Web edition: March 16, 2011
As if the magnitude-9 earthquake on March 11 and killer tsunami weren’t enough, a new round of aftershocks — psychological ones over fear of radiation — are rocking Japan and its neighbors. Severe and escalating damage to several of Japan’s nuclear reactors at a facility known as Fukushima Daiichi have released pulses of radioactive gases or particles. On Tuesday, March 15, Japan’s leaders formally told the public that some of these releases could be harmful.
To date, the only people in the direct line of fire, so to speak, have been plant workers toiling valiantly to bring the reactor crises — and there have been a steady string of them — under control.
But to play it safe, Prime Minister Naoto Kan issued an 11 a.m. call on Tuesday for a final evacuation of everyone living within 20 kilometers (roughly 12.5 miles) of the nuclear plants. He also strongly advised that people living between 20 and 30 km from the nuclear facility should stay put indoors.
Television relayed a government message over and over that people who had laundry drying outside were to leave it where it was. Don’t panic, government leaders said. But how would you feel if the government said to sacrifice your bed linens, jeans and undies to the environment as you huddled indoors waiting for the next shoe to drop?
And drop it did. Reports emerged throughout Tuesday and early Wednesday that efforts to manage the severely crippled reactors were failing. So badly were they failing that radiation levels made conditions perilous for anyone but essential safety crews to remain. So the utility operating Fukushima Daiichi sent most plant employees away to safety.
Meanwhile, television reports replayed snippets over and over from Yukio Adano, Japan’s chief cabinet member. He noted that radiation dose rates of 400 millisieverts per hour had been detected Tuesday around the number 3 reactor at the site, 100 mSv around the number 4 reactor, and a reading of 30 mSv between reactors 2 and 3. (That per-hour rate means that anybody exposed for an hour would receive that dose — or some pro-rated fraction thereof. For example, exposure to 400 mSv/hr for a minute would result in a dose of 6.7 mSv.)
Such exposure rates are potentially high, says Kelly Classic, a medical health physicist at the Mayo Clinic in Rochester, Minn. It all depends on how long someone is exposed.
For perspective, 400 mSv is about the dose people would receive from 40 computed tomography (CT) scans of the abdomen, she notes. Natural background rates of radiation — from rocks, bricks, soil and water — averages about 3 mSv per year. So 400 mSv would be “a hefty dose,” Classic says.
“Below about 500 millisieverts, in the short term, we aren’t likely to see any signs of symptoms,” she explains. So there would be no acute radiation poisoning. In the long-term, however, any exposures above even 10 mSv might elevate somewhat an individual’s risk of developing cancer.
Fires, explosions and the deliberate venting of gases to manage the pressure inside poorly cooled reactors — all of which has occurred at the Fukushima Daiichi facility — could expose workers to radiation doses substantially exceeding 10 mSv. It was something that nuclear engineer David Lochbaum of the Union of Concerned Scientists explained to reporters in a March 15 briefing.
A less obvious radiation risk, but one that emerged in Japan this week: used-fuel-storage depots found at or near most nuclear plants.
Even after nuclear fuel is ostensibly used up, it remains hot, both radioactively and thermally for years. So utilities tend to store their used fuel on site in pools of recirculating water, notes Lochbaum, who for 17 years worked at U.S. nuclear plants — three of which were similar to those at the Fukushima Daiichi facility.
When the Japanese reactor complex lost power as a result of the earthquake, and its backup diesel generators were essentially drowned by the tsunami, at least one of its pools of used fuel lost its cool.
Indeed, its contents heated to where a fire broke out. Clearly, the fuel assemblies must have lost their watery blanket, Lochbaum says — perhaps because the water boiled away or some breach in the pool allowed water to drain out. Plant managers have been working to keep the pool’s water level up. And one reason: safety.
U.S. studies performed by nuclear utilities several decades ago analyzed what would happen if used-fuel cooling ponds lost their coolant. One of those studies concluded that if the water fell to the top of the fuel — a drop of perhaps 30 feet in the water level — radiation dose rates at a railing along the top of the spent-fuel pool “would be high enough that you would receive a lethal dose in something like 16 seconds,” Lochbaum said.
Equally bad, if a fire erupts, the fumes will include radioactive gases — and ferry radioactive particles. Once these enter the environment, they can travel long distances.
Risks associated with radiation explosures tend to fall off quickly with distance from a source as contaminants become diluted. For instance, Japanese television reported Tuesday that as of 8:30 a.m., dose rates measured at Fukushima Daiichi’s gate were distinctly elevated, but still only 8 mSv.
The real concern is that plumes of radioactive gases or particles spewed by episodic events at the crippled reactors will travel with air currents until rain or winds carry them into waters or onto other surfaces — like laundered sheets hung out to dry.