Since the discovery that hydrogen gas was trapped in the upper part of the reactor vessel at Fukushima Daiichi No. 2, TEPCO decided that even though the concentration was not high enough to be dangerous (2.9% was found, while hydrogen concentration must be above 4% depending on humidity to be combustible) it was high enough to consider and then implement a nitrogen purge directly to the reactor pressure vessels themselves.
In order to make sure all of our readers (and we get new ones all the time) are clear on what is going on, let's look at some illustrations we've used before on this site but which have some new captions.
Below is a view of what the reactor plants at Fukushima Daiichi look like. By that I mean BWR/3 and BWR/4 reactors, installed in Mk I containments.
Below is a view of the drywell assembly, inside of which is the reactor vessel. The drywell is the primary containment.
We know that the reactor cores have been seriously damaged; in fact, they are in all likelihood nearly completely damaged or destroyed by melting due to overheating. This can be expected to cause the fuel elements to collapse. That is the reason that I have used, and continue to prefer, the term "demolished" for the cores in these plants. So first we know that the cores are not in their expected locations, but are in all likelihood fallen to the bottoms of the pressure vessels.
However, we also know (and knew very early on) that the cooling water being injected to the reactors was getting into the reactor buildings. The best guess of course as to how it was getting out of the reactor vessels is that the core melt caused damage to the lower ends of the vessels. It seems unlikely that the entire cores have fallen out of the vessels, into the dry well area, according to TEPCO and NISA. But damage to the many various openings at the lower vessel head area is probable in part, and there may even be direct melt openings in the vessel lower heads.
The hydrogen gas we are talking about in these plants came from two sources during the accident: Hydrogen liberated by the energetic zirconium-water reaction which is expected in a serious core melt accident, and also by more normal radiolytic decomposition. Presently the generation of hydrogen, which is very slight, is only by the latter process. However, because there has been no gas flow in the upper ends of the reactor vessels, some hydrogen has probably been trapped there a long time and any new hydrogen may congregate there. To mitigate this, TEPCO has now added nitrogen feed to the reactor vessels instead of to only the primary containments (dry wells.) TEPCO has jumpered in, if you will, the nitrogen feed to the top core spray line (normally used for cooling water) to push the hydrogen through the vessels and into the drywells. Knowing that hydrogen will be flushed, TEPCO has almost doubled the nitrogen feed rate to the drywells from the previous rate.
Below, you can see the top spray line I'm talking about at the very top of the illustration. Click all illustrations on this page to enlarge them.
Below is an illustration I've made for this post. It is very large, but must be blown up to see the detail.
These are the details at the moment. Updates will appear on the APRA FB page, and/or on Twitter (see the Twitter news feed at the right of this page) until another post here is required.
8:30 AM Eastern Wednesday November 30, 2011
ATOMIC POWER REVIEW
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