Tuesday, April 05, 2011

Recritical Thinking

You may have noticed some talk flowing around the great chatter box that is the Internet about recriticality at the reactors at Fukushima.

…Now there is definitive proof, courtesy of Tellurium 129 and a order of magnitude higher concentration of Iodine 131 in Reactor 1, that the reactor is now undergoing sporadic events of recriticality…
Simply put, recriticality means that a reactor has become critical again and reentered the fission process. But finding tellurium-129 in water in the turbine building of the reactor is hardly the smoking gun of recriticality that “Tyler Durden” would have you believe.
It is evidence of criticality, to be sure, but not re-criticality. In fact, it is evidence of past criticality at the reactor. In this case—if the reading turns out to be accurate—it could be evidence of criticality before the earthquake on March 11 when the reactor was operating normally and was, well, critical.
Here is NEI’s own Rodney McCullum with a technical explanation:
The detection of tellurium-129 is not proof of recriticality.
Even though it has a half-life of just 69 minutes it is still one of the most prevalent fission products in used nuclear fuel several months after fuel is removed from the reactor core or last criticality.
This is due to the fact that tellurium-129 also exists in a higher energy state (Te-129m, where “m” stands for metastable) with a half-life of 34.1 days prior to transforming into lower energy tellurium-129. Given the high initial abundance of both forms of tellurium-129 among fission products, it is reasonable that some would still be present months after the reactor was last critical.
The author also cites iodine-131, with a half-life of eight days, as evidence of recriticality. However, since we are about 20 days post-accident, you can still expect about 10 percent of the original iodine-131 to be present.
According to Rodney not realizing that tellurium-129 has a higher and lower energy state is “a common mistake a new nuclear engineering student would make.” Thanks, Rodney, that makes us non-technical types feel much better. 
What’s more, Tokyo Electric Power Company (TEPCO) has said that its reading of tellurium-129 is “doubtful.”
It is ascertained that the result of nuclide analysis of tellurium-129 (half life: about 70 minutes) conducted on March 30th, for water puddle collected near the trench and ground water collected near the turbine building are [is] doubtful.
To sum up, at this point, TEPCO is not sure if tellurium-129 has been found in water in the turbine building. And even if the results do show tellurium-129 in the water, it is not evidence of recriticality, but past criticality from the Fukushima reactors operating regularly before the earthquake struck on March 11.


Cyril R said...

For those interested, also see this:


M. Simon said...

The ratio of I-131 between reactors #1 vs #2 and #3 is more compelling.

BTW I'm trying to figure out how Te 129 gets produced. I can't find any ready references (Google).

Anonymous said...

M. Simon,

See here for Te-129 production:


Uli said...

Re-criticality is also highly doubtful from a physics point of view. In order to be critical, a reactor needs sufficient water to moderate the neutrons and cause fission. It is unlikely that a hot rubble pile of fuel/cladding would have enough water mixed within the pile sustain a chain reaction.

M. Simon said...


Sure it is doubtful. But the indications are there. Plus it could be critical from fast neutrons. Or the water above the slag could be acting as a moderator and reflector.

If the wells below the slag are filled with water even better.


Thanks for the link. Much appreciated.

M. Simon said...

Re: the higher/lower energy state.

Easy enough to determine by running the sample twice (or some multiple of that depending on how close you want the results - i.e. reduction of statistical errors).

Once as soon as the sample is available and a second time about 1 to 5 (short) half lives later.

You have to assume that if the operators were sharp they did this. So either the operators were not sharp or they did the determination in such a way that it could be plausibly denied.

Anonymous said...

The above is why I despise Arnie Gunderson. Through omission, deception, turning a blind eye, etc, he takes a few facts and spins a hell of a yarn. He's a scientist by profession, but a propaganda specialists by trade. He does a disservice every time he opens his mouth about Fukushima. He shamelessly manipulates this crisis at every opportunity.