This was initially reported in a TEPCO roadmap report that they were monitoring a pocket of hydrogen that has developed in the torus. The torus is almost full of water, that small air gap above the water in the torus holds the hydrogen concentration. We previously detailed TEPCO’s now ongoing work to deal with the hydrogen in the torus. TEPCO assumes the hydrogen in the air gap is left over from the initial accident. TEPCO has known about the hydrogen problem for a while, they do not state why they chose now to purge it.
The more concerning aspect of this is the air density concentration of the hydrogen in the torus air gap. At 4% concentrations hydrogen can explode. Hydrogen build up is thought to have been the cause of the explosions at the reactors at Fukushima Daiichi.
They certainly do not want this to happen again. Years earlier a hydrogen build up in a pipe at Hamaoka nuclear plant caused a pipe explosion at their unit 1. TEPCO workers previously dealt with a hydrogen build up in a containment watering pipe at unit 1 at Fukushima Daiichi in 2011. They were able to safely solve this problem. The current plan is to slowly inject nitrogen into the torus air gap to push the hydrogen out of the torus, assuming it will flow up the downcomer pipe and go into the containment structure. Nitrogen injection began on October 23rd, 2012.
This October 23rd press release makes a small yet disturbing clarification:
“At 9:37 AM on October 23, continuous nitrogen injection into Unit 1 suppression chamber was started. The nitrogen injection is planned to be continued for about a month until the hydrogen concentration in the suppression chamber is reduced to approx. 2%.“
This indicates that the current known level in the torus air gap is over 2% air density. 4% is the point where an explosion becomes possible. This plant parameter report for October 26th shows some activity that may be related to the attempt to purge hydrogen from the torus. Hydrogen in the containment bulb is at 1.36% at both sensors, up from zero a few days ago. TEPCO is also injecting nitrogen into containment at a fairly high rate of 20.48N㎥/h. The current containment injection rate at the other reactors is zero. Unit 1 also has a PCV (containment) gas outflow of 27.02㎥/h. This is higher than the other reactors and indicates the level gasses are being pulled out of containment through the filtration system.
These higher hydrogen levels in containment along with the related effort to control and purge that hydrogen may indicate the successful purge of some of the hydrogen built up in the torus air gap. It will be worth watching these levels over the next few weeks to see how they play out. We will continue to monitor TEPCO’s work on this issue.
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