First it was reported that "over 10,000 ppm" or over 1% of hydrogen gas was detected at 2 locations in the pipe that connects to the Containment Vessel of Reactor 1 at Fukushima I Nuclear Power Plant. Then it was allegedly "over 40,000 ppm" or 4%.
According to Jiji Tsushin, TEPCO thinks the hydrogen gas concentration in the pipe may be 100%. 1,000,000 ppm.
Still, TEPCO says possibility of explosion is not necessarily high because there is no source nearby that could cause sparks. (Never mind that they were going to use blow torches to cut the pipes...)
Jiji Tsushin (12:28PM JST 6/24/2011):
Concerning the detection of hydrogen gas in more than 1% concentration inside the pipe that connects to the Containment Vessel of Reactor 1 at Fukushia I Nuclear Power Plant, TEPCO announced on September 24 that it is highly probable that almost all the gas inside the pipe is hydrogen gas. TEPCO's Matsumoto said in the press conference, "Since there is no source for sparks, it cannot be said that there is a high risk of explosion immediately".
According to TEPCO, they measured the gas at the pipe exit several times in the afternoon of September 23. Each time, the result showed "flammable gas including hydrogen gas, over 100% ". The company plans to use the instrument that only measures hydrogen, in order to accurately measure the concentration of hydrogen.
It's so TEPCO. First they used the device that could only measure up to 10,000 ppm, and that maxed out. Then they apparently used the device that could only measure up to 40,000 ppm, and that maxed out. So they brought in a bit more powerful instrument, but it measures all flammable gases including hydrogen.
I suppose they were hoping that one of these devices would suffice. But now, even they have admitted that the air inside the pipe may be 100% hydrogen.