Helicopters and fire trucks proved unsuccessful at replenishing damaged nuclear fuel pools at Japan's Fukushima Daiichi plant on Thursday. The spent-fuel pools contain a large amount of radioactive material that is not contained as well as that in the reactor cores. And although information has been spotty, nuclear experts worry that this fuel—which should be submerged in circulating water to keep it from overheating—has been at least partly exposed in the pools belonging to reactor Nos. 3 and 4.

In an early attempt to refill the vital pools with water Thursday, the Japan Self Defense Forces (JSDF) dispatched a cargo helicopter—specially outfitted with lead plates to help shield crewmembers from direct radiation—to drop seawater on the plant's reactor No. 3. The unit houses MOX (mixed oxide) fuel, which can melt at lower temperatures and could release some of its plutonium, which has a half-life of 24,000 years.

Later that day, the country's National Police Agency attempted to use fire trucks to pump water into No. 3's spent-fuel pools, but owing to high radiation levels, operators were not able to get close enough. Five, more robust pump trucks, sent later by the JSDF were able to move in close enough for 24 minutes to inject some 30 tons of water into the low pools. As of 9:30 P.M. local time, the "effect of this operation [was] still under evaluation," the Japan Atomic Industrial Forum (JAIF) reported.

As of 10 P.M. local time on Thursday, the JAIF listed the following status of the six Fukushima Daiichi reactors:

•    Buildings around reactor Nos. 1, 3 and 4 were "severely damaged"; the building housing reactor No. 2 was "slightly damaged";
•    Cooling was not working for reactor Nos. 1, or 3;
•    Water levels were covering more than half of the fuel in reactor No. 2; reactor Nos. 1 and 3 water levels were covering only about half of the fuel.
•    Structural integrity of the spent fuel pools was unknown for reactor Nos. 1 and 2;
•    Reactor Nos. 3 and 4 had low water levels; pool temperature was continuing to rise for reactor Nos. 5 and 6.

The spent fuel pools are of significant concern, Marvin Resnikoff, a radioactive waste management consultant, said in a Wednesday press briefing organized by the nonprofit organization Physicians for Social Responsibility. Resnikoff noted that the pools at each reactor are thought to have contained the following amounts of spent fuel, according to The Mainichi Daily News:

•    Reactor No. 1: 50 tons of nuclear fuel
•    Reactor No. 2: 81 tons
•    Reactor No. 3: 88 tons
•    Reactor No. 4: 135 tons
•    Reactor No. 5: 142 tons
•    Reactor No. 6: 151 tons
•    Also, a separate ground-level fuel pool contains 1,097 tons of fuel; and some 70 tons of nuclear materials are kept on the grounds in dry storage.

The reactor cores themselves contain less than 100 tons of fuel, Resnikoff noted.

The fuel had been moved from reactor No. 4's core to its spent-fuel pool recently, so "that fuel is relatively fresh and hotter, thermally," Resnikoff explained. "So it's not surprising that when the water [was] no longer circulating that the water was actually boiled off in a zirconium exothermic reaction, that the zirconium burned" (occurring at about 1,800 degrees Celsius).

Scientists are not confident that they will be able to assess just how much radioactive material will have been released as this event unfolds, David Richardson, an epidemiologist at the University of North Carolina at Chapel Hill's School of Public Health, said in the press briefing. It might not be until people can safely take stock of all of the fuel that is left, and then only "by that we can make a reckoning of what was lost," he said.

As of midday Thursday, the country's Nuclear and Industrial Safety Agency spokesperson Yoshitaka Nagayama, noted that "because we have been unable to go to the scene, we cannot confirm whether there is water left or not in the spent fuel pool at reactor No. 4," The New York Times reported. As of March 16, there had been at least two fires suspected at that reactor.

If the burning-hot fuel is not covered by adequate water, the heat from the ongoing nuclear reactions can cause the water to boil off. "Water in the pool serves as shielding and cooling, and when that water is gone, that direct gamma radiation is very high," Resnikoff said.

Resnikoff was skeptical at the briefing that helicopters would be an effective way to stave off overheating in the spent-fuel pools. "Part of the roof still remains, and they cannot just dump water into the fuel pools" from the air, he said.

In a congressional testimony yesterday, Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission (NRC) said of the conditions at the plant: "We believe that radiation levels are extremely high."

In any case, the NRC recommended Wednesday that U.S. residents in Japan within 80 kilometers of the facility should evacuate. American military personnel were being kept at least at this distance from the site. The Japanese government had evacuated residents within a 20-kilometer radius (and recommended those 20 to 30 kilometers away to remain indoors).

Yukio Edano, Japan's chief cabinet secretary, said that the U.S.'s "more conservative decision" to move U.S. residents farther away from the plant is "understandable," CNN reported.

Governments, agencies and many in the public have complained about the paucity of data being made available by the Japanese government and Tokyo Electric Power, the company that owns and operates the facility. But as Resnikoff pointed out, many of the radiation sensors are located on the nuclear plant's site and may well have been damaged during or since the March 11 magnitude 9.0 earthquake. "So it's not surprising that we're not getting the numbers we want."

And whether or not the 50 tons of water dumped on reactor No. 3 was enough to temporarily cool the spent fuel pool, the efforts will need to continue to avoid a significant release of radiation. "This is a several-months problem," Resnikoff said. "The heat values will be high for months—high enough to cause an exothermic reaction. So this is going to be a continual problem."

At least, the threat of radioactive release in Japan seems to be contained to the Fukushima Daiichi facility (Japan has a total of 54 nuclear reactors at various facilities). "Other nuclear power plants in Japan are in normal operations or safely shutdown," the JAIF reported Thursday.