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 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.
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 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.