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When I was in the U.S. Navy, 1959-62, I was stationed at Naval Air Station, NAS, now called Naval Air Facility, NAF, Atsugi Japan. Naval Air Facility Atsugi is co-located with the Japanese Maritime Self Defense Force. It is located in Kanagawa Prefecture, in Ayase City, which is About 16 km west of Yokohama and about 36 km southwest of Tokyo. During that time I also was temporarily assigned, TDY, to a small gunnery range in Ibaraki Prefecture, in the small town of Mito on the sea shore. Both places have been affected by the earth quake and tsunami and will certainly be affected by the situation at the power plant.

From Science magazine, April 1:

Of all the terrible news from the crippled Fukushima Daiichi nuclear power plant, reports about the spent fuel storage pool for reactor #4 may be among the most disconcerting for scientists. The pool held the entire complement of fuel rods from the reactor's core, which had been emptied 3 months before the 11 March earthquake and tsunami struck. And yet on 15 March the building exploded, apparently fueled by hydrogen, leaving nuclear engineers to speculate about the source. Adding to the confusion are reports of fires in the pool, a worst-case scenario that had never before occurred in a working nuclear plant.

Unraveling the mysteries surrounding the #4 pool will require discovering why water levels there fell so quickly, and whether the 230 tons of spent nuclear fuel melted in addition to catching on fire. Researchers also need to quantify how much radioactive material might have been released. The events at the #4 pool could shed light on the dangers posed by spent fuel in pools at more than 350 reactors globally, and what needs to be done to assure the public that they can be operated safely.

As Science went to press, extremely high levels of radiation were hampering efforts to restore power and water lines to all six reactors at the complex. And while a possibly damaged core in reactor #3 was leaking highly radioactive water, reactor #4's pool remains among the biggest potential sources of radiation. In video released early this week, what appeared to be steam continued to billow from reactor #4's blown-out frame despite continued efforts to add water.

Hot fuel

The nuclear fuel at reactor #4 is made up of uranium pellets held in 4-meter-long tubes made of zirconium alloy. The pool holds 1331 bundles of tubes, known as assemblies; 548 were removed from the reactor in January during maintenance.

Daiichi has seven spent nuclear fuel pools: one for each of its six reactors, and a central one. They serve two main purposes: to cool the fuel, which gives off heat as a result of radioactive decay, and to shield workers and the environment from radiation. In reactors with Fukushima Daiichi's design, pools are not in sealed containment vessels but are open and accessible; operators are keenly aware of the importance of keeping water levels high. (Although the pool at reactor #3 may also have produced hydrogen, the other five pools have required constant replenishment but remained stable.)

One mystery about the #4 pool is how its water level fell so quickly. During normal operation, 7 meters of roughly 40°C water sit between the top of the fuel rods and the surface of the 1425-ton pool. The water is constantly circulated and replenished. There's little doubt that temperatures in the pool would have risen steadily after power was lost. But several scientists have independently calculated that it would take much longer than 4 days—perhaps as much as 3 weeks—for the heat of the fresh fuel in the #4 pool to evaporate or boil off the water.

Could the #4 pool's structure have been damaged in the quake or subsequent explosions or both? Among possible weak points are the large doors on the side of the pool. The doors, which allow fuel to pass underwater from the reactor into the pool, are held shut by rubber gaskets inflated by electric pumps. In 1986, at the Hatch nuclear plant near Baxley, Georgia, the water level in a spent fuel pool dropped by more than a meter after the seals were left uninflated. But engineers say that the channel between the reactor and the pool is filled with water during maintenance periods, meaning that a leak would lengthen the time it took to empty the pool. “It's surprising to me [that] the fuel became uncovered that quickly,” says Lake Barrett, a retired nuclear engineer and former U.S. Nuclear Regulatory Commission (NRC) official.

Determining the temperature of the pool has been another challenge. (It was reported to be 84°C the day before the explosion; no data have been subsequently released.) Knowing that temperatures were high enough to drive water completely from the pool could help researchers quantify how much radioactivity was released in subsequent steps. The day after the 15 March explosion, NRC Chair Gregory Jaczko said that the pool had run dry at one point, a claim that the Japanese government has disputed.

At least one fire in the pool—and possibly a second—was reported by power company officials after the explosion. Lab experiments have shown that zirconium can burn either with steam or with oxygen. Both reactions progress rapidly at roughly 800°C; the former, crucially, releases hydrogen. The hydrogen explosion at reactor #4 points to the steam reaction, which releases less energy and therefore melts the fuel more slowly. But knowing which reaction dominated could help scientists quantify how much radioactivity was released from pool #4. A 2006 study by the U.S. National Research Council said that a heat up after a loss-of-water event could melt the spent fuel, allowing the escape of volatile radionuclides, including “a substantial fraction of the cesium,” into the air.

Soil samples analyzed by the Japanese science ministry last week found cesium levels roughly equivalent to 8 million becquerels per square meter near the plant. That level, if accurate, would be higher than those found near Chernobyl. (Radioactive iodine has been found in Japanese tap water, but its 8-day half-life means that it couldn't have come from the older spent fuel in pool #4. Reactor #3 may be the source.) Scientists also don't know what fraction of the radioactive cesium released is from reactors or from spent fuel. Scientists hope more detailed isotopic measurements will shed light on the age and, therefore, the source of the radioactive particles.

Minding the pools

Some experts believe that governments and the nuclear power industry have done a poor job of sharing information on the risk of zirconium fires. Critics of NRC say that studies conducted for the agency likely contain relevant data but have been kept classified to keep the information away from terrorists. “To the extent that any experiments have been done at all, the public doesn't know about them,” says spent-fuel expert Gordon Thompson of Clark University in Worcester, Massachusetts. The National Research Council study called on NRC to “improve the sharing of pertinent information” on pool risks.

The calamity at Fukushima Daiichi has raised particular concerns about U.S. spent nuclear fuel pools, which are thought to be packed more tightly than those in Europe or Asia. “Spent nuclear fuel may be more vulnerable than we thought,” says Edwin Lyman of the Union of Concerned Scientists in Washington, D.C. The nuclear industry has added additional sprayers to pools and now mixes hot, fresh spent fuel with older fuel in the pools to redistribute the heat. But it balked at a 2008 recommendation by Jaczko—speaking in an unofficial capacity—to transfer U.S. fuel older than 5 years to dry concrete casks, where the cooled fuel is highly unlikely to catch fire. Still, the industry has begun a safety review of U.S. reactors, and NRC has launched two studies into U.S. plant safety that could lead to new rules on spent fuel.

-- Eli Kintisch, with reporting by Dennis Normile in Tokyo.


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