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Friday, November 4, 2011

Xenon at Fukushima Daiichi No 2: Case closed?

NISA and TEPCO have released fairly voluminous documents to support the theory that spontaneous fissions of isotopes such as Curium 242 and Curium 244 may have led to the Xenon detection at Fukushima Daiichi No. 2. While TEPCO's diagram does indicate that neutrons released from these spontaneous fissions can and do trigger occasional fissions in the U-235 fuel material, TEPCO and NISA also indicate (as did I from the beginning) that criticality --- a sustained fission chain reaction in Uranium fuel ---is not actually possible with the core in its present condition.

Here is some of the material released today covering the development of the xenon in the PCV and its concentration.


Xe135 density comparison of spontaneous fission and actual measurement
1. Presumed radioactive density of the Xe135 by spontaneous fission
Even in a non-critical (shut down) state, a nuclear reactor usually has spontaneous
fissile nuclides in it. Well-known are Cm-242 and Cm-244, and currently in Nuclear
Reactor of Unit 2, the fission is as follows:
Cm242: 8.3E8 times/sec
Cm244: 7.4E8 times/sec (Appendix 1-1)
We don’t take into consideration the nuclear fission caused by the U-235 absorption of neutron. This is an evaluation conservative enough to show that the Xe135 detected this time was produced in a non-critical state.
Although Xe135 is also produced by Xe134 (n, γ) reaction, we don’t evaluate it due to the presumably small contribution. (Appendix 1-2)

Yield of Xe135 produced by Cm-242: 2.66% Generation speed: 2.2E7 p/sec
Yield of Xe135 produced by Cm-244: 1.22% Generation speed 9.0E6 p/sec
That follows in total: 3.1E7 p/sec

(Source:Fission product yields, http://www-nds.iaea.org/wimsd/fpyield.htm#T5)

As the present gaseous phase volume of the reactor vessel is 3,000 m3 (Appendix 2), and the amount of injected N2 is 4m3/h, it takes 214 hours to fully ventilate.
The amount of spontaneous fission is steady. On the other hand, because the amount of
the injected N2 from September to October is 14 m3/h, varying little, we think the amount of Xe135 in Nuclear Reactor is well balanced as the following formula illustrates:.

Yield of Xe135 per second = λ・N + 14・N/(3000×3600)
λ:Xe135 decay-constant (2.12E-5)
N:Number of Xe135 atoms in Nuclear Reactor
3.1E7 = 2.12E-5×N + 1.3E-6×N
N = 1.4E12 particles

Nuclide density of Xe135 in Nuclear Reactor per 1cc
= 2.12E-5 × 1.4E12 /(3000×1E6)
= 9.9E-3 Bq/cc

2. Xe135 Radioactive density measured in the primary containment vessel
Xe135 Radioactive density was measured from the charcoal filter, which was installed in the gas management system, is 1.7E-5 Bq/cc.
Collection efficiency of the charcoal filter is 1566 times as determined from the ratio of the value measured in the charcoal (5.3E-1 Bq/cc) and the value measured in a vial.
Thus, the density of the Xe135 Radioactive in the primary containment vessel, obtained from the measurements, is

1.7E-5 × 8.3E2 / 5.3E-1
= 2.7E-2 Bq/cc

3. Conclusion
The density of the Xe135 Radioactive in the primary containment vessel, obtained from
the measurements, is the same order of the Xe135 density estimated to result from the
spontaneous fission of Cm.
Therefore, Xe135 detected this time, arose from spontaneous fission of Cm.


This should put to rest all of the furor that's occurred on anti-nuclear propaganda sites, pages and forums everywhere concerning supposed recriticality / restart of the core, or a continuing meltdown.

7:16 PM Eastern Friday November 4, 2011

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