As night fell on Friday in Japan, workers and soldiers continued heroic efforts to douse the potential meltdown underway at the Fukushima Daiichi nuclear power plant. The covering darkness is not the only reason for confusion: vital systems monitors have lost power, making the status of critical elements—such as the integrity of the nuclear fuel rods in reactor No. 2 or of the steel vessel containing them—unavailable.
But what measurements are available are worrying. Electrical pump systems for cooling water remained inoperative as of 6 pm Eastern, although Reuters reported that Tokyo Electric Power Company succeeded in laying a giant extension cord to the stricken nuclear power plant early Saturday morning in Japan, which could be used to power up the site's pumps for cooling water. That might represent the best hope for cooling down the fuel rods in reactors No. 1, No. 2 and No. 3 as well as the used rods stored in pools in No. 3 and No. 4.
At the same time, the earthquake, subsequent tsunami, hydrogen explosions and seawater flooding, among other recent incidents, may have damaged the electrical equipment necessary for those pumps to run. Otherwise, portable diesel generators transported to the site should have enabled pumping to resume.
The buildings housing reactors Nos. 1, 2, 3 and 4 have all suffered damage from hydrogen explosions, caused by the high-temperature, high-speed interaction of fuel rods and steam. With the roofs off and other paths for hydrogen to escape—hydrogen is the only element capable of escaping Earth's gravity—the chance of suffering another such explosion has been diminished, except at reactors No. 5 and 6. But those reactors' spent fuel pools are benefiting from a diesel generator that is still working to keep cooling water in place, according to World Nuclear News, though temperatures are beginning to rise in these pools as well.
Steam continues to rise in the spent fuel pool at reactor No. 3, indicating that at least some water is reaching the hot rods. Workers spray, allow the radioactive steam to dissipate and then spray again—radiation levels near reactor No. 3 are the highest at the plant, roughly 400 milliSieverts per hour. Worse, reactor No. 3 also employed so-called mixed oxide fuel—or uranium fuel rods made from recycled materials that include plutonium—creating extra cause for concern of nuclear fission restarting itself. Reactors employing mixed oxide fuel may require extra control rods, made of boron or other neutron-absorbing materials, to slow down more fissile plutonium.
The water level is low at reactor No. 4's spent fuel pool as well. The Fukushima Daiichi site in total has some 11,000 such fuel rods in the seven spent fuel pools—500 or more of which in reactor No. 4's spent fuel pool are still quite hot having only been removed from that reactor in December, according to The New York Times. TEPCO may have made the problem worse, according to some experts, by storing more spent fuel in some of the pools than they could safely cool. Such "re-racking" is a common practice in the U.S. nuclear industry, under regulation from the U.S. Nuclear Regulatory Commission, given the lack of an off-site solution for storing used nuclear fuel rods. In the event of a loss of water, such re-racking can make it more difficult to prevent used fuel rods from overheating, according to a 2006 study by the National Academy of Sciences.
The Japan Atomic Industrial Forum also estimates that the fuel rods actually in reactors Nos. 1, 2 and 3 have boiled away even the seawater and boric acid workers have injected via lines that pass into the sealed concrete and steel containment structure to put out fires. Those fuel rods are now likely exposed and generating heat, on their way to a potential meltdown.
Such a boil off would suggest that pressure should be rising inside the steel and concrete structures that contain each of these nuclear reactors. In fact, atmospheric pressure rapidly increased eightfold before the explosion in reactor No. 1 on March 12 (nuclear power plants typically operate at roughly 4 atmospheres of pressure). But pressure readings—and none are available for reactor No. 1, the first to melt down—are low for reactors No. 2 and No. 3, suggesting that cracks or other holes in containment may have formed.
And that means there may be two direct paths for radioactive particle byproducts of nuclear fission, such as cesium 137 and iodine 131, to escape and spread radiation—cracks in containment as well as the spent fuel pools now open to the air.
Worse: without cooling, the fuel rods continue to meltdown and may completely burst the zirconium cladding—a ceramic material—that holds them together. At that point, the uranium pellets would fall to the bottom of the containment vessel and continue to overheat, potentially melting through the steel vessel itself eventually.
That, in itself, would not cause an explosion. The danger is that the hot melted fuel would fall into water, instantly vaporizing it and causing a steam explosion. Such an explosion would then waft the uranium and other radioactive particles into the air.
"We are making utmost efforts to prevent further explosions or the release of radioactive materials," the Japanese Prime Minister, Naoto Kan, said in a televised address to the nation on Monday morning, and those efforts continue.
But even should such a steam or hydrogen explosion somehow occur, the radioactive plume created would likely not travel high enough into the atmosphere to spread the dangerous materials very far.
"If the Japanese fail to keep the reactors cool and fail to keep the pressure in the containment vessels at an appropriate level, you can get this, you know, the dramatic word 'meltdown.' But what does that actually mean?" Sir John Beddington, chief scientific officer to the U.K. government told British embassy workers on March 16. "In this reasonable worst case you get an explosion. You get some radioactive material going up to about 500 meters up into the air."
He continued: "If you then couple that with the worst possible weather situation, i.e. prevailing weather taking radioactive material in the direction of Greater Tokyo and you had maybe rainfall which would bring the radioactive material down, do we have a problem? The answer is unequivocally no. Absolutely no issue. The problems are within 30 kilometers of the reactor." After all, 30 kilometers was the extent of the spread of dangerous radioactive material even at Chernobyl, a far worse nuclear accident that included an intense fire that wafted radioactive particles more than 9,000 meters into the air.
As a result of these ongoing problems, Japan's Nuclear and Industrial Safety Agency has raised the International Nuclear Event Scale rating for the Fukushima disaster from a 4 to a 5, the same level as Three Mile Island and meaning an accident with "wider consequences" that might include "several deaths" from radiation. Of course, even in the worst case, the Fukushima nuclear crisis pales in comparison to the 7,000 dead and 10,000 missing as a result of the March 11 earthquake and subsequent tsunami—no one has yet died from the Japanese nuclear accident.
And time is on our side. The steam indicates that at least some cooling is going on and the heat of such fuel rods drops off dramatically over a period of days or weeks. Though cooling will need to continue for months, the risk of a catastrophic meltdown diminishes with each passing hour.
Distance also helps, radiation decreases exponentially with distance from the source. While levels are dangerously high near reactor No. 3—400 milliSieverts per hour—levels at the border of the power plant grounds drop to just 0.6 milliSieverts an hour. But any radioactive particles that escape will carry their own radiation with them wherever the wind blows, until the rain washes them out of the sky. Hence the distribution of iodine pills to local residents in Japan in order to block any uptake of radioactive iodine. The Japanese diet may also help here: seafood and seaweed carry high levels of safe iodine.
Ultimately, Fukushima may share the fate of Chernobyl, entombed in concrete and sand while radioactive decay—and heat generation—continue. Such shielding is the final line in radiation defense.