This article first appeared in the St. Louis Beacon, April 8, 2011 - In the days since the twin earthquake and tsunami struck Japan, news of the resulting leaks of radioactive material from Japanese nuclear facilities and worries about food contamination have flooded the airwaves. That has left many in the U.S. fearing the worst about friends and loved ones in Japan -- and about the health risks here at home.
Last week, a trio of Washington University scientists sought to clear up misconceptions and give the public a general overview of the science behind Japan's nuclear crisis in a public lecture at the university. The speakers were Michael Wysession, a seismologist and associate professor of earth and planetary sciences; Lee Sobotka, a nuclear scientist and professor of chemistry; and Dr. Henry Royal, a physician and professor of radiology at the Washington University School of Medicine.
Radiation can be harmful, but it depends on the exposure
Some reports on the devastated Fukushima Daiichi nuclear power plant compare the event to the 1986 nuclear accident at the Chernobyl nuclear plant in the former Soviet Union. A nuclear meltdown there leaked enough radioactive material to kill more than 50 people from direct exposure. It also contaminated nearby areas for years to come.
Royal understands why people might be concerned. "What could be more frightening to a mother than to have someone check their child to see if they're contaminated by radioactive material?" he asks.
One of the common elements leaked in nuclear disasters is iodine-131, a radioactive form of iodine that is a common byproduct in spent nuclear fuel. About 2 million times more iodine-131 escaped in Chernobyl than in the Three Mile Island disaster. Fukushima probably falls somewhere in between, Royal says. He says the Fukushima disaster will likely be worse than the 1979 Three Mile Island nuclear incident in Pennsylvania. But he adds that Fukushima won't be anywhere near as bad as Chernobyl -- mainly because far less radiation has leaked from the Fukushima plant than in the Chernobyl disaster, and the Fukushima reactor vessels have so many more structural safeguards than did Chernobyl's.
A common measure of radiation exposure is the millisievert. Royal says that full-body exposure to 2 millisieverts corresponds to an additional 1 in 10,000 chance of dying. That works out to about an average of 0.03 millisieverts a year in a 70-year lifespan.
Humans get about 1 millisievert a year just from background radiation, such as cosmic rays from outer space. With radiation from the radioactive element radon included, that number is about 3 millisieverts a year, according to Royal.
For a dose to be considered lethal, it must be several thousand millisieverts or more in a short period, according to experts. Radiation exposure briefly reached that level in the Chernobyl disaster, eventually killing a number of nuclear plant workers and firefighters.
Some media reports have raised concerns that workers at the Fukushima plant will die within weeks just from the radiation doses they have been getting during their efforts to control the nuclear crisis.
Dr. Henry Royal says the workers' radiation exposure may increase their risk of cancer in the long term, but it's almost certainly far from lethal in and of itself. "I can't imagine any situation in which anyone working at the Fukushima plant would die from radiation exposure," he says.
In cities near the plant, radiation doses in millisieverts an hour have been far higher than the normal background radiation dose of 0.1 microsieverts per hour -- one-ten-thousandth of a millisievert -- but nowhere near the kinds of doses released in Chernobyl. One town northwest of the plant was found to have some radiation levels that met International Atomic Energy Agency criteria to prompt an evacuation. IAEA data from April 4 show some hourly doses around 0.7 to 12.5 microsieverts an hour in cities 30 to 40 kilometers south and southwest of the Fukushima plant. Graphs from the IAEA have shown a gradual decline in dose rates since the disaster started.
Despite concern about food contamination, IAEA measurements show that most of Japan's food supply is safe. Food samples taken from 11 prefectures in Japan either had no detectable levels of radioactive iodine and cesium or the levels were below Japanese regulatory standards.
Royal says it's not surprising that some spinach might have suffered contamination. That's because individual leaves are extremely thin, leaving a high percentage of each leaf's mass susceptible to being covered with radioactive material. He's more skeptical about claims that water has been contaminated severely, largely because reservoirs are so deep, leaving only a tiny proportion of water exposed on the surface.
As for recent reports that radioactive water had leaked from the plant into the ocean, Royal says it's not likely to cause the public any harm. Not only will the ocean vastly dilute the radioactive material, but also any seafood that comes from the ocean will be monitored to make sure it's safe to eat, he says.
While radiation doses will be relatively low near the Fukushima plant, it will be even lower for those in the United States. As Royal says, "Distance is our ally," because the radioactive material will get diluted in the air the more it spreads out from the plant.
Still, reports have emerged of people across the country stocking up on food and supplements to protect themselves from the worst.
The U.S. Environmental Protection Agency has detected trace amounts of radioactive material in the atmosphere in the continental U.S. But the agency has recently said, "These detections were expected and the levels detected are far below levels of public-health concern," and only slightly above normal background levels.
Even if the radiation levels were high, how much can it harm health? "Radiation is a relatively weak carcinogen, and it's hard, even when large populations are exposed to radiation, to measure these effects," Royal says.
Humans are exposed to so many carcinogens in their daily lives -- chemicals in food and household products, pollutants, among others -- making it hard to figure out which ones, if any, are causing the cancer.
Not only that, but cancer is very common. About 40 percent of people will get cancer in their lifetimes, and 20 percent will die of it, he says. That makes it difficult to detect minute increases in cases that might result from radiation exposure.
That's not to say that radiation doesn't pose any health risk. Royal says some additional cancer risk does arise from radiation exposure, and that risk does increase directly with the amount. He says a person's risk of thyroid cancer has been shown to be higher the more they are exposed to iodine-131. Still, a U.N. commission that studied Chernobyl found "no scientific evidence of increases in overall cancer incidence or mortality rates or in rates of non-malignant disorders that could be related to radiation exposure" in the two decades after the incident.
The problem in containing the disaster
The nuclear disaster in Japan was a classic case of Murphy's law: What could go wrong did go wrong.
But what exactly went wrong, and why has it been so difficult to contain the disaster? The short answer: Even though the reactors automatically shut down after the earthquake, certain nuclear processes that release heat continue naturally. Unfortunately, the tsunami knocked out the system that was supposed to regulate reactors' temperatures after they went offline.
The Fukushima plant produced electricity when fission of uranium and radioactive decay of the fission byproducts boiled water in the reactor cores. In these types of reactors, a cooling system condenses the steam back into water and sends it back to the reactor core to start the process all over again. The cooling systems in these types of reactors serve another critical purpose: temperature control.
The reactors were designed to shut down automatically in the event of an earthquake, ending fission. But the radioactive decay reactions continue on their own, releasing heat. That means the cooling system needs to keep working even if the reactors shut off.
The cooling system has one major flaw; it must get its power from a separate source once the reactors go offline. What if that separate source of electricity stops working? The reactors' temperature will rise, boiling off more water in the reactor core and causing the water level to drop. That process leaves the fuel rods and the reactor core itself at risk of overheating.
That's where backup mechanisms at the Fukushima Daiichi plant were supposed to assist. The cooling pump system could have been powered by the electric grid that is connected to the plant. If the grid were to go offline, the next line of backup mechanism -- diesel-fuel generators -- could step in. The final line of defense was a battery system that could power the cooling system for a few hours until another power source could be restored.
When the earthquake hit, all three reactors that were operating Fukushima plant successfully shut down. The only problem: "They would have turned to the power grid to cool down, but the whole region was hit," says Michael Wysession.
All the backup options had failed.With no electric-grid power, the diesel-fuel generators were next. That might have been the end of the problem, Sobotka says. But soon the tsunami arrived and wiped out that system. Only the temporary batteries remained, but those soon ran out of juice.
With no way to cool the reactors, the water level fell, exposing the delicate fuel rods in the reactor cores. The New York Times recently reported that water levels in some of the reactor cores dropped by as much as three-quarters and their temperatures rose 5,000 degrees Fahrenheit, "hot enough to burn and melt the zirconium casings that protect the fuel rods." The Japanese have since sought to flood the reactor cores with seawater in a last-ditch cooling effort.
The protective zirconium casings then started to oxidize, triggering a reaction that made hydrogen gas, which can explode when exposed to air. It's no wonder, then, that several hydrogen explosions occurred in four buildings, three of which had reactors that were operating before the disaster.
These explosions may not have destroyed the reactors' containment systems, Lee Sobotka says. But they did destroy or severely damage the spent-fuel ponds' management systems.
News reports have suggested that some of those rods have indeed melted, spewing radioactive material into the atmosphere.
With all the damage to the plant and the continuing delicate situation there, the obvious question is this: How does Japan end its nuclear crisis, and how long will it take?
Sobotka says it could take months or even years to put an end to it once and for all because the radioactive decay process continues and releases heat in the reactors, radiation levels at the plant are making work more difficult, and the spent-fuel ponds' management systems suffered severe damage from hydrogen explosions.
He says two things must occur. First, the cores in the three reactors that were running before the tsunami will need to have some sort of "closed-loop" cooling system restored until the cores can be removed months or years down the line. Luckily, some backup power systems have been restored to the plant since the initial disaster, according to the IAEA, but water levels remain dangerously low.
Second, the spent-fuel ponds will have to be kept cool until the debris zone around them can be cleaned up and their management systems can be fixed. After that happens, the fuel-pond assemblies can be removed one-by-one; again, this will take months or years.
Sobotka says it's not clear what state the spent-fuel ponds are in now. According to the IAEA, water either is being or has been injected into the affected ponds to keep their temperatures under control. Either way, with the management systems damaged by hydrogen explosions, "their ability to manage the spent-fuel ponds is in dire straits," Sobotka says.
What caused the tsunami?
A magnitude-9.0 earthquake was bad enough for Japan, but the tsunami simply made matters worse. The tsunami that struck the Fukushima Daiichi nuclear power plant was nearly 50 feet high, and enough to take out the diesel-fuel generators that could have otherwise prevented the country's nuclear crisis. What caused that devastating tsunami?
To answer that question, it helps to understand the geology of earthquakes.
The Japan island group is located on four separate tectonic plates -- giant jigsaw puzzle pieces of crust that float and move alongside each other. Japan and nearby areas are such a hotbed for earthquakes and volcanoes because of all the tectonic plate interfaces that are in the area.
Plates can slide along, away from or into each other. Sometimes, two plates get locked together because of friction. But the forces that produce the plates' motion continue, building up a lot of stress in the "locked-up" plates. Eventually, one of the two plates may slip, "unlocking" the two and releasing tons of energy in the form of an earthquake.
The Japanese earthquake happened at the intersection of the North American and Pacific plates, southeast of the island of Honshu. The Pacific plate is gradually moving west and "subducting," or sliding underneath, the North American plate. Part of Honshu is on the North American plate, so subduction is gradually pushing the island up over time.
Earthquakes that have occurred along the interface between those plates have been severe -- in the 7.0 and 8.0 range -- but nowhere near as severe as magnitude 9.0. Additionally, certain geological features called faults -- giant fissures in rock near intersections of two plates -- were not big enough to suggest that the area could experience a 9.0 quake.
As a result the nuclear plant was built to withstand an 8.0 quake, but not necessarily a 9.0 quake and a tsunami, says Wysession.
"It posed an embarrassment for us seismologists," he says. "We never thought this size of fault could have a magnitude-9 earthquake."
Not all earthquakes generate tsunamis, however. What exactly are tsunamis, and why did the earthquake near Japan generate such a big one?
It turns out that the popular depiction of tsunamis as giant tidal waves isn't quite accurate, Wysession says. They're actually giant changes in the water level that occur if large amounts of crust at the bottom of the ocean are pushed upward in an earthquake. When that rock is forced upward, the seawater directly above is pushed up, too, increasing the seawater level and propelling water at the surface out in all directions. That moving wall of water is the tsunami.
The tsunami travels so quickly that it can wash away entire towns -- not by crashing on them like a tidal wave, but by overrunning them with a rapidly moving deluge of water.
The two plates that caused the Japan earthquake interacted in the perfect way to cause a tsunami. The energy released in the earthquake pushed rock from the two plates upward. Wysession says the earthquake rupture lasted for a full three minutes -- far longer than the average of 10 seconds for most other earthquakes.
"That meant an enormously broad area of the seafloor lifted up, generating this very large tsunami," he said.
And that tsunami ultimately spelled doom for the Fukushima power plant, devastated hundreds of miles of coastline in northeastern Japan and sent thousands of people to their deaths.
The future of nuclear power in Japan seems uncertain. Wysession says the Japanese islands lack fossil fuel reserves that North America and Eurasia have. So while public appetite for more nuclear plants in Japan may have soured in the wake of the crisis, Wysession says the Japanese likely won't have any choice: "Nuclear is going to be an important component for them."