Tuesday, May 10, 2011

Recriticality at #Fukushima I Nuke Plant: Japanese Researcher Says Nuclear Chain Reaction May Have Reignited Long After Tsunami Disabled the Plant

Tetsuo Matsui is a professor at University of Tokyo's Institute of Physics. On May 2, he submitted his paper titled "Deciphering the measured ratios of Iodine-131 to Cesium-137 at the Fukushima reactors" to arXiv.org, in which he says "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." (quote taken from his paper)

Chain nuclear reactions reignited = recriticality

That is what some of the non-mainstream researchers like Japan's Hiroaki Koide of Kyoto University and the US's Arnie Gundersen of Fairewind Associates have been saying all along, that there may have been recriticality in some of the reactors at Fukushima I Nuclear Power Plant.

Now the Japanese mainstream Tokyo University professor is saying it. TEPCO's and the Japanese government's acknowledgement can't be that far away. (And no doubt it will be presented as "it's nothing, it's safe.")

From The Physics arXiv Blog at Technology Review at MIT (5/9/2011):

Chain Reactions Reignited At Fukushima After Tsunami, Says New Study: Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded

..... 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.

He says the ratios from drains at reactors 1 and 3 at Fukushima are consistent with the nuclear reactions having terminated at the time of the earthquake.

However, 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.

The blogpost above includes some imprecise information that does not appear in the original paper. For those of you who want to read Matsui's original paper (9 pages with calculations), the link is this: http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.0242v1.pdf

ArXiv.org was originally hosted at the Los Alamos National Laboratory and is currently hosted by Cornell University. Researchers submit electronic preprints in the fields of mathematics, physics, astronomy, computer science, quantitative biology and statistics, which can be accessed via the world wide web. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv. Though it is not peer-reviewed, there are moderators in each field who review the submissions. (from Wikipedia.)

(h/t あ)


Anonymous said...

We need to be clear here: Arnie Gundersen speculates that a prompt criticality happened in SFP #3, and this, his hypothesizes, is what caused the explosion of #3 on March 14. In an earlier video, he hypothesized that recriticalities were happening in reactor #1, on the basis of reports from Tepco, which have since been revised (or falsified, depending on your bent). Professor Koide, on the basis of the same data, also speculates that recriticalities were happening in reactor #1. Professor Matsui is conjecturing (apparently) that fission reactions were occurring in #2 as late as 10 days after the reactor was shut down. There is by no means any agreement or consensus as to what is going on.

netudiant said...

Professor Matsui may be correct in his analysis, but we should pay attention to his caveats, which are essentially that he assumes the data provided is representative and accurate.
Given that we have seven reactors and spent fuel pools, all in less than perfect condition after numerous earthquakes, explosions and cooling failures, that may be a false assumption. The water samples may be contaminated by leaks from anywhere in the site, as well as by particles that have been generated by the explosions and structural failures.
Recreating criticalities in a damaged reactor is not simply a matter of melting the core fuel rods. That will produce a hot mess, but no criticality. So unless some plausible mechanism is laid out, it is more plausible to Prof Matsui's measurement interpretation than to believe his conclusion.

arevamirpal::laprimavera said...

So, Reactor 1 RPV, Reactor 2 RPV, Reactor 3 SFP, Reactor 4 SFP may all have had recriticality. Or not. TEPCO's data could be false in both directions, overstating and understating.

Anonymous said...

Have there been any more neutron beams reported? Hard to imagine recriticality without neutrons.

Anonymous said...

I think we'd be seeing dead people all over the place if there were recriticalities happening. But the iodine ratios are puzzling. Could be a lot of things happening that we won't know about until we get deeper into the buildings and get more reliable instrumentation.

Anonymous said...

Robbie001 sez:

The Tokaimura recriticality only managed to kill two people through overexposure they were extremely close to the action a third person that was close survived. The rest of the exposures were measured at 23 mSv and below to people in and outside the facility. Tokaimura’s criticality event ran for off and on for 20 hours. Fukushima has a much larger safety distance offset than Tokimura and most of the closest locals were evacuated before the neutrons began to fly. Remember at Fukushima a nuclear emergency was declared by the Government on March 11th. initially it was a 2 km circle it was quickly expanded to a 10 km evacuation zone on Mar 12th. As for reports of additional neutrons it wouldn't surprise me in the least if they stopped announcing their neutron findings I have absolutely no faith in official proclamations anymore. These neutron detections weren’t accompanied with any useful information like intensity or energy level just a vague direction of travel. Remember the facility was repeatedly evacuated in the early days of the disaster neutron flux could have play a part in some of those early evacuations. The NRC had people on the ground and they recommended an 80 km exclusion zone due to the condition of the SFP's.

If a prompt criticality event occurred in SFP#3 the ejection of the spent core material would have turned it off. Since no one knows the state of the fuel before the explosion it is hard to rule out Gundersen's assertion. The high ratio of I-131 points to intermittent recriticality somewhere at the facility regardless of how unlikely some experts may feel such an event could be. I seem to remember an event like Fukushima being so unlikely as to be impossible until nature proved our highly paid expert opinions very wrong. I find it laughable that people continue to tout what is impossible or unlikely when it comes to a nuclear disaster.

Something to keep in mind is the neutron emissions were recorded between Mar. 13th - 16th SFP #3 exploded on Mar.14th it is possible one of these emission coincided with SFP #3 blast.


M. Simon said...

I just found this. It is pertinent to this question (which fortunately was kept in my Firefox history):.


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