Nuclear officials believe a hydrogen explosion may have occurred at Japan's Fukushima Dai-ich nuclear plant crippled by massive earthquake. Fukushima I is one of the 25 largest nuclear power stations in the world. It was the first nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO). In Japan, the desperate moves to avert a nuclear disaster continue, with emergency resources also heavily stressed by the impacts of the initial earthquake and tsunami - the catastrophe named by the Japanese government as the worst endured by the country in over 50 years. Engineers are working to keep control of cooling of reactor cores, with three facilities badly damaged as a result of the natural disaster, causing urgent concern. Japanese stations are constructed to specifications for resisting earthquake shocks well beyond the high point of 7 on the Richter scale, though the one that struck on Friday was at the high end for all records kept in the world since 1900, at 8.9. The Japanese government declared a "nuclear power emergency" due to a loss of coolant and evacuated thousands of residents living close to Fukushima I.