Japanese officials are weighing a set of difficult challenges in trying to contain the nuclear crisis within the Fukushima power complex. In addition to the extremely powerful earthquake, the nuclear industry has rarely faced simultaneous partial or complete meltdowns in multiple reactors at one site.
Here are three key questions experts are grappling with:
Can Fukushima Reactor #2 be cooled without blowing the containment vessel?
While explosions have rocked two of six nuclear reactors at the Fukushima complex, designated #1 and #3, the most challenging aspect for officials is preventing a meltdown in reactor #2, where coolant levels have fallen so low today that fuel rods are exposed to air, beginning a partial or complete meltdown. (A meltdown is well explained here.)
Adding water to the reactor is crucial to maintaining safe temperatures and halting any meltdown that may have started. But that creates steam, which raises the pressure within the reactor vessel. "You don't want to destroy" the vessel, Robert Alvarez, a former senior policy adviser with the U.S. Department of Energy, explained to reporters on a conference call this afternoon.
Adding seawater, which is what crews have done to try to stop the overheating, is "the least bad option," he said (it generally makes the reactor inoperable). But an hour after Alvarez spoke, it emerged that this option failed, at least temporarily. Trouble with vents that relieve the pressure within the vessel had complicated the shutdown effort, because more water cannot be added if the pressure cannot be regulated.
Have spent fuel storage facilities been damaged?
The explosions that rocked reactors #1 and #3 were caused by high temperature hydrogen in steam reacting with oxygen. The explosions may have damaged cooling pools. The pools, as deep as 30 meters, hold fuel rods that have been used in the reactor. The rods are cooled for several years to prevent a fire or the release of radionuclides.
But after its outer building exploded today, satellite photos of reactor #3 showed fallen fuel cask cranes and damage to various concrete structures, said Alvarez. The pools are massive, reinforced structures built of concrete and steel as much as a meter thick. Were they damaged? Alvarez wondered whether apparent steam plumes visible in the photos were water coming off of the spent fuel pool.
Loss of water from the pools could lead the spent fuel to rise in temperature, causing the zirconium cladding which encases the fuel rods to catch on fire. Zirconium fires could lead to massive amounts of radioactive cesium being emitted from the spent fuel and spewed into the atmosphere as part of the smoke. "It's worrisome, but I don't think there's evidence of a zirconium fire," said Alvarez. But that risk remains real.
Why have four methods of cooling failed in the Fukushima reactor complex?
Shaking from the earthquake—or water from the tsunami rushed into the reactor complex—damaged the normal pumping system, powered by the electrical grid, that keeps coolant flowing through the reactor. That should have triggered startup of an on site diesel-powered pumping system, but that system failed as well. A third backup effort, which uses steam formed in the running of the reactor to make water for cooling, either failed or worked for only a short amount of time in several of the reactors. Finally, efforts to bring outside power through mobile generators apparently failed as well. The reason for the final failure is "unclear," says retired nuclear engineer Howard Shaffer, a member of the American Nuclear Society's public information committee, although a problem with the plant's electrical system is one possibility. One thing that seems certain, he says, is that the earthquake and tsunami were "much more than what [the reactor complex] was designed for."