Guest Post: Are Nuclear Reactions Still Occurring at Fukushima?

Washington’s Blog

You know that Fukushima reactors 1, 2 and 3 all melted down within hours of the Japanese earthquake.

You also know that at least some of the subsequent explosions could have been caused by small-scale nuclear reactions called “prompt moderated criticalities”.

But you might not know that nuclear reactions may still be ongoing.

Specifically, it is well-known by nuclear scientists that the ratio of iodine 131 to cesium 137 tells a lot about when nuclear reactions have stopped. For example, on May 2nd, University of Tokyo physics professor Tetsuo Matsui published a scientific paper with the following summary:

We calculate the relative abundance of the radioactive isotopes Iodine-131 and Cesium-137 produced by nuclear fission in reactors and compare it with data taken at the troubled Fukushima Dai-ichi nuclear power plant. The ratio of radioactivities of these two isotopes can be used to obtain information about when the nuclear reactions terminated.

Technology Review summarizes Professor Matsui’s findings as follows:

Nuclear reactors produce radioactive by-products that decay at different rates. One common by-product is iodine-131 which has a half life of about 8 days while another is cesium-137 with a half life of about 30 years.

When a reactor switches off, the iodine decays more quickly so the ratio between these two isotopes changes rapidly over a period of days. That’s why measuring this ratio is a good way to work out when the nuclear reactions terminated.

There are some complicating factors, however. The most important of these is that the ratio of iodine-131 and cesium-137 to start with depends on how long the reactor has been operating and so is not constant.


Today, Tetsuo Matsui at the University of Tokyo, says the limited data from Fukushima indicates that nuclear chain reactions must have reignited at Fuksuhima up to 12 days after the accident.

Matsui says the evidence comes from measurements of the ratio of cesium-137 and iodine-131 at several points around the facility and in the seawater nearby. He has calculated what the starting ratio must have been by assuming the reactors had been operating for between 7 and 12 months.


The data from the drain near reactor 2 and from the cooling pond at reactor 4, where spent fuel rods are stored, indicate that the reactions must have been burning much later.

“The data of the water samples from the unit-4 cooling pool and from the sub-drain near the unit-2 reactor show anomaly which may indicate, if they are correct, that some of these fission products were produced by chain nuclear reactions reignited after the earthquake,” he says.

These chain reactions must have occurred a significant time after the accident. “It would be difficult to understand the observed anomaly near the unit-2 reactor without assuming that a significant amount of fission products were produced at least 10 – 15 days after X-day,” says Matsui.

So things in reactor 2 must have been extremely dangerous right up to the end of March.

As Time Magazine blogger Eben Harrell pointed out on March 30th:

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.”

On April 18th, nuclear expert Arnie Gundersen said that iodine 131 readings indicated ongoing nuclear reactions.

Indeed, Gundersen says today:

Unit 3 may not have melted through and that means that some of the fuel certainly is lying on the bottom, but it may not have melted through and some of the fuel may still look like fuel, although it is certainly brittle. And it’s possible that when the fuel is in that configuration that you can get a re-criticality. It’s also possible in any of the fuel pools, one, two, three, and four pools, that you could get a criticality, as well. So there’s been frequent enough high iodine indications to lead me to believe that either one of the four fuel pools or the Unit 3 reactor is in fact, every once in a while starting itself up and then it gets to a point where it gets so hot that it shuts itself down and it kind of cycles.

Similarly, a Daily Kos writer points out today:

Radiation levels in water inside the silt fence near reactor 2 are high and rising, despite large amounts of dilution. Continued very high levels of Iodine 131 with a half life of 8 days are very hard to explain for a reactor that has been “shut down”. Normally Iodine levels would drop several orders of magnitude below cesium activity levels over the sixty day period shown in the graph, but instead they continue to track each other. The level of 10,000 Bq/liter I-131 is very problematic. It is much higher than would be expected for a reactor in cold shut down for 2 1/2 months.

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George Washington is the head writer at Washington’s Blog. A busy professional and former adjunct professor, George’s insatiable curiousity causes him to write on a wide variety of topics, including economics, finance, the environment and politics. For further details, ask Keith Alexander…


  1. Industrial Failure

    Kyodo, AP:

    “Tepco said Saturday it has detected radiation of up to 4,000 millisieverts per hour at the building housing the No. 1 reactor at the Fukushima No. 1 nuclear plant.”

    Physicist Louis Slotin died after he received a lethal dose (around 2100 rems, or 21 Sv) of neutron and gamma radiation.

    (1 rems = 10 millisieverts)

  2. gruntled

    The title is a bit silly, since the nuclear reactions (spontaneous decay of unstable isotopes) go on all the time in radioactive materials.

    1. Eric

      That was my first reaction to the headline, but a nuclear chain reaction is very different than spontaneous decay. This article indicates chain reactions are still going on, hence the continuous creation of fission byproducts.

      1. psychohistorian

        It is this ongoing release of radioactive particles that will define what the health effects are, near and far.

        As a friend of mone with MS in both Bio and Chem said when I sent him to the Gunderson interview:

        “and of course generally, but not always, the most important is the increase compared to background….if we inhale/ignest 2000 hot particles a day from local sources, cosmic radiation, radon, radium dials, ec…and ingest 10 from FuckYu-Shima…well that is not a particular cause for concern…so just saying hot (and I didnt read it to see hot they defined “hot”) doesn’t mean a lot.”

        I don’t know that the background is and if my friend characterized it correctly but if the radioactive release is ongoing then, if not contained, the accumulation locally and possibly at other unlucky spots around the world will be deadly. How long will that take? I would be happy to see some serious public discussion of this problem so maybe we can get another BP type fix from some plumber to this ongoing catastrophe.

        1. ambrit

          Dear Psychohostorian;
          You have a seldon seen moniker good sir. However…
          As a plumber who lived on the Gulf Coast during the Deepwater Horizon debacle I’m here to tell you that we most certainly do not want a BP type reaction. (I.E. one that involves lots of criticality.) The basic response to the oil release was to ‘throw money’ at it. No one had any usable response planning easily available. (If someone did it appears they were sidelined.) One of the primary consequences of the ‘throwing money at it’ strategy was a mad scramble by local ‘authorities’ to get a piece of the ‘action.’ Thus, local politics to a great degree negated any useful central planning that might have gone on. The programs that actually did useful things, mainly the offshore clean ups and containment schemes, were visibly being run by the Coast Guard and National Guard. One of the main staging areas for the clean up was at a public boat launch near where we lived at the mouth of the Pearl River. It was pure and simple a military base; Guard launches, Corps of Engineer tugs, National Guard traffic wardens, etc. Most of the workers were short term civilians, but the organizers and directors were uniformed personnell. This effort seems to have accomplished its’ objectives. Seeing this first hand for several months I would suggest that the Japaneese Army or Navy should be given control of the Fukushima situation. The last I heard, the Japaneese military has a very good reputation for professionalism and probity. In short, the present lot have failed in their responsibilities and should be shown the door.
          On a related matter; what is this I have been hearing about a sister atomic complex up the coast called Daimi? Is it doing all right, and what about its’ fate if it’s within the exclusion zone around Daichi? Inquiring minds want to know.

      2. Up the Ante

        Correct, Eric. Fission byproducts are telltale, and with anomalous iodine-131 levels are the smoking gun. The only way to hide these facts is to not measure for them, curiously similar to TEPCOs performance for the first, say, month.

        Combined with the lack of neutron measurements, ..

        Certainly well understood in the nuclear industry.

        1. Boiling Bubble Toiling Trouble

          hide these facts is to not measure for them, curiously similar to TEPCOs

          ~~Up the Ante~

          Did Germanese measure isotope level of bean sprouts, Gurgen, und so weiter? Was that escherichia coli from mutationVille? Mutation from Japanic isotopes busy at chromosome-atom-smashing? Who knows? Can you measure one smash? Is one smash truly all it takes to start a clone of e. coli?

          Think about it

          1. Up the Ante

            assuming your comment was an offering of ‘language salad’

            “Is one smash truly all it takes ..”

            Yes, you were ‘smashed’.


  3. Mark P.

    Continued fission reactions are obviously happening at the Fukushima site. There’s not much evidence for prompt criticality.

    Prompt criticality occurs when with each nuclear fissioning, one or more of the immediate (or ‘prompt’) neutrons released then triggers an additional fission event. This rapidly translates into an exponential increase in the number of fission events. Obviously, accidental prompt criticality incidents are bad news: Chernobyl’s reactor #4, and the Soviet sub K-431 were both examples, both producing physical explosions of the reactors involved.

    God knows a lot could be going on in the stew of corium and radioactive debris at the Fukushima site. We’re in uncharted territory, with comparisons with Chernobyl less than helpful.

    Nevertheless, if prompt criticality has occurred somewhere at Fukushima the radiation doses would be from neutron contributions, which IIRC we have not been seen. Specifically, there would be a spike ~ 1e18 fissions, with a spike width of about 1 millisecond or shorter. This would translate into a 10 GW spike, which would have been picked up by any neutron monitor.

    1. propertius

      Nevertheless, if prompt criticality has occurred somewhere at Fukushima the radiation doses would be from neutron contributions, which IIRC we have not been seen.

      It’s possible that we have, although it’s really hard to tell from this poorly-written, nearly incomprehensible article (and I haven’t heard anything about this since):

  4. Mission Accomplished

    Who cares about rems and millisieverts.

    The Gulf of Mexico is collapsing under the BP oil spill and an ocean of oil is floating just under the surface ready to contaminate the ENTIRE NORTH ATLANTIC COASTLINE according to respected oil industry insider Matt Simmons!!!!!!!!

    What? Matts dead. Leak is plugged. World didn’t end?

    Now what the heck can I panic about?
    Oh yeah Fukushima! Do tell me more!

    1. ambrit

      Hey MA;
      This is how the King of the Monsters got woken up. Gojira (Godzilla to us round eyes,) was a semi mythical attempt by the Japaneese to make sense of the new Atomic Age they had become the slightly mutated poster children for. The Godzilla movies have gone on, and on, and on, from that day to this. Fukushima seems to be on track to do likewise. The reason to worry about this is that IT IS NOT GOING TO GO AWAY. Same as all the deep water oil plumes the NOAA people are finding in the Gulf of Mexico, this one is going to hang around and haunt our descendents. We have a lot to answer for.

  5. Arizonan

    Independent analyses by Chris busby and Arnie gunderson both concluded that there was enoough evidence to support the idea that a promopt criticality event event caused the #3 spent fueld explosion in March 14. See archives,

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