[Japan]TEPCO: Melted fuel eroded containment vessel floor at Fukushima reactor [diagram inside]

Asahi Japan Watch ^ | 12/01/11

[TigerLikesRooster]

TEPCO: Melted fuel eroded containment vessel floor at Fukushima reactor

December 01, 2011

Most of the fuel rods that melted in the pressure vessel of the No. 1 reactor of the stricken Fukushima nuclear power plant ate into the containment vessel, according to Tokyo Electric Power Co.

It said on Nov. 30 the melted fuel did not breach the containment vessel but partially eroded its concrete floor.

As for the No. 2 and No. 3 reactors, most of the melted fuel stayed within the pressure vessel, TEPCO added.

Water levels have submerged the fuel rods at all three reactors, which is helping to cool them, the plant operator said.

TEPCO said its latest findings were based on water levels in the containment vessels, temperature readings and other data.

It admitted earlier that melted fuel partially breached the reactors, a potentially catastrophic development.

During the 1979 accident at the Three Mile Island nuclear power plant in the U.S. state of Pennsylvania, a core meltdown occurred but the pressure vessel was not breached.

[justa-hairyape]

Thanks for that image. So it is 10.2 meters from the top of the concrete to the bottom ? I was guessing 15 meters on another thread.

[justa-hairyape]

My guess is the concrete floor erosion in not a nice square pattern as TEPCO illustrates. Probably looks more like this.

[justa-hairyape]

Maybe they can cool the concrete structure itself with liquid nitrogen ? Then perhaps they would not have to be right next to the corium ?

[NVDave]

This is largely consistent with what was seen at TMI.

[LukeL]

But it looks like the containment building did its job and held the nuclear material.

[justa-hairyape]

So far that is apparently true. But we do know that the corium was and may still be eating its way through the concrete core catcher. This is based on a simulation study. Not actual measurements or observations. There is some debate as to whether it breached the dry wall which is a steel wall embedded in the upper section of the concrete.

[Razzz42]

Depends if the concrete is not cracked and can hold water.

Also, once the corium has dug itself a hole it prevents any coolant (water) from getting to it and is free to heat up via internal reactions.

In reality they are still not sure where the corium is and these are just educated guesses. No one can go near to find out either with a visual inspection. Even bots can take only so much exposure to radiation and there are literally tons and tons of the stuff in each unit now in corium form, radioactive blobs.

[WL-law]

You wrote:

“But we do know that the corium was and may still be eating its way through the concrete core catcher.”

But (unless I’m misunderstanding) the article admits (I say ‘admits’ because it’s dramatized in a way that tortures the truth, i.e., “China Syndrome”) that the rods are still contained in the steel structure, ie, they are not in contact with the concrete superstructure.

[justa-hairyape]

Technically there are two steel structures. That is what is causing the confusion. The inner steel structure is the RPV Reactor Pressure Vessel. That is where all the control rods and fuel rods are located. It is normally under high pressure. That has been pierced in all three reactors. #1 has anywhere from 70-90 % of the core escaped out the RPV. #2 and #3 have less. The RPV is located within a larger steel structure defined by a steel dry wall. This outer part of the reactor is supposed to remain relatively dry under normal operations. We do not know if the outer dry wall has been breached. Tepco is saying it has gotten close, but has not breached.

In #1 the rods and the RPV internals have almost completely melted and have dripped out of the RPV. Actually under pressure it can actually partially jet out the bottom of the RPV. That molten mass is now eating the drywell floor and has yet to get to the last steel wall which is called the dry wall. There is concrete on the dry well floor above the dry steel wall. That is the concrete that Tepco admits the corium (melted rods and stuff) is eating through.

Now that Tepco has admitted a significant amount of concrete floor erosion, there is another problem that must also be occurring. Gets much more technical and has to do with the drywell sumps and suppression chambers. Basically the corium mass could have flowed across the drywell floor and down the downcomers into the suppression pools. That is where the wet wells are located. That can cause a steam explosion and may have been what happened in Unit #2.

[justa-hairyape]

Forgot to note, the outer steel shell is called the CV Containment Vessel. The RPV is located inside the dry CV. So we have RPV breach, but no CV breach. However, if the explosion in #2 was indeed caused by the corium overflowing into the downcomers and into the wet well, CV has been breached. The Wet Wells are located outside and below the CV Dry Well. Tepco will have to be dragged kicking and screaming into admitting that, assuming it did occur.

[RoadGumby]

Having spent many hours over many years in seceral different drywells around the country, I would like to applaud you for your synopsis. Factual and clear.

[justa-hairyape]

I cheated. Used info from this link. Was always good at open book tests. Later.

Notes On Corium And Containment