(UPDATE 9/24/2011: It may not be even 40,000 ppm or 4%. TEPCO now says it may be 100%, or 1 million ppm, hydrogen gas. See my post.)
Just last night I reported on the hydrogen gas detected inside the pipe leading to the Reactor 1 Containment Vessel. TEPCO, according to the report and by their own press conference, said the density was "over 10,000 ppm".
Well. It was an understatement if the worker who tweets from Fukushima I Nuke Plant is correct. He says they went back to test the locations again with the meter that can measure up to 40,000 ppm. The meter maxed out.
It is over 40,000 ppm, or over 4%.
The media and TEPCO reported the number as "over 10,000 ppm", which is technically correct. "Over 40,000 ppm" is definitely over 10,000 ppm.
The worker tweeted:
And today [September 23; they knew about "over 10,000 ppm" the day before] we went to measure using the meter that can measure up to 4%. It was 4% max. So the work [to cut the pipe] was canceled, and TEPCO made an announcement. I didn't know, as I'm not very knowledgeable about chemistry, but it seems there's a chance of explosion if hydrogen gas is more than 4%.
He is wondering if zirconium cladding is still melting and reacting, producing hydrogen.
But as one reader of this blog points out after putting two posts from yesterday together (the one about Reactor 4 SFP water radiolysis and the one about the hydrogen detection in the Reactor 1 CV pipe), it may be possible that the water radiolysis is happening somewhere deep inside the reactor building where boiling water is getting irradiated and generating hydrogen gas.