It was the part of the Fukushima Daiichi nuclear power plant that spooked American officials the most, as the complex spiraled out of control two and a half years ago: the spent fuel pool at Reactor No. 4, with more than 1,500 radioactive fuel assemblies left exposed when a hydrogen explosion blew the roof off the building. In the next 10 days, the plant's operator, the Tokyo Electric Power Company, is set to start the delicate and risky task of using a crane to remove the fuel assemblies from the pool, a critical step in a long decommissioning process that has already had serious setbacks. The operation addresses a threat that has hung over the plant since the crisis started. It is still dangerous to have the fuel high up in a damaged structure that could collapse in another quake, experts warn. But removing it poses dangers, too. The fuel rods must remain immersed in water to block the gamma radiation they emit and allow workers to be in the area, and to prevent the rods from overheating. An accident could expose the rods and - in a worst-case scenario, some experts say - allow them to release radioactive materials beyond the plant. There are potentially very big risks involved," Shunichi Tanaka, the head of Japan's nuclear regulator, said last week. "Each assembly must be handled very carefully." "All I can do is pray that nothing goes wrong," said Yasuro Kawai, a former plant engineer who now heads a group that is independently monitoring the decommissioning process.