Hydrogen gas / Fukushima Daiichi

TEPCO has detected a higher than previously recorded level of hydrogen gas at its Fukushima Daiichi No. 2 plant. This hydrogen, according to TEPCO, was probably located high in the reactor pressure vessel and primary containment until this time, and was flushed out when TEPCO placed its (improvised) gas handling system into operation. This new gas system changed the flowpaths for gas and is able to purge more of the previously mentioned volumes.

The highest hydrogen concentration noted thus far was 2.9%. The concentration has dropped. Roughly 4% is combustible, vapor concentration notwithstanding.

TEPCO has made a few alterations to its plans in order to mitigate the possible effect of this hydrogen; it plans to alter the nitrogen purge system feed flow path to inject nitrogen directly into the pressure vessel, instead of into the primary containment. This will take some time. In order to mitigate possible deleterious effects from hydrogen until such time as the new nitrogen feed is available, TEPCO plans to alter the feed injection rate to the No. 2 plant (this means cooling water) to make certain that there is enough steam present to make whatever hydrogen is available not flammable. Upon restart of nitrogen injection to the RPV, the nitrogen flowrate will be doubled. This procedure should flush the hydrogen and prevent any risk of burn.

Finally, TEPCO made a reference in a press handout to not beginning alternate water injection to Unit 1 until that unit, also, has direct nitrogen feed to the pressure vessel; I assume this means the addition of the core spray line to the feed line. (Number 1 plant is the only one of the three only using the feed line and not both the feed line and core spray line.)

5:30 AM Eastern Friday November 25, 2011
ATOMIC POWER REVIEW