the 9.0 magnitude earthquake

northeast Japan. Although the plant’s three functioning reactors shut down automatically when they detected the quake, the tsunami that followed swamped the diesel generators that provided backup power to the reactor cooling systems. Crews eventually were able to restore backup power, but problems keeping the reactors cool eventually forced plant officials to take the drastic step of flooding them with seawater in a bid to keep the temperatures down. Still, pressure buildups, problems with valves and even a failure to fill a generator’s gas tank have led to explosions and other problems with keeping the reactors under control. Tuesday’s incidents appeared to escalate the situation: Edano said the radiation releases from the explosion and fire were the first that appeared to pose a threat to human health, if only briefly. Radiation levels also spiked Monday, after workers vented steam to release pressure, but the levels quickly dropped, officials said. Also Monday, an explosion in the building housing the plant’s No. 3 reactor apparently damaged both a water-filled chamber at the base of the reactor and the reactor containment unit itself, IAEA Director General Yukiya Amano said at a news conference Tuesday. Damage to the core involved about 5% of the core’s nuclear fuel, Amano said.