There is growing evidence that uranium and plutonium fuel at the Fukushima nuclear plant may have continued nuclear fission chain reactions long after the reactors were shut down almost three weeks ago. This worrying development may explain the continued release of some shorter-lived radioisotopes from the stricken site.

Tepco, the plant operator, said earlier this week that it had – on 13 occasions – detected beams of neutrons near the reactors. Neutrons are produced during fission of nuclear fuel, and are the key driver of the chain reaction that sustains continuous fission reactions in a reactor.

Japan Today reports that “the neutron beam was measured about 1.5 kilometers southwest of the plant’s No. 1 and 2 reactors over three days from March 13.”

The neutron beam didn’t pack much of a punch – if anyone got in its way, it would likely deliver a dose of just 0.01 to 0.02 microsieverts per hour. But the finding tallies with a recent analysis of other isotopes found at the plant, published in the Asia-Pacific Journal: Japan Focus

Ferenc Dalnoki-Veress, at the James Martin Center for Non-Proliferation Studies of the Monterey Institute of International Studies in California, hones in on the significance of a very short-lived radioisotope, chlorine-38, in the water in the turbine building of reactor 1.

In an introduction to the analysis, Arjun Makhijani, president of the Institute for Energy and Environmental Research, an energy and environment information-provider based in Takoma Park, Maryland, explains:

Chlorine-38, which has a half-life of only 37 minutes, is created when stable chlorine-37, which is about one-fourth of the chlorine in salt, absorbs a neutron. Since seawater has been used to cool [the reactors], there is now a large amount of salt – thousands of kilograms – in all three reactors. Now, if a reactor is truly shut down, there is only one source of neutrons – spontaneous fission of some heavy metals that are created when the reactor is working that are present in the reactor fuel. The most important ones are two isotopes of plutonium and two of curium.

But if accidental chain reactions are occurring, it means that the efforts to completely shut down the reactor by mixing boron with the seawater have not completely succeeded. Periodic criticalities, or even a single accidental one, would mean that highly radioactive fission and activation products are being (or have been) created at least in Unit 1 since it was shut down. It would also mean that one or more intense bursts of neutrons, which cause heavy radiation damage to people, have occurred and possibly could occur again, unless the mechanism is understood and measures taken to prevent it. Measures would also need to be taken to protect workers and to measure potential neutron and gamma radiation exposure.

There’s a great debate about the implications of all this going on over at Arms Control Wonk.

And also see below in our comments thread – some very good counterarguments.

For full coverage of the Fukushima disaster, go to Nature’s news special.