As the massive earthquake and Tsunami in Japan has left 1600 people dead or unaccounted for, a nuclear power plant has exploded triggering a nationwide atomic alert.With the country already reeling under the impact of giant tidal waves, the main reactor at Fukushima nuclear plant in northern Japan, 250 kilo metres north of Tokyo, exploded this afternoon leading to radiation leaks. Initial reports say that the leak has injured four people. Authorities there have told 45,000 residents living within a 10-km radius of the leaking plant to evacuate their homes. Kyodo news agency reported that the plant is experiencing a nuclear meltdown. This has been confirmed by Japanese nuclear scientists confirmed, after Prime Minister Naoto Kan visited the site in a helicopter early today.Reports said the main building of the plant housing the reactor and the outer walls were blown away in the high intensity explosion, which occurred at noon Indian Standard Time. The cooling water levels dropped alarmingly through evaporation after the quake had damaged the main plant. Jiji Press reported that the loss of cooling water is leading to a meltdown of the reactor.TV footage showed smoke billowing from the site and reported that the reactor building had been destroyed. Nearby residents have been warned to turn off air conditioners and not to drink tap water.People going outside were told to avoid exposing their skins and to cover faces with masks and wet towels.Kyodo quoting police said that another nuclear plant adjacent to the one which exploded is also malfunctioning with the radiation level reaching almost 1000 times the normal level.The authorities said, an emergency has been declared at all five nuclear reactors as the units had lost cooling ability, while rushed plane loads of coolants to the disaster struck nation. They said that Fukushima, Daiichi plant unit no. 1 had exploded after failing to bring down heat and pressure inside the reactor which had suffered extensive damaged due to tidal waves set off by the earth quake which cut down the power supply to the plant. Japan's Nuclear Safety Agency described the situation at the Fukushima plant dire.Defence Ministry officials said dozens of troops and fireman trained for chemical disasters have been dispatched to the plant, which is located South of the Miyagi Prefecture which was hardest hit by the quake. Despite Japan has strict set of regulations designed to limit the impact of quakes on nuclear plants,10 of the Japan's 54 commercial reactors were shut down because of the quake.Meanwhile, four trains running in a coastal area of Miyagi and Iwate prefectures remained unaccounted for. Train operator said it is not known how many people were aboard the trains that were running on East Japan Railway Co.'s Ofunato, Senseki and Kesennuma lines on the Pacific coast when the quake hit northern Japan. The company said earlier that another train on the Senseki Line was found derailed near Nobiru Station after the quake. The Miyagi prefectural police today rescued nine passengers from the train by helicopter.The number of partially or completely destroyed buildings reached 3,400, while there were 200 fire incidents at quake-affected areas. Some 181 welfare facilities, including nursing homes, have been damaged.As reported, around 200 to 300 bodies were found in Sendai, the closest town, which bore the brunt of the disaster. Some 1,800 houses in Fukushima Prefecture were found to have been destroyed.As rescuers have not been able to completely access the tsunami-hit areas, the overall picture of the destruction remained unclear.A municipal official of the town of Futaba, Fukushima, said, more than 90 per cent of the houses in three coastal communities have been washed away by tsunami. In the quake-hit areas, around 5.57 million households have lost power, while 6 lakh had their water supply cut off. Nine expressways were closed and at least 312 domestic flights cancelled. The Tokyo police said more than 1 Lakh 20 thousand people in the capital were unable to return home last evening due to the suspension of train operations and traffic jams.Rescue teams from South Korea, Australia, New Zealand, Singapore and other countries were set to arrive in Japan, after 50 nations offered support following the powerful earthquake, the Japanese Foreign Ministry said. A mammoth relief mission is swinging into action in north-east Japan.The Japanese Prime Minister said that today is a critical day for rescue teams to find survivors. He said the 50,000 rescue personnel deployed to the hardest-hit regions, including Japan's Self Defence Force, will do their utmost to help those in need.