Nitrogen inerting drywells..

Apparently the nitrogen injection to the drywell on No. 1 plant continues... and TEPCO has made a fairly revealing press release which includes information given to NISA prior to this time, for approval regarding this operation.

As mentioned here many times before, hydrogen is generated continuously in operating plants through radiolytic dissociation of water (high energy gamma.) In an accident scenario such as experienced at Fukushima Daiichi, it's also released through a very energetic metal-water reaction involving Zircaloy (an alloy containing zirconium) in the presence of high temperature steam. At this point, one might imagine either could take place even now, in different amounts.

TEPCO is now admitting something assumed here previously; that is, that the reactor pressure vessels are not fully intact. This could allow the hydrogen that's generated inside them to escape and build up in the drywells. Further, there seems to be very low pressure in the drywells due to condensation of core cooling water that's leaking out as steam. This action will tend to draw a vacuum on the drywells, further enticing the hydrogen to exit any small above-water penetrations or openings (or gaps between the pressure vessel lid and vessel itself) and collect in the drywells. Hence the need to inert them with nitrogen.

TEPCO now states that the drywells for No. 1 and No. 2 plants are damaged.. the damage to No. 1 being "relatively light compared to No. 2." This means that hydrogen that gets out of the reactor vessel due to the slight vacuum, but the atmosphere enters the drywell as well and the mixing of a concentration of greater than about 4% hydrogen (the value for burn or explosion is altered somewhat by the value of moisture content present in the air) could lead to another burn or explosion.

Put together this all sounds like a downturn, but taken each as its own fact none of them is particularly surprising and in fact the action TEPCO is taking, knowing for a fact there is serious core damage, is one of the things you'd expect them to be doing as soon as possible after higher priorities are taken care of. This may have been another good reason why TEPCO has been rushing to get access to all the normally occupied buildings and equipment spaces.

8:10 PM Eastern Wednesday 4/6
ATOMIC POWER REVIEW