Meltdown 101: What are spent-fuel pools and why are they a threat?

An explosion and fire at Japan's stricken Fukushima I nuclear power plant has added spent fuel to the roster of crises that workers there are struggling to control.

The explosion, which hit at 6:14 a.m. local time Tuesday, was likely caused by a build-up of hydrogen gas inside No. 4's reactor building, according to Japan's Kyodo News Service, citing officials with the Tokyo Electric Power Company, which operates the facility. Others posit that the fire involved oil and other lubricants in the building.

The explosion blasted a pair of holes, 26 square feet each, in the reactor building wall. Just over two hours later, fire broke out.

The concern: Water levels in the pools holding spent fuel may be boiling off, threatening to expose the fuel. The explosion and fire, as well as an explosion at the No. 2 reactor, reportedly pushed radiation levels at the plant to levels that prevented workers from pouring additional water into the pool.

Spent-fuel pools shielded only by outer reactor building

Spent-fuel pools at the plant sit in the upper reaches of the reactor buildings, near the top of the reactors so cranes have easy access to load and unload fuel.

Most of the radiation shielding at the reactors comes from the thick steel reactor vessel itself, which sits inside a steel-and-concrete containment structure. The upper reaches of reactor buildings represent the weakest barrier in the plant's "defense in depth" approach to keeping dangerous levels of radioactivity out of the environment.

As a result, under the right conditions, the spent-fuel pools at the plant potentially pose as large a threat of environmental contamination – if breached – as the multiply-shielded reactor cores themselves. In particular, two of the reactors – No. 1 and No. 3 – have experienced explosions that blew holes in their roofs and upper levels.

Many nuclear power plants in the United States have a beefier final layer of protection – containment domes of very thick concrete.

How do these 'spent-fuel pools' work?

Each of the six reactor buildings at the Fukushima I plant has its own spent-fuel pool, which holds fuel rods that no longer produce enough energy to be useful, but are still too hot – and too radioactive – for safe disposal.

Ordinarily, pumps circulate water through each spent-fuel pool to carry the heat away. But if the water level in the pool drops too low, the decay heat can begin boiling off the water. If the water level drops enough to expose the metal housing containing the fuel assemblies, the assemblies can heat to the point where the fuel-rod casings catch fire, releasing radioactive gases and particles to find their way into the environment.

Since the magnitude 9.0 earthquake and subsequent tsunami wiped out the sources of emergency power that would have kept cooling pumps operating, the only way to keep cool water in the pools is through fire hoses. Despite workers' best efforts, temperatures Tuesday were rising in the spent-fuel pools of reactors No. 5 and No. 6 at the plant, as well.

Fuel rod assemblies

Each "fuel assembly," roughly 15 feet long, is a unit containing 82 fuel rods full of the reactor's fuel: uranium oxide pellets. During periodic refueling shutdowns, workers typically replace 20 to 30 percent of the fuel assemblies.

Once extracted from the reactor, the used assemblies are housed in close-fitting steel containers that are treated with boron, to ensure they don't resume the chain reactions necessary to generate electricity.

The used assemblies are then submerged in 45 feet of water in a spent-fuel pool. The water acts as a radiation barrier, in addition to serving as a coolant.

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