Fukushima fuel rods may have completely melted

[alp227]

Source: The Guardian

Fuel rods inside one of the reactors at the Fukushima Daiichi nuclear power plant may have completely melted and bored most of the way through a concrete floor, the reactor's last line of defence before its steel outer casing, the plant's operator said.

Tokyo Electric Power (Tepco) said in a report that fuel inside reactor No 1 appeared to have dropped through its inner pressure vessel and into the outer containment vessel, indicating that the accident was more severe than first thought.

The revelation that the plant may have narrowly averted a disastrous "China syndrome" scenario comes days after reports that the company had dismissed a 2008 warning that the plant was inadequately prepared to resist a tsunami.

Tepco revised its view of the damage inside the No 1 reactor – one of three that suffered meltdown soon after the 11 March disaster – after running a new simulation of the accident.

Read more: http://www.guardian.co.uk/world/2011/dec/02/fukushima-fuel-rods-completely-melted

[Renew Deal]

1. Is this what's considered a nuclear meltdown?

It's amazing the quiet the press has gotten on this since around 3-4 weeks after the tsunami.

[Poll_Blind]

3. Not only is this a meltdown, if it hits groundwater (assuming it hasn't yet) then it would...

...be the China syndrome.

Basically the China syndrome (rods melting down and boring down "all the way to China") doesn't happen but the final stage of this is much worse. It may already be happening for all I know. With molten material like this it can flow like a fluid. Apparently it's still concentrated enough to form occasional criticalities. Usually a criticality like this would blow itself up but I've been getting the impression that wherever it is, it's in some kind of granular substrate which doesn't absorb the material so much as mix with it.

Think of water in a bucket of beach sand. Yes, they're together but the water and sand don't really mix with each other. The sand sort of spreads the water out.

PB

[izquierdista]

4. Like silly putty is a fluid

This stuff is like The Blob looking for a hole to hide in. Unless it hits something denser that contains it (shale or clay layer?) it is unlikely to go critical. The more boron they can inject into the meltdown pit, the more they can moderate the nuclear reaction and cool it off. I expect they will try for the same solution as Sneferu did (although his problem was not nuclear, but where to spend the afterlife).

[jtrockville]

2. "may have...", "appeared to have..."

One thing's for certain: the absence of certainty. Now doesn't that inspire confidence?

[AtheistCrusader]

5. Most of the way through the floor my ass.

There's no evidence it's even gotten through the steel layer of the primary containment, let alone 7 meters of concrete.

[Yo_Mama]

6. The concrete is on the bottom of the drywell inside the containment

That's what they are talking about. These units were designed that way. Once the fuel goes through the bottom of the RPV, it drops onto the concrete drywall floor, where it is kind of supposed to spread out and slow down. Flooding of the drywell was designed to be the next step, and they did do that. They also injected boric acid.

It's not news at all that some of the core wound up in the drywell. That's been the assumption for quite some time, and it was strongly supported by the rate of water flow through the RPV. There had to be some pretty big holes in the bottom, not just small leaks.

The steel shrouds that go down into that concrete weren't breached according to this study, and the bottom of the primary containment is still intact according to this simulation.

I don't know why people are reacting like this. Maybe they are not thinking in terms of process at all, or maybe they are envisioning a structure somewhat like a flimsy condo and thinking that a beachball of corium just crashed right down through the basement ceiling onto the dryers, rapidly burning its way to the floor while vaporizing all the odd socks.

The truth is somewhat different:
http://neinuclearnotes.blogspot.com/2011/12/on-containment-vessel-damage-at.html

In addition to the 2.6 meters (about 8.5 feet) of steel reinforced concrete inside the containment vessel, underneath the steel shell of the containment vessel lies another 7.6 meters (about 25 feet) of basemat reinforced concrete and steel. Altogether, that means there was 10.2 meters (about 33.5 feet) of reinforced concrete and steel standing between the reactor core and the outside of the plant before the accident.

Even if 2 meters (about 6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27 feet) of reinforced steel and concrete lies between the melted fuel and the external environment.

When these things were built they were designed to be massive and have a lot of protection, and they are designed to be extremely quake resistant.

Even if it did make it through the bottom of the primary containment, it would be stuck in the base infrastructure layer.

I can't see how anyone can possibly theorize that it's made through more than 30 feet of concrete and steel and is burrowing on down to the water table. Nor can I envision what they expect to happen when it hits the water table under that scenario, since it's been sitting in water for months.

The way that I took this study was that TEPCO believes the simulation shows that structural integrity of the whole building system isn't compromised. The most worried speculation I saw around recently seemed to be about the possibility that chunks of core might be through or close enough to structural steel supports to destabilize parts of the structure, and that is an understandable concern given the frequent quakes.

[AtheistCrusader]

7. It would make it harder to clean up too.

If the remains of the RPV falls over, well, that's one more damn obstacle to cleaning this thing out, using the processes we used at 3 mile island.

I read the headline as hyperventilated-ly implying the corium was almost through the 7-meter outer layer of concrete, not in the inner 'platform'.

[Yo_Mama]

8. Yeah, it was.

Hyperventilated, that is.

I figure it will be decades before they can get in there to clean this up. It's not the huge wreck that Chernobyl is, but the only thing you can do is isolate it in situ until it's much, much less radioactive. The stuff is dribbled around, and probably amalgamated into the structure in places.

Given what we believe to be the case, it would really suck big time if the remains of the RPV tipped. That would tend to expose a lot more core remnants at least in the reactor building, and those are certainly so hot already they can't effectively do much in them anyway. I think they already have the outer shroud built around reactor 1 to provide more isolation. They are going to build those shrouds around 2 and 3 also.

So I read this simulation as relatively good news. We knew the core was out of the reactor pressure vessel anyway, and this seems to be a pretty credible simulation. If it's wrong, it would pose further risks. If it's right, it's giving a little more safety margin than we thought we had.