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Tuesday, November 1, 2011

BREAKING: Xenon detected at Fukushima Daiichi No. 2


-TEPCO has injected borated water (boric acid suspension) into No. 2 reactor at Fukushima Daiichi after detecting Xenon-133 and Xenon-135 in the primary containment of the plant.

-The borated water injection lasted one hour, from 3 AM to 4 AM on Wednesday November 2, 2011.

-According to information released to NHK Japan, TEPCO cannot rule out (and this is my rewording of their release) some measurable fission rate in the demolished core structure.

-A look at the indication by TEPCO to NHK and a quick perusal of graphical data to the present shows no rise in PCV pressure or any of the measured temperatures at the plant. However, as I've noted many times, the state of derangement of the lower pressure vessel head... and the amount of fuel material exit from the pressure vessel... cannot be guaranteed or even accurately estimated. Thus, lower vessel head temperature may not be indicative of the temperature of the entire (demolished) core mass.

-TEPCO states that the Xenon concentration in the samples from the gas handling system is "low."

-NHK indicates a statement from NISA that there is little or no chance that temperature has risen enough to cause further melting ... which seems obvious at this point. (For those not familiar, it would be difficult for the mass to achieve sustained criticality at this point; what we might be looking at is very limited increase in fissions per unit time, leading to generation of these Xenon gases which are fission products.)

Xenon background:

Xenon is produced in a reactor as a product of fission; when the U-235 nucleus splits, the resulting pieces or "fission fragments" can be a number of different elements.

Xe-135 is produced directly from fission in a very small amount (~.2 to .3% of fission products) but is produced indirectly in larger amount from Tellurium 135 since the Te-135 decays in less than two minutes to I-135 which then decays with a half life of 6.7 hours to Xe-135. This Xenon, unless it absorbs a neutron (which it has a very high propensity to do - it is an impressive neutron absorber or "poison") will then decay with a half life of 9.13 hrs to Cesium 135.

It seems obvious then, considering the Xe-135 alone, that this finding indicates that there are fissions occurring. Fissions are indeed normal even in a shut down reactor, of course; they occur naturally.

TEPCO has injected borated water because boron is a significant neutron absorber; it is thus a "poison" and will absorb neutrons before they can be absorbed by U-235 and cause fission. Clearly TEPCO has considered that an increase in fissions per unit time may have occurred, or perhaps even a recriticality in a limited volume of the core material.. having stated explicitly that it cannot be ruled out.

Having said all this, the net probable effect on the total operational aspects of the plant itself, on safety, and on environmental release is probably ZERO. This is assuming a number of things; we will certainly be awaiting a press release or sitrep update from TEPCO in the later hours tonight.


Update 1 - 9:45 PM Eastern - Both NHK and Kyodo News are now carrying news flashes on this story.

EDIT: November 3rd... Boric acid injection took place from 2:48 AM to 3;47 AM on November 1st.


  1. Absent any numbers on this, a small rise in fission might be associated with successful cooling, if there is some disordered mass of fuel away from the thermometers. If that area has so far been well above 100C but has now dropped down towards boiling point, that could allow liquid water into an area that has the potential geometry for fission given the presence of a moderator (of course). The area would then heat up again if any fission started and drive off the water again. This was the mechanism for the natural reactors at Oklo, of course.

    I can see the mainstream headlines jumping to conclusions now...

  2. I too followed your exact same reasoning, joffan, and immediately thought of some pocket with fissile material and some free neutrons, with some coolant water that seeped in and acted as a moderator, starting a very short-lived fission, which would immediately stop as soon as water would vaporize.

    Exactly the same configuration as the natural reactors at Oklo. Quite amazing.

    Obviously, no harm to the public, and nothing to worry about. But nuclear always hits the headlines, and ignorance on complex topics like this never helps...

    Luca Bertagnolio
    Futuro Nucleare