Reactor 1 water level is more than one meter below where the bottom of the fuel rods should be

[bananas]

Just want to make sure everyone is clear on how low the water level is:

"the coolant water in the reactor is at a level which would completely expose nuclear fuel rods if they were in their normal position"
"the water level was more than one meter below the bottom of the fuel rods"
"the fuel has fallen and is being cooled in the water below"

http://www3.nhk.or.jp/daily/english/13_03.html

No.1 reactor is in a "meltdown" state

Tokyo Electric Power Company says the No.1 reactor at the Fukushima Daiichi nuclear power plant is believed to be in a state of "meltdown".
...
The utility company said on Thursday that most of the fuel rods are likely to have melted and fallen to the bottom of the reactor. Earlier in the day, it found that the coolant water in the reactor is at a level which would completely expose nuclear fuel rods if they were in their normal position.
...

Friday, May 13, 2011 05:21 +0900 (JST)

http://www3.nhk.or.jp/daily/english/12_23.html

Water likely leaking from No.1 reactor

...
On Thursday morning, it was found that the water level was more than one meter below the bottom of the fuel rods, suggesting a large volume of water is leaking into the containment vessel.
...
Tokyo Electric says temperatures at the bottom of the reactor are between 100 and 120 degrees Celsius, suggesting that the fuel has fallen and is being cooled in the water below.
...

Thursday, May 12, 2011 15:08 +0900 (JST)

[bananas]

1. They couldn't believe it and thought the gauge was wrong

From one of the articles in the OP:
"The utility had suspected the gauge wasn't working properly because the water level hasn't been rising despite pumping in 150 tons of water daily to cool the reactor."

[madokie]

2. Incompetence all the way around

In China they'd be off with their heads by now.

[Javaman]

3. I keep thinking of that scene in "the china syndrome" where one of the workers...

flicks his finger against a meter because it "sticks".

[FBaggins]

4. IIRC, it's the same thing in K-19 "Widdowmaker". n/t

[bananas]

6. I was thinking of that too

IIRC that actually happened at TMI, they thought the gauges were wrong.

[Throckmorton]

8. The control room had terrible human factors engineering

The PORV (Pilot Operated Relief Valve, or sometime Power Operated Relief Valve) position indication was on the rear of the operators desk. Not out in front where it was easy to see. This is a valve connected to the top of the pressurizer that relieves over-pressures, before the code safety valve lifts.

They didn't believe the PORV tail piece temperature, because the valve had been leaking by all cycle, and it was always reading high. The high temperature was indicative of flow in the pipe due to the valve being open.

The was also no direct reading, accident grade, reactor vessel level indication, so it was just a guess as to how much water was in the reactor pressure vessel.

[Throckmorton]

7. Actually it was a chart recorder.

And the older pneumatic/electric ones do indeed stick, but are for the most part no longer used.

The lesson from TMI is that the reactor core will be far more damaged than expected. The "studies" concluded that 15 to 20% of the core at TMI had melted, when they finally pulled the reactor head, about 5 years after the melt down, it was "discovered" that 50 to 60% of the fuel had melted.

While I don't usually like to guess about these things, my personal experience with this generation of reactor cores leads me to believe that the melted fuel, now given the idiotic name "Corium" has pool in the bottom of the vessel. From there, it has undoubtedly overheated and failed the Grafoil seals on at least some number of CEDM (control rod) penetrations, allowing melted fuel to exit the reactor pressure vessel into the primary containment.

These failed penetrations will of course also allow coolant to leak by as well, along with the entrained/dissolved fuel fragments.

[kristopher]

10. Did you switch to Fukushima in the middle of the description?

I get the impression this is about Fukushima, "While I don't usually like to guess about these things, my personal experience with this generation of reactor cores leads me to believe that the melted fuel, now given the idiotic name "Corium" has pool in the bottom of the vessel. From there, it has undoubtedly overheated and failed the Grafoil seals on at least some number of CEDM (control rod) penetrations, allowing melted fuel to exit the reactor pressure vessel into the primary containment.

These failed penetrations will of course also allow coolant to leak by as well, along with the entrained/dissolved fuel fragments."

If that is correct and if you have a moment, would you repeat it with more detail aimed at lay readers?

[Throckmorton]

11. yes, I did switch topics, bad form I know,

In this generation of BWR, the control rods enter the core from the bottom up, in essence, gravity is not your friend in this design. This was done to simplify the upper head structure, which needed the space for the steam separators and steam dryers.

Because of this, each control rod, (which are actually not a rods at all, but a plus sign shaped plate made of stainless steel and boron carbide )has an extension shaft that penetrates the bottom of the Reactor Pressure Vessel through a hole. This hole is sealed with a packing gland, and a series of graphite packings called Grafoil (a trade name). The packing gland tightens the packing against the extension shaft, and prevents leaks (mostly). Because of the pressures and temperatures involved in an operating reactor (1100 PSI and 550 degrees or so), small leaks do develop and are captured by a seal leak-off collection system.

The graphite that composes the packing was never intended to remain functional for long at the 2200+ degrees F that the melted fuel will present.

Now for the conjecture part:

I believe that the graphite packing on several of these extension shafts have now failed, allowing some of the fuel to exit the core, under the force of gravity. This may also have created a series of leaks around each extension shaft, allowing cooling water to spray out the bottom of the reactor. This cooling water will be caring alone with it small particles of spalled fuel, akin to the mud in a river. This area of the reactor containment structure (called the dry well) is difficult to access under normal circumstances, and would be unenterable under this scenario. I would guess that dose rate over 100 Severt/hour would be common right under the reactor vessel. It may also explain the high doses received by the electrical workers 6 weeks ago. The leak off collection system ultimately finds its way to the feedwater system, and I am personally aware of issues at operating BWRs where the coolant collected by this system has caused contamination incidents in the past. The overflowing leak-off collection system flows into the feedwater system, and then leaks past the main feedwater pump seals (which have no seal water available due to the loss of power for the seal water pumps).

Once again, this is my informed guess as to what has happened, and i suspect facts to support it will take forever to come out.

[flamingdem]

5. Some are nitpicking total meltdown vs. state of meltdown

both seem to have the same result.

[Throckmorton]

9. Tomato, Tomatoe N/T