Fukushima - Why did Unit 2 release so much more radioactivity than Units 1 and 3?

SteveElbows

I expect that the term dry venting is supposed to mean deliberate venting by one very specific route, as opposed to unplanned escapes from drywell containment due to some kind of containment failure. Most importantly this route still involves the stack, but as you mentioned does not include scrubbing via the S/C so it has larger environmental implications.

Sadly unlike the initial vents at reactors 1 & 3, we have no visual confirmation of anything leaving the stack during any of the reactor 2 venting attempts, mostly because the attempt at dry venting happened at night, and the old 'one published image an hour' camera was no use at night.

What I find especially annoying is that we don't even know exactly when steam was first noticed to be coming from the blowout panel hole in the side of reactor 2. We certainly know that plenty of steam etc came out of this hole later, as we have footage, but not knowing when it began is frustrating. Assuming it was from the reactor rather than the fuel pool, which is a reasonable bet given much later footage showing steam from the reactor arena and the robot-measured radiation readings showing high levels by one side of the floor above the reactor well, we may well imagine that this started on March 15th. But it would be good to know for sure, especially as events at reactor 4 building on the 15th also got in the way of really clear explanations of radiation data on that day.

zapperzero

I feel a bit guilty for being unclear - I meant to say that there is a number of routes by which one could achieve this kind of venting

One could route the steam through the SGTS or not; if the suppression pool water level is lower than that of the steam pipes for some reason, dry venting is also achieved, by default; there is also a choice of RPV valves that could be opened.

Yamanote

Probably the venting of unit 2 happened in the way Arnie Gundersen explained in one of his videos:

http://vimeo.com/36492960

clancy688

Nope, most definitely not.

That's only a "containment vent", not a reactor vent. Moreover, it doesn't vent high pressure. It only ensures, that, at high pressure, the pressure doesn't get any higher. And that only goes for the containment.
So when pressure inside the containment reaches a certain value (I think it was twice the normal pressure), the containment cap lifts and pressure escapes. The pressure falls slightly and then the cap closes again. So the pressure can't go over twice the normal pressure, but it also can't go below twice the normal pressure.
But that still doesn't vent the reactor pressure vessel. And the unfiltered release ("dry venting") can't happen that way. Because for reactor pressure to escape via a lifting containment cap, the gases have to travel through the wetwell (torus). Which they obviously didn't, that's why we have such a dirty release for Unit 2.
So there must've been an other path.

zapperzero

We don't know that. The torus may have been dry.

SteveElbows

Now you are confusing me a bit clancy688, either I misunderstood or else I don't think I agree with you about several things.

I thought that the terms wet and dry venting applies only to how the gases are released from drywell containment to the outside world, not how those gasses got into containment in the first place. If the gases were vented from the reactor vessel to the s/c, but then ended up in the drywell, and the drywell was then vented directly, I think that would still count as a dry vent.

A quote from the interim investigation report gives us some info about the venting of the reactor vessel into containment, with emphasis on how poorly the s/c seemed to handle this:

and

and

(from pages labelled as numbers 257,258 and 259 of http://icanps.go.jp/eng/120224Honbun04Eng.pdf )

Obviously what we can't tell from this is whether the bulk of the depressurisation of the reactor was achieved through the deliberate operation to depressurise the reactor vessel, as opposed to the fuel falling out of the bottom of the reactor vessel. Its been ages since I looked at the pressure charts for this period, I will have to refresh my knowledge on this front and comment again if anything seems relevant.

In any case I find the approach of looking at containment failure in terms of a very simple model of a cap rising and falling at certain pressures to be just a bit too flawed. We know that there are a range of potential failure points in containment, such as gaskets & flanges around not just the cap but also pipework points and equipment & personnel airlocks. And we know that heat contributes to degradation of such seals, so we aren't just dealing with pressure. Now it may turn out that the cap rising was a main culprit at one or all of the affected reactors, but its going to be a long journey to discover this. Certainly we saw some visual evidence of stuff escaping from locations around the top of containment at all three of the reactors. Reactor 3 offered the clearest view of this, could see it emerging from at least the area where the drywall concrete pit gates meet the floor area above the reactor. But we saw some signs at reactors 1 and 2 as well, along with the robot readings around that area of reactor 2 that I mentioned earlier. But this isn't proof that the cap lifted, cap or another part of containment could have degraded in a different manner.

SteveElbows

Does anybody know where a Japanese version of the following document might be found?

It is described on Tepco's English website as '- Fukushima Nuclear Accident Investigation Report (Interim Report Supplementary Volume) (PDF 212KB) '

http://www.tepco.co.jp/en/press/corp.../111202e16.pdf

The reason I ask is that in the section dealing with reactor 2, on page 39 it says the following, which as you can see is suffering from poor wording just at the moment when it could reveal something about the status of the rupture disk:

Sorai

This one: http://www.tepco.co.jp/cc/press/betu...es/111202g.pdf

Seems to be the same info. They couldn't confirm whether or not the rupture disk was open:

etudiant

There has been speculation on EX-SKF that the rupture disk did rupture, but that debris from within the RPV clogged the piping for the vent. Given the violence of the events, this does seem plausible.

SteveElbows

Thanks very much, this is perfect, just what I needed. The Japanese version is much clearer than the English version. If I google translate the relevant sentence I get this:

Whereas the English version fails to make that point clearly at all, due to using rupture words three times.

So now we have heard them say that the rupture disk status is unknown, the theory on ex-skf that there may have been a vent for a couple of minutes, is not incompatible with official statements.

SteveElbows

I've just been saying elsewhere that it might be useful to look again at some graphs in this document:

http://www.kantei.go.jp/foreign/kan/...ttach_04_2.pdf

Specifically the CsI distribution graph for reactor 2 Tepco Case 2 on page 36, and Cs graph on page 37.

Note the presence of reactor building, FHB (fuel handling building I guess) and environment in these graphs, as well as the timings. Then compare to the other reactors.

Yamanote

It seems to me that successful venting through the designated path is almost impossible under accident conditions and without electricity, at least for the Mark I containment (don't know if it would work out better for other containment designs).

Is the "blow out panel" opened at unit 2 actually an improvisation or a feature?
Because units 1 and 3 explosions showed us that trying to contain gases and Hydrogen (which must be released anyway) within the secondary containment might not be the right approach in an emergency.

zapperzero

Design feature. But it was popped from the outside, by hand, as it were, to avoid the accumulation of hydrogen. Seems to have worked, to a point.

Nothing is supposed to be released into the reactor building. The very worst case scenario designed for is to vent some steam from the RPV through the S/C, then through the hardened vent (bypassing the undersized SGTS) and out the stack.

rowmag

Are you sure? My recollection is that they announced that they intended to open it, but then discovered that it had already popped off by itself. (As a result of the Unit 1 explosion?)

zapperzero

No, I am not sure. This is my recollection however. I would be grateful if you can dig up a source, of course.

Yamanote

I agree that it was not supposed but happened for all three units. And like Steve Lochbaum said, even unit 4 paid sympathy to the others and exploded as well.

So in the future we have to suppose that unintended venting into reactor building will happen in such kind of accidents. Opening the blow out panel in advance might be a good idea and venting into another unit through shared equipment must be avoided.

zapperzero

You got that right, I think.