Has Fukushima’s Reactor No. 1 Gone Critical?

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On March 23, Dr. Ferenc Dalnoki-Veress, a Research Scientist at the Monterey Institute of International Studies saw a report by Kyodo news agency that caught his eye. It reported that Tokyo Electric Power Company (TEPCO) had observed a neutron beam about 1.5 km away from the plant.  Bursts of neutrons in large quantities can only come from fission so Dalnoki-Veress, a physicist, was faced with an alarming possibility: had portions of one of Fukushima’s reactors gone critical?

To nuclear workers, there are few events more fearful than a criticality accident. In such a scenario, the fissile material in a reactor core–be it enriched uranium or plutonium–undergoes a spontaneous chain reaction, releasing a flash of aurora-blue light and a surge of neutron radiation; the gamma rays, neutrons and radioactive fission products emitted during criticality are highly dangerous to humans. Criticality occurs so rapidly–within a few fractions of a second–and so unpredictably that it can suddenly kill workers without warning. There have been 60 criticality incidents worldwide since 1945. The most recent occurred in Japan in 1999, at an experimental reactor in Tokai, when a beam of neutrons killed two workers, hospitalized dozens of emergency workers and nearby residents, and forced hundreds of thousands to remain indoors for 24 hours.

Dalnoki-Veress did not see any further reference to a neutron release. But two days after the Kyodo agency report, on March 25, TEPCO made public measurements of different isotopes contributing to the extremely high measured radioactivity in the seawater used to cool reactor No 1. Again, a piece of the data jumped out at Dalnoki-Veress: the high prevalence of the chlorine-38 (CL-38) isotope. CL-38 has a half-life of 37 minutes, so would decay so rapidly as to be of little long-term safety concern. But it’s very presence troubled Dalnoki-Veress. Chlorine-37 (CL-37) is part of natural chlorine that is present in seawater in the form of ordinary table salt. In order to form CL-38, however, neutrons must interact with CL-37. Dalnoki-Verress did some calculations and came to the conclusion that  the only possible way this neutron interaction could have occurred was the presence of transient criticalities in pockets of melted fuel in the reactor core.

Yesterday, he published those calculation in a paper for the blog ArmsControlWonk. The paper makes clear that if a criticality accident occurred at Fukishima, it could happen again—and while such a possibility poses minimal danger to Japanese citizens outside of the 20km exclusion zone, it means the emergency workers at Fukushima are operating in even more dangerous conditions than anyone realized. “It is important for TEPCO to be aware of the possibility of transient criticalities when work is being done; otherwise workers would be in considerably greater danger,” the paper concludes. “This analysis is not definitive proof but it does mean that we cannot rule out localized criticality.”

The paper is now open for comments at the ArmsControlWonk website and can be found here.

Update: Edwin Lyman, a nuclear safety expert at the Union of Concerned Scientists, told Ecocentric that he is skeptical of Dalnoki-Veress’s thesis, not because the math or physics was faulty but because he does not trust the accuracy of TEPCO’s reporting of high levels of CL-38. In an email, he wrote, “I think, given the error they committed in Unit 2 (first reporting a huge concentration of I-134, which wasn’t actually there), I’d be wary of attributing too much significance to a single anomalous measurement.”

Update 2: The IAEA has said that the Fukushima nuclear power plant may have achieved re-criticality.  “There is no final assessment,” IAEA nuclear safety director Denis Flory said at a press conference on Wednesday, according to Bloomberg News. “This may happen locally and possibly increase the releases.”