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Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety
Report of Japanese Government
to the IAEA Ministerial Conference on Nuclear Safety
- The Accident
at TEPCO's Fukushima Nuclear Power Stations -

Any idiot can face a crisis - it's day to day living that wears you out.
by ceebs (ceebs (at) eurotrib (dot) com) on Sun Jun 12th, 2011 at 02:10:20 PM EST
That's a massive report. Even just the chapter on the accident itself. But so far, regarding No. 1, I found:
  • they think the earthquake did not break reactor circuit pipes, based on pressure/steam flow readings before the tsunami hit;
  • at last there is a confirmation that the electric switchboxes of the rectors were submerged by the tsunami, too (thus making initial attempts to restore AC power futile);
  • in addition to the generators and the switchboxes, the seawater pumps of the cooling system were damaged, too;
  • elevated radiation levels inside the building were first detected at 23:00 local time on 11 March, that is seven and a half hours after the tsunami hit;
  • TEPCO's difficulty in following up on the government's order to vent was also because of elevated radiation levels, which forced workers to use improvised methods to open the valves;
  • they mention some confusion in the line of communication between TEPCO's higher and lower echelons and the government regarding a supposed suspension of seawater injection after starting it on the second day, but TEPCO's current position is that at the order of the plant director, there never was an actual suspension.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Jun 12th, 2011 at 03:44:30 PM EST
[ Parent ]
From the next section I'm reading, it appears that basically none of the gauges installed inside the reactor pressure vessel are to be trusted, making estimates about the conditions inside guesswork. Take this last one for example:

In the present state, it is thought that steam continues to escape from the gas phase part of the RPV, but the RPV pressure is higher than the D/W pressure, so it is assumed that the opening is not large. However, the pressure changes after March 23 are changing in parallel with the changes in PCV pressure, so the possibility cannot be denied that there is a problem with the measurements.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Jun 12th, 2011 at 04:12:56 PM EST
[ Parent ]
Why do they think (most of?) the fuel still in the pressure vessel? Temperature readings:

...after the injection water amount was dropped, temperatures in some areas increased, so it is thought that the fuel is inside the RPV.

Apparently not all the fuel at the bottom of the vessel is under water (have you read this before?):

The temperature of part of the RPV (the feed water nozzles, etc.) is higher than the saturation temperature for the PRV pressure, so at the present stage it is estimated that part of the fuel is not submerged in water, but is being cooled by steam.

What is the rate of aerial loss to the environment?

The inclusion of nitrogen, which started on April 7, was measured to increase the pressure by approx. 0.05 MPa, so at that stage it was estimated that the leakage rate from the D/W was approx. 4%/h.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Jun 12th, 2011 at 04:22:27 PM EST
[ Parent ]
Reading on, I found the following interesting details regarding the No. 2 reactor's accident:

  • Apparently, they think hat the problems started with a water leak from the reactor pressure vessel, which shut down the steam-powered cooling system at 13:25 on 14 March (this system needs battery electricity only to operate valves). However, they say they won't know for certain that the cooling system worked properly even before the shutdown until they don't dismantle it to check its parts.
  • Here we have a data on how long it took to install and activate seawater injection: from 16:34 to 19:54 on 14 March (that's exactly three and a half hours). However, they are still not certain whether seaweater injection worked at all, against the high pressure.
  • In TEPCO's simulation, the fuel was uncovered five hours after the loss of cooling, with core damage from the second hour.
  • I can't make sense of what they say about NISA's cross-check simulation (possibly bad translation): they agree on the fuel uncovering and meltdown timeline, however, conclude that the pressure vessel was damaged five hours after the earthquake (that's 70 hours before the fuel was uncovered!). Regarding material release, NISA estimates gives a wide range: 0.4-7% for iodine, 0.4-3% for tellurium, and 0.3-6% for caesium.
  • There was one venting on 13 March (before the loss of cooling and the now assumed meltdown) and one at the start of 15 March (after the meltdown), both without achieving a reduction of dry well pressure.
  • The fuel conditions are the same as at No. 1: temperature changes indicate that (most of?) it is cooled at the bottom of the pressure vessel with part of it above water, and some may be in the containment vessel.
  • Regarding the suppression chamber explosion, without being able to get there, they still don't know the cause. However, there was associated external damage: to the neighbouring waste processing building.

As for a final conclusion:

At this point, we cannot indentify to what extent each component functioned, and therefore, cannot determine how the events of the accident have developed.

Still in the dark, three months on.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Mon Jun 13th, 2011 at 07:20:43 AM EST
[ Parent ]
Regarding No. 3:
  • It suffered less tsunami damage, for example the DC switchbox remained intact, and multiple cooling systems were operational.
  • The first cooling system failed probably due to batteries running out, the second because the water level gauge stopped and thus there was no reference value. The water level gauge was restored, but the cooling system probably didn't start again due to low pressure.
  • Again, the rate of water injection was low. According to TEPCO simulations, the fuel got uncovered about four hours after the stoppage, and core damage another two hours later, and another 20 hours later (66 hours after the earthquake), the corium damaged the pressure vessel.
  • According to NISA, the above events progressed slower (pressure vessel damage 79 hours after the earthquake).
  • The iodine and caesium that escaped was less than one percent.

Regarding No. 4:
  • Taking evaporation of the spent fuel pool into account only, the hydrogen explosion cannot be explained. Lack of significant damage to the fuel rods (as indicated by the water sample) and lack of visible cracks on photos also point against this possibility.
  • So they don't say explicitly but imply that the other theory now looks more likely: hydrogen from No. 3 entering No. 4 via the piping to the joint exhaust stack (there is a diagram showing the route of hydrogen from No. 3 to different exit points in No. 4).

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Mon Jun 13th, 2011 at 08:16:26 AM EST
[ Parent ]


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