Workers at Japan's stricken nuclear plant today pumped nitrogen into a crippled reactor in a bid to prevent a possible explosion and contain the world's worst atomic accident in 25 years. With the crisis at the Fukushima Daiichi plant approaching the end of its fourth week, operator Tokyo Electric Power Company said it was concerned a build-up of hydrogen gas at the No. 1 reactor could cause another explosion at the site. The fresh challenge underscored the fragility of the situation at the stricken plant, after rare progress was seen yesterday when workers plugged a hole spewing highly radioactive water into the ocean. TEPCO officials said hydrogen building up in the housing around reactor No. 1 could mix with incoming oxygen, creating an explosion. Experts said the risk of a detonation could rise as the nuclear fuel rods cool and as the steam inside the containment vessel condenses into water, reducing pressure inside the unit and drawing air in through cracks. Workers began pumping in nitrogen, an inert gas abundant in the atmosphere, which they hope will displace the oxygen. The process to inject 6,000 cubic metres will take around six days. TEPCO said, it was also planning to inject nitrogen gas into reactors number 2 and 3 as a protective measure. Meanwhile, Chinese authorities have detected extremely low levels of radiation from Japan's crippled nuclear power plant on spinach grown in parts of China. The health ministry issued a statement late last night saying tests on the vegetable grown outdoors in Beijing, the northern city of Tianjin and in the central province of Henan had revealed traces of radioactive iodine-131, but the amount of contamination posed no health hazard.