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Fukushima update: did nuclear chain reactions continue after shut-down?

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.


  1. Report this comment

    Kingsley Jones, PhD, CFA said:

    There is a fairly detailed study of recriticality in a variety of reactor not dissimilar to those at Fukushima:

    The link is to a Scandinavian study dealing with the issue of recriticality following a Loss of Coolant Accident (LOCA).

    Of course, this is what happened at Fukushima.

    What is noteworthy about the study is the interaction between the rate of coolant introduced to an overheating core and the incidence of fission reaction re-start.

    This study seems significant to me, since it involves the use of several reactor modelling codes to corroborate findings.

    The most disturbing feature of the study is that if the coolant rate is too high, then severe power excursions are possible at a level exceeding the rated nominal power of the reactor.

    I would like to see more public discussion of such modelling. It seems to be very well founded and careful.

    I am surprised we hear so little from the nuclear industry concerning this.

    Then again, maybe I am not surprised.

  2. Report this comment

    Andy Dawson said:

    Of course, if you check back, you’ll find the actual rate of neutron detection was at rates consistent with naturally occuring neutron emitters. And, also, that the “beam” issue in the original translation (which is what seems to have started off the various conspiracy theorists) was a mistranslation.

    It’s worth a look at the continued monitoring records from the site and the prepiphery. No neutron activity.

    There’s also no evidence temperature/pressure spikes in the reactors or containment drywells, as you’d expect to see were criticality “flashes” occuring.

    One obvious last point. How much iodine, etc. would you actually expect to be produced by “flashes”? Not a great deal, I’d suggest. From memory, it’s about 3% of fission events that produce Iodine 131 as a decay product. It takes months of sustained operation to build up a significant inventory in fuel.

    Nice theory, chaps, but it really doesn’t bear examination.

  3. Report this comment

    Uncle Al said:

    Reactive uncladding of fuel assemblies (re hydrogen release) allows fuel pellets to collect at the containment vessels’ bottoms, away from control rods, where they can critically assemble. Borate in cooling water will not circulate through a (crusted) hot bottom slough.

    Ultrasonic pinging through containment would characterize bottom accumulation. Send in a radiation-hardened robot.

    One requires control rods to be at the bottom, penetrating the slough, without dangerously withdrawing from remaining core.

  4. Report this comment

    Will said:

    Thanks for the links and the table mimyselfandi.

    This isn’t my area, but looking at the numbers I can’t say I am entirely convinced that the detection of short lived isotopes in reactor 1 is definitive proof of periodic criticality.

    For example, in your table reactors 1, 3 and 6 all have measurable levels of I-132 (2 hour half life) and reactor 6 also had detectable (but much lower) levels of the same Te-129.

    Some possible hypothesis:

    1) Yours requires that all three of these reactors have encountered temporary criticality events.

    2) An alternative is that some other process (neutron radiation from longer lived fission products?) is creating these shorter lived products.

    Without much knowledge, your hypothesis 1 seems unlikely because reactor 6 spent the least time without power, was kept relatively cool, and has been in cold shutdown since August. Hypothesis 2 isn’t perfect either. I can’t think of any reason why side reactions would create differing levels of isotopes among the reactors, but maybe those three had more significant core damage (again though, why reactor 6?)

    My point is that short lived isotopes can be (maybe even strong) evidence for recent criticality, but it doesn’t seem to definitively “confirm” anything.

    Of course in light of even the possibility, appropriate tests and safety precautions should be taken!

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