(AP Photo/Kyodo News) JAPAN OUT,... (Associated Press) 
 
 Fukushima Dai-ichi nuclear power plant is pictured before helicopters dump water on the stricken reactor to cool overheated fuel rods inside the core in Okumamachi, Fukushima Prefecture, Japan, Thursday... (Associated Press) 
 
 Four of the troubled Fukushima Dai-ichi plant's six reactor units have seen fires, explosions or partial meltdowns in the week since the tsunami. Japanese and U.S. concerns were increasingly focusing on the pools used to store spent nuclear fuel: Some of the pools are dry or nearly empty and the rods could heat up and spew radiation. The U.S. government said "minuscule" amounts of radiation were detected in California consistent with a release from Japan's damaged facility, but there were no levels of concern. “This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel pool,” said Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission , briefing reporters at the White House. He defended the U.S. decision to recommend a 50-mile (80-kilometer) evacuation zone for its citizens, wider than the 30-mile (50-kilometer) band Japan has ordered. Tokyo Electric Power said it believed workers were making headway in staving off a catastrophe both with the spraying and, especially, with efforts to complete an emergency power line to restart the plant's own electric cooling systems. But they couldn't be sure any of it was easing the peril at the tsunami-ravaged facility. "It's frustrating, because we don't have a goal, something to strive for. American officials, meanwhile, remained fixated on the temperature readings inside that reactor and two others that had been operating until the earthquake shut them down, as well as at the plant’s spent fuel pools, looking for any signs that their high levels of heat were going down.  YAMAGATA, Japan -- Emergency workers seemed to try everything they could think of Thursday to douse one of Japan's dangerously overheated nuclear reactors: helicopters, heavy-duty fire trucks, even water cannons normally used to quell rioters. Nearly a week after the earthquake and tsunami, police said more than 452,000 people were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short. Both victims and aid workers appealed for more help, as the chances of finding more survivors dwindled. In Washington, the State Department warned U.S. citizens to consider leaving the country and offered voluntary evacuation to family members and dependents of U.S. personnel in the cities of Tokyo, Yokohama and Nagoya.
– Japan is considering burying its troubled nuclear plant in sand and concrete—the same move made in Chernobyl 25 years ago, Reuters reports. “It is not impossible to encase the reactors in concrete. But our priority right now is to try and cool them down first,” said an official, as authorities work to restore power. Smoke has been rising from the No. 2 reactor at the plant, and authorities aren’t sure of the cause—though it may be linked to the explosion there Tuesday, notes the AP. New damage has been discovered in the fuel pool at the plant’s No. 4 reactor, hampering cooling efforts to refill it with water (click for more on the alarming situation). US data-collecting flights suggest that radiation hasn’t expanded outside a 19-mile area—but Japan has raised its threat level from 4 to 5 out of 7, pointing to danger that extends beyond the local. The crisis could continue for weeks, said a US official. Meanwhile, the Huffington Post notes, authorities are using every cooling method available, from fire trucks to water cannons.