Fears of Fission Rise at Stricken Nuclear Plant in Japan
By HIROKO TABUCHI
Published: November 2, 2011
TOKYO — Nuclear workers at the crippled Fukushima power plant raced to inject boric acid into the plant’s No. 2 reactor early Wednesday after telltale radioactive elements were detected there, and the plant’s owner admitted for the first time that fuel deep inside three stricken plants was probably continuing to experience bursts of fission.
The unexpected bursts — something akin to flare-ups after a major fire — are extremely unlikely to presage a large-scale nuclear reaction with the resulting large-scale production of heat and radiation. But they threaten to increase the amount of dangerous radioactive elements leaking from the complex and complicate cleanup efforts, raising startling questions about how much remains uncertain at the plant, the site of the world’s worst nuclear disaster since Chernobyl. The Japanese government has said that it aims to bring the reactors to a stable state known as a “cold shutdown” by the end of the year.
On Wednesday, the plant’s operator, the Tokyo Electric Power Company, said that measurements of gas from inside Reactor No. 2 indicated the presence of radioactive xenon and other substances that could be the byproduct of nuclear fission. The presence of xenon 135 in particular, which has a half-life of just nine hours, seemed to indicate that fission took place very recently.
Trade Minister Yukuo Edano censured Japan’s nuclear regulator, the Nuclear and Industrial Safety Agency, for failing to report the discovery to the prime minister’s office for hours, according to local media reports.
The developments added to the disquiet over handling of information related to the disaster. For almost two months after the March 11 earthquake and tsunami, disaster, both company and government officials declared it was unlikely any meltdown had occurred at all at the Fukushima Daichi nuclear complex, finally conceding that the fuel had indeed slumped and had likely breached containments in three reactors.
The amount of detected xenon was small, and there was no rise in temperature, pressure or radiation levels at the reactor, Tokyo Electric said. Researchers were double-checking the data to make sure there were no errors, the company said. Experts concurred that it was possible that Tokyo Electric had made a simple error in its measurements.
But the urgent injection of boric acid underscored that the company was operating on the assumption that the measurements were valid. A naturally occurring element, boron soaks up the neutrons released when an atom is split so that those neutrons cannot go on to split other atoms in the process of fission. Nuclear power plants harness the energy released in the form of heat to produce electricity.
It is impossible to determine exactly what state the fuel is in, given that even an intact reactor can offer only limited gauges in the form of temperature, pressure readings and neutron flow, but not visual observation. That lack of clarity is one of the most resonant lessons of the Fukushima disaster, where those trying to guide the response and assess the danger operated by what amounted to educated guesswork.
In reactors of the design used at Fukushima, that chain reaction is normally stopped when the operator gives a command to insert control rods, which rise up from the bottom of the core and separate the fuel assemblies. But when the cores of three reactors at Fukushima melted, a large part of the fuel presumably formed a jumbled mass in the bottom of the vessel, and without a strict gridlike geometry, the control rods cannot be inserted. Some of the fuel has escaped the vessel, experts believe, and is in spaces underneath, where there is no way to use control rods to interrupt the flow of neutrons.
The jumble of material and conditions had seemed very unlikely to be able to produce sustained fission, but intermittent criticalities have long been suspected.
Junichi Matsumoto, a Tokyo Electric spokesman, acknowledged episodes of fission, telling a news conference: “There is a possibility that certain conditions came together temporarily that were conducive to re-criticality,” and that the measurements indicated a burst that occurred at a slightly higher rate than prior cases. “It’s not that we’ve had zero fission until now,” Mr. Matsumoto said. “But at this point, we do not think there is a large-scale and self-sustained re-criticality.”
A criticality could produce energy that would rearrange the wrecked fuel into a configuration that would no longer support fission, but gradually the material could come together in a form that would support a new burst of fission. That has been the case in previous so-called inadvertent criticalities in other accidents.
He said detailed measurements had not yet been taken at two other severely damaged reactors on the Fukushima site, but acknowledged the possibility of episodes of fission there too. The Fukushima complex, about 160 miles from Tokyo, was struck by a devastating earthquake and tsunami on March 11, which knocked out vital cooling systems and caused the nuclear fuel at three of the plant’s six reactors to melt, with radiation leaks and releases whose damage is still being calculated. A 12-mile exclusion zone is still in effect around the plant. Over 80,000 households were displaced.
The three reactors — together with spent fuel rods stored at a fourth damaged reactor — have been leaking radioactive material since the initial disaster, and new episodes of fission would only increase their dangers.
“Re-criticality would produce more harmful radioactive material, and because the reactors are damaged, there would be a danger of a leak,” said Hiroaki Koide, assistant professor at Kyoto University’s Research Reactor Institute, whose prescient warnings about nuclear safety have won him respect in Japan.
Mr. Koide holds that the nuclear fuel at the three reactors probably melted through containments and into the ground, raising the possibility of contaminated groundwater. If much of the fuel was indeed in the ground early in the crisis, the “feed and bleed” strategy initially taken by Tokyo Electric — where workers pumped cooling water into the reactors, producing hundreds of tons of radioactive runoff — would have prevented fuel still in the reactor from boiling itself dry and melting, but would not have done anything to reduce danger from fuel already in the soil — if it got that far. Workers have now put in place a circulating cooling system that recycles water, which results in less runoff.
Tokyo Electric does not deny the possibility that the fuel may have burrowed into the ground, but its officials say that “most” of the fuel likely remains within the reactor, albeit slumped at the bottom in a molten mass.
But even in their most dire assessments, some experts had not expected even bursts of re-criticality to occur, because it was unlikely that the fuel would melt in just the right way — and that another ingredient, water, would be present in just the right amounts — to allow for any nuclear reaction. If episodes of fission at Fukushima were confirmed, Mr. Koide said, “our entire understanding of nuclear safety would be turned on its head.”
Some nuclear experts have debated for months whether nuclear reactions might be continuing, either in the fuel inside the reactors, or in the spent fuel pools at the plant. They have pointed, for example, to the continued reports of short-lived iodine in the spent fuel pool at Reactor No. 3.
A former nuclear engineer with three decades of experience at a major engineering firm, meanwhile, said that sustained re-criticality remained highly unlikely. But his main concern was that officials could not pinpoint the exact location of the nuclear fuel — which would greatly complicate the cleanup.
The engineer, who has worked at all three nuclear power complexes operated by Tokyo Electric, spoke on condition of anonymity because he did not want to be identified by his former employers. He said that tiny fuel pellets could have been carried to different parts of the plant, like the spaces under the reactor during attempts to vent them in the early days. That would explain several cases of lethally high radiation readings found outside the reactor cores.
“If the fuel is still inside the reactor core, that’s one thing,” he said. But if the fuel has been dispersed more widely, then we are far from any stable shutdown.”