Experts fear that damage to the Fukushima reactors is worse than partial meltdown.
It came as no surprise when the Tokyo Electric Power Company (TEPCO) admitted last week that it will scrap its stricken Fukushima Daiichi reactors. After explosions, copious radioisotope leaks and a liberal dousing with sea water, it has been clear for weeks that the reactors were a write-off. But just what will workers encounter when they finally start decommissioning the shattered plant?
It may be years before the true state of the reactors is known and a full clean-up can begin. But already scientists have been piecing together a picture of what happened inside the reactors from fragmentary and sometimes incorrect data about pressure, radiation levels and most of all the radioisotopes leaking from the plant.
After the tsunami on 11 March knocked out backup generators — preventing cooling water from circulating around the hot cores of reactors 1–3 — the fuel rods inside began to warp, split and at least partially melt. Steam reacted with the rods' outer sheath of zirconium, creating hydrogen gas that caused a sequence of massive explosions (see Nature 471, 417–418; 2011).
But data from Japanese regulators and TEPCO suggest to some researchers that conditions inside the core could be far worse than a partial meltdown. Some believe that molten fuel may have flowed into the outer concrete containment vessel, whereas others suggest that nuclear chain reactions are still happening inside the fuel.
The most worrisome evidence comes from water found in a building next to reactor 1. On 26 March, Japan's Nuclear and Industrial Safety Agency reported the presence of chlorine-38, a radioisotope with a half-life of just 37 minutes. The short-lived isotope, which forms when natural chlorine-37 is hit by neutrons from fission, could be evidence that fuel inside the reactor has clumped together into sufficiently large chunks to briefly restart nuclear reactions, says Ferenc Dalnoki-Veress, a physicist at the Monterey Institute of International Studies in California. Such bursts could put workers at extreme risk of radiation exposure during clean-up, he warns, and seriously complicate work at the site.
But Paddy Regan, a nuclear physicist at the University of Surrey in Guildford, UK, is sceptical. Other radioisotopes have a similar γ-ray spectrum and could be mimicking chlorine-38, he says. Indeed, in the same week as the reading was taken, TEPCO reported alarmingly high levels of iodine-134, a similarly short-lived isotope, before retracting the announcement soon after. Dalnoki-Veress agrees that the evidence is circumstantial, adding that he is frustrated by the lack of clear data coming from the plant.
Other theories also rest on tentative evidence. Richard Lahey, an emeritus professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, New York, believes that the core of reactor 2 may have melted its control rods, which are designed to stop nuclear reactions. Pressure readings and high levels of radioactivity suggest to him that molten fuel has flowed through the control-rod system like lava and dripped into the containment vessel below. If this had happened, clean-up would be far more difficult.
Wiktor Frid, a researcher at the Swedish Radiation Safety Authority in Stockholm thinks the tops of the control rods themselves may have melted. This would allow nuclear chain reactions to restart each time the reactor is filled with water, which slows neutrons sufficiently to allow fission. Simulations conducted by Frid and his colleagues have hinted that such an event is possible. But Frid is quick to add that the data he has seen may not be reliable. "A lot of information is missing and some is contradictory," he says.
The confusion recalls the weeks that followed the partial meltdown of a reactor at Three Mile Island in Pennsylvania in 1979. In the immediate aftermath of that emergency, the state of the reactor core was subject to "an ongoing debate that went on for months", recalls Jack DeVine, an independent nuclear consultant who spent six years cleaning up that accident.
Many scientists believed that the fuel rods at Three Mile Island were more-or-less intact, on the basis of computer models and simulations, says DeVine. But when a camera was eventually lowered into the core in 1982, the damage was far worse than anyone had predicted. "It looked like my gravel driveway — a mess," he says. Engineers hoping to remove fuel rods in a process akin to conventional decommissioning of a nuclear core had to rethink their clean-up strategies, though they eventually managed to clear most of the fuel out of the reactor.
DeVine says that the Japanese authorities should have no desire to shroud Fukushima's reactors in a protective concrete sarcophagus like the one built over the exploded Chernobyl reactor in 1986. The risk of another earthquake or tsunami damaging the structure and creating more problems would be too great. Instead, Fukushima's fuel will have to be removed in a process similar to Three Mile Island's recovery.
That effort took 14 years. Based on what he has seen so far, DeVine believes that one thing is certain: decommissioning Fukushima will probably take longer.
A shorter version of this story appears in this week's edition of Nature magazine.
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Brumfiel, G. Japan faces more than a decade of nuclear clean-up. Nature (2011). https://doi.org/10.1038/news.2011.211