APR: your source for nuclear news and analysis since April 16, 2010

Sunday, May 15, 2011

Core totally demolished at Fukushima Daiichi No. 1

We now have a fairly thorough report from TEPCO which details the extent of core damage at Fukushima Daiichi No. 1, and gives some times for certain events on the first several days. This report contains, again, some serious information and I will detail it for you as best as possible for a very wide audience. Before going into details, let me first give some of the most important revelations and analytical impressions off the top.

1. The entire core at No. 1 plant was essentially demolished within 16 hours or so after the scram.

2. Data indicate maintained water level in the core until the time of the arrival of the tsunami.

3. TEPCO feels that the core material is still inside the reactor pressure vessel, albeit all at the bottom resting on the lower pressure vessel head.

4. Analytically, the vast majority of studies and symposiums on BWR plants in a long term SBO situation have been proven correct in their findings. By this I mean the generally assumed time until core damage after loss of battery power, and the plain fact that extended SBO leads to core melt. (SBO = Station Blackout, loss of all AC power and then eventually loss of DC power in the long-term SBO sequence. There is also the chance of short-term SBO, but this is not our focus here.)


Unfortunately TEPCO has printed "all rights reserved" on its illustrations in this press release, but we can quote data.

Here from the data available now from TEPCO is a timeline of events at Fukushima Daiichi No. 1 plant, adding in historical data from our filed earlier reports:

MARCH 11 2011
14:46 Earthquake; automatic reactor protection action (full scram.)
15:30 Tsunami hits plant
15:42 Report of loss of all AC power
16:36 Report of inability to inject water to core
18:00 Water level is down to top of the fuel rods
19:30 Water level at bottom of active fuel. Fuel temp rapidly approaching melt point

MARCH 12 2011
01:20 Report of rapid rise in primary containment pressure
05:50 Begin injection of fresh water to core
06:00 All fuel in core is in some state of damage or melt
14:30 Began venting primary containment to atmosphere
14:50 Stop fresh water injection to core
15:36 Hydrogen explosion heavily damages reactor building
20:00 Begin injection of sea water (borated) {one source says 20:20}


TEPCO indicates that even though the core was destroyed about 16 hours after the protective trip (scram) it is still inside the pressure vessel, and has been cooled to some extent by water most of the time. TEPCO also feels that the relatively continuous injection of water has prevented the core from exiting the pressure vessel. At this time, the core is considered adequately cooled to prevent further release, at the very least.

The high temperature seen at the feed nozzle is due to exposed fuel in the core superheating injected cooling water.

TEPCO indicates that damage to the pressure vessel is likely. However, it indicates that damage isn't so severe that the corium could exit. The rationale for this assumption is the grouping of similar temperatures on the lower head and two different parts of the CRDM.

TEPCO also feels that since temperature and pressure in the containment seem to move in step with primary plant changes, there is enough damage to the pressure vessel and/or piping to permit what would appear to be fairly rapid communication between the two. However, TEPCO has backed off from saying that there is a serious large breach of the lower vessel head... instead referring to a number of smaller openings.

Not specifically mentioned, but important... It is also clear that the primary containment is damaged and that injected water has been issuing from the primary system to the containment, and then out of the containment to various locations for a long time.


-The occurrence of core damage as predicted in studies and computer modeling is as would be expected.

-The timing of beginning of fresh water injection to the core again on the 12th as stated in one source correlates well with the new TEPCO indication of all the fuel being damaged or melting a half hour or so later. This is because the readmission of water would rapidly damage the already super hot fuel cladding... as surmised in numerous studies covering core re-flood.

-Injection of borated seawater at 20:00 on the 12th was far too late. The first source used should have been borated.

-In order to be really meaningful, we need both many more data points than the times and events given above, and reasons for each. For example, what was the reason that fresh water injection was stopped at 14:50 on the 12th? Did the injection system fail? Did the site run out of fresh water? We can go on, but it must be stressed that we cannot read TOO much into the timeline given above. It is only a first working model.

As we now see, the previous BWR conclusions done in many years' worth of studies have proven essentially correct. Not that anyone thought that they would not; no one figured that the event would actually occur (long-term SBO) because of sufficient backup power. The tsunami (the true accident-triggering event, even with the latest data) was the real death blow to the TEPCO efforts on site. After the tsunami forced the plant into the true long-term SBO event sequence, it does now appear that plant behavior even with the best possible TEPCO operator action followed the expected curve and resulted in a complete core melt.

What is fortunate is that it appears as if the ongoing efforts at the site during that time ensured that the core debris (corium) remained in the pressure vessel, even though the vessel was damaged. It now appears that with the core totally destroyed, that the metal-water reaction generated hydrogen, coupled with likely added hydrogen from radiolysis and perhaps even methane (as mentioned twice or more on this blog previously) would have been enough to destroy the reactor building when ignited.

We will be watching very closely for further updates and particularly for estimation of the damage to the vessel and primary containment. We will be issuing some further illustrations in updates coming shortly.

4:00 PM Eastern Sunday 5/15


  1. what is the reason for the 10 hour delay between water levels dropping below the fuel and the stare of water injection?

  2. @jl: That is just one of the things which I find, going through past reports, to be covered by conflicting and highly contradictory data. There is a document we have, which is marked not for reproduction in any way, from NISA that seems to describe the actions during this time period but which makes it look as if there were in fact injection on and off during this period. We will have to wait for a more complete timeline from TEPCO for better details. These are the sorts of fine, specific, minute by minute details from which we will learn a great deal about future reactor safety -- as we did from a number of previous reactor accidents, ONLY after a long enough time had passed that good solid time and event lines could be established and analyzed.

  3. have you seen this report? I have only the news story http://www.asahi.com/english/TKY201105130370.html

    link hat tip to Greg Laden's blog

    "The Asahi Shimbun obtained a 100-page internal TEPCO report containing minute-to-minute data on radiation levels at the plant as well as pressure and water levels inside the No. 3 reactor from March 11 to April 30...."

    The data has never been released by the company ...."