TEPCO posted a section in a larger roadmap in Japanese about an investigation into the torus of unit 1 over gas build up concerns. Below is a rough machine translation of the document section.
Update: Fukushima-Diary just posted an NHK video explaining the hydrogen & krypton buildup in the torus. Apparently it has been an issue since April and TEPCO thought they needed to intervene.
For the nitrogen injection to the S / C Unit 1
October 22, Heisei 24
Tokyo Electric Power Co., Inc.
Method of estimation and verification mechanism
Pressure drop ① S / C pressure (← D / W
Due to) lowering of water level and PCV
Water level in the S / C that is reduced,
The top gas in the closed space (water
Iodine, Kr85) is vacuum break device
Discharged to the D / W through the 置管
At the top of the gas ② S / C emissions
When, inland S / C again
Place to rise, stop the spill
Repeat ③ ① ⇔ ②
In order to validate the estimation mechanism, which is filled with nitrogen into the S / C, check the response
2. As a result of the nitrogen injection to S / C (implementation inclusion 9/4)
After the start of inclusion N2, pressure S / C rise
Pressure gauge (S / C is the same as the line inclusion N2
Pene has been removed from the X205)
Pressure D / W minutely rise
N2 is sealed through the X205 Pene
Has been determined that the
With a lag time of about 1 hour, 12
Kr85 concentration, hydrogen concentration is up from at around
No increase in oxygen concentration
Street-estimation mechanism, or S / C
Those that are extruded to D / W from
– The concentration of oxygen in the S / C is almost zero
3. Continuous inclusion of nitrogen into the S / C
For gas retention in the S / C, which is confirmed by nitrogen injection 9/4 last
Carried out by the inclusion of continuous nitrogen purge.
※ root inclusion S / C is the same as when carrying out 9/4
4. The basic idea of the continuous inclusion of nitrogen
The hydrogen concentration in the (PCV is sufficiently low hydrogen concentration in the S / C is about 2% until the estimated
I do a continuous inclusion) until.
Upper limit of the concentration of hydrogen in the PCV is 2%.
From the results of simulation, the nitrogen injection flow rate is 1Nm3
/ and degree h, about one month
Carried out in the period hydrogen concentration (S / C drops sufficiently, 3Nm3
/ increase in degree h).
<Process at the start of nitrogen injection more>
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