Sunday as a reactor of Fukushima No.1 nuclear power plant is close to explode after the devastating 9-magnitude earthquake on March 11.

The government rated the incident at Fukushima No.1 nuclear power plant at level four on an international scale of zero to seven on Sunday, in a sign that radiation risks are mounting in Japan.

Two radioactive substances, cesium and radioactive iodine, have been detected near the No.1 reactor of the plant, where radiation rose to as high as 1,204 micro sievert, compared with the legal limit of 500, according to Tokyo Electric Power Co.(TEPOC), operator and owner of the Fukushima plants.

The No. 3 reactor at the plant lost its ability to cool the reactor core earlier Sunday, becoming the sixth reactor that lost the function. And a hydrogen explosion is possible at the No. 3 reactor, Japanese Chief Cabinet Secretary Yukio Edano said Sunday.

Sea water has been injected into the reactor to prevent it from overheating, said Edano. "This will result in some radiation leakage, although at a level that won't affect peoples' health. It will help stabilise the situation," he said, acknowledging the core of the reactor may have been deformed due to overheating.

Pressure has been successfully released at the reactor following the injection of water, according to the latest Sankei Shimbun report.

Earlier, Edano said at an emergency press conference that the radiation briefly jumped to 1, 204 micro sievert at the plant. The number of people exposed to radiation has risen to 22. Authorities prepared to distribute iodine to protect people from radioactive exposure.

Rescue workers were scanning people arriving at evacuation centers for radioactive exposure. In the neighboring Ibaraki Prefecture, radioactive monitor was set up.

Two experts from the US Nuclear Regulatory Commission were headed for Japan to check on the problem.

The top government spokesman warned that there are still 114 people staying within a 10-km radius of the Fukushima No.1 nuclear power plant and 180,000 in the 20-km evacuation zone.

All inhabitants have been removed from a 3-km radius of the No. 2 plant, and authorities have begun evacuating more than 30,000 from a 10-km zone around the plant, he added.

A hydrogen explosion occurred at the No. 1 reactor at the plant Saturday, blowing away the roof and walls of the building that houses the container of the reactor and injuring four people.

The No.1 reactor was also filled with sea water and was poured in boric acid to prevent an occurrence of criticality.

The Japanese government continued Sunday to wrestle with extensive damage from the great earthquake and ensuing enormous tsunami that hit northeastern and eastern regions Friday.

Japanese Prime Minister Naoto Kan planned to double the number of Self Defense Forces personnel sent to quake-hit areas to 100, 000.

Foreign countries have started to send disaster relief teams to help Japan.

Police said that more than 2,000 have died or remained unaccounted for so far while the government death toll has risen to almost 800. More than 170,000 people have been evacuated from the northeastern coast affected by the quake and the tsunami that followed.