Tuesday, November 1, 2011

More on Xenon Detection in Reactor 2 (TEPCO's Press Conference)

Xenon was detected in the order of 10^-5 (ten to the power of minus 5) per cubic centimeter, says TEPCO's Matsumoto in the press conference that is on-going right now in Japan. It is significant enough to conclude it is not from March. (TEPCO by the way started to netcast its press conference live, here.)

Matsumoto does think a localized, small-scale, and/or temporary nuclear fission may have occurred in Reactor 2. But not a large, sustained re-criticality, judging by the parameters (temperature, pressure, etc.).

Xenon-133's half life is 5 days, and xenon-135's half life is 9.2 hours. As to whether the amount of xenon is large or not, the company is evaluating with the help of experts.


  • Neutrons that exist inside the reactor hitting uranium or plutonium

  • Curium-244, -242, causing "spontaneous fission"

  • (I missed the third possibility that Matsumoto mentioned.)

It would be difficult to deny that the same thing is not happening in Reactors 1 and 3, says Matsumoto.

He thinks the possibility of re-criticality is low with the damaged fuel melted together with control rods and other things. Even if it does happen as the reactor is cooled further and the amount of water increases, TEPCO thinks it can be controlled by carefully monitoring the temperature and pressure.

(Reporters seem to want to reassure themselves by saying "In a nutshell, even if it is in re-criticality, TEPCO can control it, right?")

TEPCO's handout for the press on November 2, the result of gas analysis in Reactor 2 is reproduced below. The numbers are still being evaluated, says Matsumoto:


netudiant said...

The essential element for fission is slow neutrons.
The fast neutrons emitted from fission are unreactive unless they are slowed, which happens when they collide with the water molecules surrounding the fuel.
One can envision a heap of fuel at the bottom of the reactor vessel emitting neutrons and some of them getting slowed enough by the surrounding water to fission some of the fuel. Because the fuel is not carefully arranged any more in the surrounding water, it would not be able to ramp up, but it may also be more difficult to shut down completely than thought.
Boron injections seem a good idea in any case.
They were probably stopped because the boron is not cheap and perhaps because it may interfere with the water decontamination process.

Anonymous said...

Is anybody that surprised?

arevamirpal::laprimavera said...

@netudiant, thank you for clearer explanation.

It was almost comical to read what the so-called experts in Japan (so-called "goyo" experts - "doing the bidding of the government") were trying to say, on hearing this news. First they tried to change the definition of "re-criticality". When that failed when even TEPCO's Matsumoto admitted it may have happened, one resorted to deleting his tweet claiming there was no re-criticality.
(If you read Japanese, here's a togetter file: http://togetter.com/li/208588)

Anonymous said...

What does recriticality imply in laymans terms? The damn thing is running by itself, uncontrollable in fits and starts? So this is all about to get a lot worse? So much for the 'decontamination' work. Job creation scheme.

Anonymous said...

Not uncontrollable, by any means. If enough boron is added, there will be not enough slow neutrons to sustain a reaction and the system will become sub-critical again.

Darth3/11 said...

That's a mighty big "if"...

Anonymous said...

"Curium is typically quite insoluble .."

"Curium is a byproduct of plutonium production activities and results from the successive capture of neutrons by plutonium and americium, generally in nuclear reactors."


How much plutonium would be expected to be produced in Reactor 2 w/out MOX fuel?

Fish said...

Xenon isotopes confirm my suspicion that localized pulses of recriticality have been occurring at unit 2 since the initial failure of the reactor.

Because TEPCO has been very slow to release information, such as noble gas activity levels, that would indicate what has happened to the failed reactors, we are left to speculate.

Atomfritz said...

Where have these samples been taken, what has been sampled exactly?

These high measurements of Xenon-135, which is generated by the decay of the precursor fission product nuclide I-135 with a half-life of 6.6 hours really call for some official investigation.

Basically the first commenter is correct.
But, the truth is possibly more complicated than we'd like.

To explain this in a very simplified manner:

Neutrons are not being slown down by sort of "braking". They are slowing because they bounce with other atoms. Like billiard balls.

There are two possible things that happen when a neutron bullet hits another atom.
Either the neutron gets bounced off (and slowed).
Or, if the neutron hits the critical "neutron capture cross section", the victim atom either becomes a heavier isotope or falls apart into fission products.

Water, for example, is so well-suited as moderator because it usually bounces neutrons back, eating only a few (resulting in tritium).

Some atoms like some boron isotope etc thus work like pacmans that can eat up a single neutron each. (Like hafnium, gadolinium etc that are being used in control and moderator rods).
After this single neutron capture these "fission inhibitors" get dysfunctional, becoming moderators themselves, because they are now another isotope with a way smaller neutron capture ability.

So you can see, neutron moderation, reflection and capture is not limited to water and boron. These just happen to be most suitable technically. Other materials can do such too, even if less effective.

However, we are not dealing with many fuel rods finely separated by inhibitor rods.

We are dealing with a big, big blob of fissionable material that can possibly "moderate" itself, like a nuclear bomb does.

You could possibly think of it like a giant nuclear bomb exploding in ultra-slow motion.

Anonymous said...

"However, we are not dealing with many fuel rods finely separated by inhibitor rods.
We are dealing with a big, big blob of fissionable material that can possibly "moderate" itself, like a nuclear bomb does.
You could possibly think of it like a giant nuclear bomb exploding in ultra-slow motion."

And criticality events are actually quite more common in reactors due to fuel rod bowing in 2nd and 3rd cycle rod use.
They use 2nd and 3rd cycles because it's cheaper for them, instead of one-time rod use.


Many reactors today are above design for power, making them even more unstable.
Criticality power surges shut down reactors.

Anonymous said...

''You could possibly think of it like a giant nuclear bomb exploding in ultra-slow motion.''

That sounds pretty awful, Atomfritz, but good enough for this layman to understand.

Can it get any worse? (genuine question).

Beau Bergquist said...

Thanks for the report - we will work on modeling and plotting these source terms ASAP.

Any possibility you can post a detailed translation of this? For example what is the Cs137 7.0e-1 bq/cm-3 column? Additionally, does the document indicate flow rate (bq/hr), total mass (bq) of the emission, or total volume (m-3) of the measured location? Any of this info is greatly appreciated (you can email me datapoke->gmail.com)

Funny how Tepco is now changing units on us. Its important to understand 10^-5/cm-3 is 10^1/m-3.

We've recently plotted the plutonium and neptunium dispersion utilizing the leaked Tepco documented indicating the total amount of p-239 and np-239 emitted from the plant. This stuff sticks around for millions of years...

If you haven't seen the dispersion maps for plutonium they are published here (you can click cancel at the login):



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