Reactor 3 at the Fukushima Daiichi plant, on March 24
What's the News: A non-peer-reviewed study
(pdf) publicized last week by radioactivity-detection expert Ferenc Dalnoki-Veress
suggests that nuclear fission reactions continued
at Japan's Fukushima nuclear power station well after the plant's operators had allegedly shut down the reactors there. The paper says there may be what are called "localized criticalities" have occurred
in the plutonium and uranium left in the reactors---little pockets of fuel that have gone critical, propagating the nuclear chain reaction and generating potentially harmful radiation. The existence of criticalities is controversial: some researchers say there are certainly none; Dalnoki-Veress himself says it's only a possibility. How the Heck:
Over three days beginning March 13---two days after the earthquake and resulting tsunami---Tokyo Electric Power Company detected a neutron beam, a stream of radioactive particles that could be evidence of continued chain-reaction fission.
The company observed the neutron beam 13 times, about a mile away from the reactors. The beam itself doesn't pose a health risk, with radiation levels between 0.01 and 0.02 microsieverts per hour. (You'd get about as much radiation exposure from eating one-tenth to one-fifth of a banana.)
After seawater was used to cool the reactors, the water had unusually high levels of chlorine-38, a radioactive isotope of chlorine. Chlorine-38 isn't much of a radiation risk; its half-life is 37 minutes, so it disappears quickly. What's strange is that chlorine-38 is formed when an atom of chlorine-37 (the stable, common chlorine isotope) absorbs a neutron. High levels of chlorine-38 mean there were lots of neutrons around, raising the possibility that melted bits of fuel may have gone critical.
What's the Context:
An explosion at the Fukushima nuclear plant came shortly after the earthquake and tsunami hit Japan. Workers are still pumping in seawater to keep the reactors' fuel rods cool, with leaks and disposal of the now-radioactive water presenting a new set of problems.
These localized criticalities, if they're happening, could cause surges of radiation and heat, making cooling and containment work at the reactors even more perilous for workers.
Not So Fast:
Other experts are divided as to whether there's even a chance that there are accidental fission reactions occurring. The dangerous conditions at the reactor make it difficult to get a good read on what, exactly, is going on. Nuclear safety expert Edwin LymantoldTime that he'd "be wary of attributing too much significance to a single anomalous measurement." But Denis Flory, head of the International Atomic Energy Agency's nuclear safety department, said in a press conference that such reactions could potentially be occurring.
Image: Wikimedia Commons / derek visser