That put a lot of responsibility on John Holdren and Steve Fetter, who know a thing or two about nuclear plants and radiation.
Holdren is Obama’s chief science adviser. A physicist and former professor of environmental policy at Harvard and the University of California–Berkeley, he is renowned for his work on climate change, energy technology, and the dangers of nuclear weapons and materials. Fetter, who was assistant director at-large in Holdren’s office, had even more specialized expertise in the science of nuclear accidents. His Ph.D. thesis at Berkeley (where Holdren was his adviser) was titled “Radiological Hazards of Fusion Reactors,” and he conducted more research on that subject after joining the University of Maryland faculty in 1988.
The biggest source of concern, both men agreed, was the spent fuel pool in Reactor No. 4, which U.S. nuclear experts in Japan believed had lost its capacity to retain water following a nearby explosion. A lack of water in the pool could mean that the 1,000-plus rods of fuel would heat up, catch fire, and emit so much radiation that any workers in the immediate vicinity would fall fatally sick within hours. Such a fire would lift large amounts of dangerous and long-lived radioactive particles into the atmosphere. Supplying water to the other reactors could then become impossible; their fuel would melt and more radiation would be emitted.
"We are now looking, as you’ve probably surmised from information available publicly, at a high likelihood of releases as large as Chernobyl or even larger," Holdren wrote in an email to a fellow scientist on March 16, 2011, citing the possibility of a spent-fuel fire.
Even so, the science advisers weren't swayed by the simple model the Navy nuclear experts were relying on. It was based on assumptions that the wind would blow steadily in one direction, carrying radioactive iodine and cesium in a very concentrated plume—a worst case that was beyond worst cases likely to occur in reality. Although Fetter had himself reached similar conclusions using similar models, he attached little importance to them. “These kinds of models are fine for 10 to 20 miles out, but not 200 miles,” said Fetter, now back at the University of Maryland, in an interview.
This is why the White House called upon the team at Livermore. Formally known as the National Atmospheric Release Advisory Center, or NARAC, the team has assessed such disasters as Chernobyl, the 1991 Kuwaiti oil fires, and the 2010 Deepwater Horizon oil rig explosion.
The Livermore scientists combine high-speed computing capacity, high-resolution weather forecasting, and stored databases about weather patterns and terrain to generate three-dimensional maps of hazardous plumes. They can project with far greater precision than simpler models how airborne particles are likely to travel over long distances and long periods involving changeable weather. Their judgments would depend on input from government experts about what might happen at Fukushima Dai-ichi.
"There's what's worst-case, and then there's what's possible. We should produce a worst case that's actually possible," Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, told staffers at his agency in a March 18, 2011, conversation. "I mean, a worst case would be that you eject the core and somebody puts it in a bag and carries it across the ocean and puts that in ... California." As his staffers chuckled, Jaczko concluded: "So I think we should produce a source term [i.e., an estimate of radioactive particles assumed to be emitted] that is truly what I would call a worst case but a possible scenario."
Lengthy and sometimes fierce technical arguments ensued among experts at the various agencies involved. In addition to multiple reactor meltdowns, should the assumptions include the much-dreaded fire in the Reactor No. 4 spent fuel pool even though it turned out to be in much better shape than initially thought? For the “plausible worst case,” the answer was yes, because unseen cracks might still cause the pool to empty, and a severe aftershock might lead to new structural problems.
The most critical factor, according to Fetter, turned out to be assumptions about weather. “We didn’t want to point the plume directly at Tokyo and leave it going there the whole time; that wasn’t realistic, because the wind always changes,” he said. “So [the Livermore researchers] looked back in their weather data and found some worst cases—periods in which the wind blew toward Tokyo for a long while.” Precise modeling of atmospheric dispersion and “plume wander” showed that radiation far from the plant would be substantially reduced; even a light rain would wash many particles out of the air.
By the last three days of March, the computer modeling produced results that settled the debate: A plume delivering radiation doses exceeding U.S. standards would come no closer to Tokyo than 75 miles, so Americans should stay put. In an April 1, 2011, email to Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, Holdren spelled out details. "Our optimism, such as it is ... comes not from any assumption that the situation at Fukushima is under control but rather from modeling that shows the worst-plausible releases from one or more reactors at Fukushima would not cross [the U.S. guidelines] in Tokyo even in the event of adverse weather," Holdren wrote. "Only with big releases from the spent-fuel pools, combined with even more perverse weather than [the scientists deemed realistic], could the [guidelines] be crossed in Tokyo, and even then, according to the modeling to date, not by much," so "even in these extreme circumstances, sheltering in place might be all you'd want to do."
Readers who have closely followed Fukushima developments may find this story about the U.S. government's worst-case scenario interesting though not necessarily conclusive. After all, didn't the Japanese government draw up its own worst-case scenario? Indeed it did. Didn’t many news articles report that this scenario would have necessitated an evacuation of Tokyo? Indeed they did—wrongly.
* * *
The timing of Prime Minister Naoto Kan's request for a new analysis—more than a week and a half after the March 11 disaster—seemed odd to Shunsuke Kondo, chairman of Japan's Atomic Energy Commission. "Since things are starting to calm down, would you consider a worst-case scenario?" the prime minister asked, according to investigative reports published later, to which Kondo responded: "If things are calming down, then isn't a worst-case scenario unnecessary?"
Still, Kondo, a 69-year-old former professor of nuclear engineering at the University of Tokyo, had been urging others in the government to develop such scenarios so that they could prepare countermeasures. He promised to deliver a scenario in three days.
Like the U.S. experts, Kondo focused heavily on the Reactor No. 4 spent fuel pool, although Japanese officials believed (correctly, as it turned out) that the pool was full of water. He imagined a "chain of incidents," starting with a new hydrogen explosion and ending in the kind of radiation-spewing spent fuel fire that the Americans worried about. Then he projected where that radiation might travel, using methods that he has described as "very rough"—models that did not involve Japan's own computerized system for forecasting plume movement.
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