As teams of Japanese engineers scramble to prevent a disastrous release of radioactive material from the Fukushima nuclear plant, scientists are already preparing for a challenge that may unfold over several years: Tracking the spread of radioactive material beyond the reactors and monitoring the radiation it emits in the environment.
"The good news," says Tony Roulstone, a nuclear engineer at the University of Cambridge in the United Kingdom, is that radiation "is simple to measure, and to measure at levels vastly below that significant to human health." But that still leaves the question of where to look for it. The answer requires modeling the release and spread of radioactive material through the environment.
Those models are "adequate," says Raúl Periáñez, a nuclear physicist at the University of Seville in Spain. "The problem is the lack of data to feed the models," He says. For example, if a fire or explosion sends material into the air, "it may be released as an aerosol and/or attached to particles." The size of those particles can make a huge difference in the range of spread.
So far, researchers have little data from the Fukushima disaster to work with.
Gerhard Proehl is scientific secretary of a program at the International Atomic Energy Agency (IAEA) in Vienna that creates these computer models. The physicist says that his research group has offered its expertise to colleagues in Japan but has received only a trickle of data. Most of it is radiation measurements taken at monitoring stations positioned around the nuclear plant. "Using these numbers with our models is very difficult," he says, because they do not distinguish between air and ground radiation. "What we really need are separate measurements," he says.
Beyond the radiation monitoring, Proehl says that scientists will need to map the distribution of different radioactive species in the environment. The worst of these is the plutonium in the reactor cores. But absent a massive explosion or an intense fire, he says, the plutonium "will not go far." The more immediate worry is the release of more volatile species such as cesium-137 (with a 30-year half-life), cesium-134 (with a 2-year half-life), and iodine-131 (with an 8-day half-life).
Another group monitoring the spread of radioactive materials is the Comprehensive Nuclear-Test-Ban Treaty Organization. The CTBTO has built up a worldwide network of sensors to pick up the signals of clandestine nuclear tests, including seismic detectors, acoustic sensors in the air and oceans, and radionuclide stations as part of its mission to ensure that no countries carry out nuclear tests. Among the 60 radionuclide stations currently operating, CTBTO has ones scattered across the Pacific and around the Pacific Rim. Each station contains an air filter that collects specks of dust from air. The filters are checked once daily with a gamma-ray detector.
Once a radionuclide station detects something that may have come from a nuclear test, researchers use sophisticated atmospheric models to wind back the clock and so finger the possible nuclear cheat. Those models can run forward equally well and estimate where debris from Fukushima might end up. CTBTO is making those calculations, but its results may go only to the governments that have signed up to the treaty. Hence none of its predictions has been made public, although The New York Times says it has seen a CTBTO report from Tuesday.
It is too soon to say how far the radioactive material has spread beyond the Fukushima plant, says Proehl. "There have been a series of plumes released at different times, with different weather conditions," he says. "It's extremely complex." A small IAEA team will visit Japan tomorrow.