TEPCO retracts criticality call

[FBaggins]

The operator of the Fukushima nuclear power plant has retracted an earlier assessment that a continuous nuclear reaction, or a criticality, could have taken place in the damaged Number 2 reactor.

The Tokyo Electric Power Company, or TEPCO, said on Thursday that the small amount of xenon-135 it detected in gas taken from the reactor's containment vessel was the result of the spontaneous nuclear fission of radioactive curium-242 and -244. The two substances are contained in nuclear fuel.

The amount of xenon-135 detected almost matched the amount that would have been produced if the radioactive curium in the fuel had undergone spontaneous fission. TEPCO says a criticality event would have resulted in higher levels of xenon concentration.

Spontaneous fission refers to the nuclear fission of radioactive materials other than uranium, and it does not lead to criticality. Such fission is said to occur constantly.The earlier detection of small amounts of Xenon-135 had suggested the possibility of a criticality occurrence in the melted fuel in the damaged reactor.


http://www3.nhk.or.jp/daily/english/03_19.html

A couple errors in the reporting, but still worth the read.

[OKIsItJustMe]

1. TEPCO Press Release

http://www.tepco.co.jp/en/press/corp-com/release/11110309-e.html

…

· We found a possibility to detect short-half-life radionuclide such as
  Xe-133 and Xe-135 according to our radionuclide analysis sampled on
  November 1 by the gas management system of the reactor containment
  vessel of Unit 2. We continued to monitor the temperature, pressure and
  data from monitoring post and there was no significant fluctuation from
  those data. As we can't be denied a possibility of fission reactions,
  we decided to start injecting boric acid water from reactor feed water
  system at 2:48 am on November 2 and stopped it at 3:47 pm on the same
  day. At around 7:20 pm on the same day, Japan Atomic Energy Agency
  evaluated that the TEPCO's analysis result of the short-half-life
  radionuclide such as Xe-133 and Xe-135 detection was valid. We consider
  that they were generated by the spontaneous fission on the grounds that
  the concentration of detected short-half-like radionuclide (Xe-135) is
  low, that short-half-like radionuclide (Xe-135) was detected even after
  the boric acid, which stops nuclear fission chain reactions, was
  injected, and that the parameters of the reactor were not significantly
  changed.
· At 10:00 am on November 3, we started transferring accumulated water
  from the basement of the Unit 6 turbine building to temporary storage
  tanks.
· At 12:40 pm on November 3, we started the work to install an additional
  flow meter into the nitrogen gas injection line into Unit 2 to improve
  the reliability of the nitrogen injection. It was finished at 2:00 pm
  on the same day, For this work the nitrogen injection was suspended for
  approximately 10 minutes, however, there were no significant changes in
  the parameters of Unit 2.

…

[FBaggins]

2. Interesting that they didn't immediately question the lack of other signs of fission

Seems that we've seen this happen half a dozen times already. They identify (occasionally in error) an isotope that has a short enough half-life that it hints that fission may have occurred recently... but the scenario always lacks all the other expected signs of ongoing fission. Which, of course, never keeps the usual suspects from leaping to the inevitable "See? Now... NOW... I'm finally right!!!" (as if this would have demonstrated that even had it actually occured as first suspected) .

It only further complicates things that the reporting hardly ever gets the science right and people have a tough time distinguishing between a brief intermittent criticality (a big deal if you're standing over an open reactor, but otherwise not very significant) and the resumption of an actual ongoing self-sustaining fission (far FAR more significant).

They're damned if they do and damned if they don't. One would hope that they would hold off on speculation/announcement until they could analyze how much Xenon there was... and how that compared to other possible explanations. But even the short delay in the report that WAS made just got their hand smacked again for waiting too long.

[OKIsItJustMe]

3. It seems to me like they acted prudently

We see undeniable evidence of fission, so…

As for getting the story right, well… that’s why I like to get as close to a primary source as I can (e.g. TEPCO press releases)

[FBaggins]

4. When something is not an immediate safety concern, "accurate" is more important than "quick"

They got plenty of things wrong in the other direction (when it WAS potentially a safety issue), so they probably overcompensate now. ("They" not being restricted to TEPCO, but also to government and pseudo-governmental agencies).

And as I said, even this accelerated-but-incomplete reporting was criticized by the government for delay.