 JaPaN was fighting to stop a nuclear meltdown last night at two reactors crippled by a giant earthquake and tsunami that are thought to have killed thousands of people.  a huge explosion blew apart the containment building walls around one of the reactors at the Fukushima 1 nuclear plant yesterday afternoon, panicking local people and raising the alarm around the world. It was triggered by an aftershock from Friday's 8.9-magnitude earthquake. Last night the cooling system of a second reactor at the Fukushima 1 plant failed and there were fears about the safety of a second plant nearby.  The blast occurred as engineers tried to cool the hot core of the 40-year-old reactor after automatic coolant systems failed. It could have been caused only by a partial meltdown of the reactor's core, said Japan's Nuclear and Industrial Safety agency.  Initially, four workers were injured and three people were exposed to radioactive material but, by early today, 160 people were reported to have been contaminated.  about 200,000 residents have been moved from the area around the two nuclear plants in the Fukushima prefecture amid fears of a large-scale release of radiation.  By early today 3,000 people had been rescued from the quake area.  as the number known to have died throughout the disaster zone rose to 1,700, it emerged that more than 9,500 people were officially listed as missing from one town alone. The figure, released by officials in Minamisanriku, suggested more than half its population of 17,000 may have been lost. Only a few concrete structures, including the hospital, remain standing.  a powerful series of aftershocks that affected buildings in Tokyo last night rocked the area around the Fukushima 1 plant. Radioactive caesium and iodine were detected around the facility.  The Japanese government was reported to be distributing potassium iodide tablets to prevent radiation sickness - a highly sensitive subject in the only nation ever to have come under nuclear attack.  an unprecedented operation began to pump sea water into the reactor container in an attempt to avert a meltdown. a turbine made by Hitachi will be brought into speed the process.  Boric acid will also be added to stop the atomic chain reaction, according to Yukio Edano, Japan's chief cabinet secretary, who predicted that the shutdown could be completed within a week. He claimed that the explosion had not damaged the reactor itself and said overall radiation leakage would be low.  However, Walt Patterson, a nuclear physicist at the Chatham House think tank in London, warned that the operation was not guaranteed to succeed and the risk of a meltdown remained.  Too little was known about the status of the fuel in the reactor core, warned Mark Hibbs of the Carnegie Endowment for International Peace. "The issue is whether the core is uncovered, whether the fuel is breaking up or being damaged, or whether the fuel is melting," he said.  John Large, a British nuclear consultant, disputed the authorities' claims that the reactor was undamaged. "This plant has been devastated by an explosion, it's lost all its containment and I would expect to see a significant amount of radioactive release," he said.  Japanese officials said last night that levels of radiation had fallen at Fukushima 1 but admitted that they could not control the pressure inside the nearby Fukushima2 plant, raising the spectre of a multiple meltdown.  a meltdown occurs when uranium fuel rods become so hot during atomic fission that they burn through the floor of the heavy steel reactor shell and spread radiation into the environment.  Chris Huhne, the energy and climate change secretary, said Britain was ready to send nuclear physicists and other experts to help the Japanese authorities tackle the worst incident in the history of their nuclear programme. Hundreds of Britons are unaccounted for and British humanitarian aid teams are due in Japan today.  The two plants in Fukushima prefecture were in the p 