For those who missed my previous diaries on this topic, I have a background in physics and worked at Oyster Creek Nuclear Generating Station providing computer support for the reactor core engineering group. For the entire qualifications spiel and/or some background on Boiling Water Reactor (BWR) containment structures, see
Everything You Never Wanted to Know about Nuclear Containments
That diary also contained a review of the then current status of each of the reactors at Fukushima. This diary is intended as an update reflecting what is known through today around noon. While much of the information has remained unchanged, I will discuss new developments, clarifications and corrections prior to summarizing the current status of each unit. Anyone wishing to trace the evolution can look back through the previous update diaries here, here and here.
In continuing diaries on this topic I will update this information based on information from a number of sources including the UN's International Atomic Energy Agency, IAEA, The Japanese Atomic Industrial Forum, JAIF, and media reports which quote directly from organizations such as Japan's Nuclear and Industrial Safety Agency. My intention here is to tie together the various strands of information to provide an overall picture of things and explain it in a way that is accessible to those without scientific training.
In this process there are a large number of details to track. In many cases, just as in life, we cannot know things with 100% certainty. This means determining what is the most probable situation, what its relative likelihood is in comparison to other possibilities, and also being open to changing that view or assessment as new information becomes available. I typically put new information through a few tests to evaluate its likeliness. One is the reliability of the source, and the other is the physical plausibility of the statement given what is known and what has happened to date.
This diary and others like it are not intended as a substitute for the ongoing liveblog diaries, but rather, to pull all of the info together in summary form.
I have also written previously on the topic of meltdowns in the diary
What, exactly, IS a nuclear meltdown?
Also, I would like to note an observation from this diary by gmoke
Seasaw Coverage of Nuclear Power
This was a collection of notes from a seminar at Harvard's Kennedy School which the diarist attended. This was the note regarding Matt Wald from the New York Times response to a question about the boundariess of uncertainty:
"We gravitate to worst case but perhaps should be more concerned with likeliest case."
This observation helped me understand why some commenters here are so ready to believe the flimsiest evidence that something terrible is happening despite having ample physical evidence to the contrary. They expect something terrible because all the media keeps feeding them is worst case scenarios. These remote possibilities have become the "true reality" for them and those saying anything different are involved in spinning and a massive coverup. This is unfortunate because having power in this situation to affect change requires that we first have a realistic and fact-based understanding of the situation. I guess I am fortunate in not having a TV in the house, not having the cable news talking heads to feed me rumors and frightening scenes of a radiological nightmare 24/7.
New Developments, Clarifications and Corrections
The report of power being connected to all six units was accurate but comes with a caveat. According to the IAEA site, the connection to unit 3 was only run to the control room initially. No power was connected to the various control systems of the plant. I've seen no update on this status. I've also not seen the reason for this delay reported as of yet.
There were indications that the reactor vessel at unit 1 was running very hot, on the order of 400 C. In response TEPCO increased their rate of water addition which in turn increased the internal steam pressure, steam which needs to be released to maintain stability and remove heat from the system. For this reason steam has now been coming from unit 1, just as it has from units 2-4. It is highly likely that the torus suppression pools at each of these units has heated to the point where they are largely ineffectual in removing heat from vented steam or significantly reducing its pressure. This is why we now see steam whenever they vent as opposed to early in the emergency when their venting was largely invisible gases.
Also, an update and correction regarding the spent fuel pools. IAEA released a table today with the inventory of each of the spent fuel pools. This answers some questions and also raises some new points. I misconstrued the report I was recalling regarding MOX fuel in the fuel pool at unit 3. From the best information I can find to this point it appears that it was all loaded into the core when the reactor was refueled this past Fall. This was what TEPCO claimed. As with everything they've announced, I take that as a starting point for consideration, but don't necessarily hold it as gospel truth. Given that there are 52 unirradiated fuel bundles in the pool at unit 3 this is the only guarantee I've seen that none of them contain MOX fuel. We know that none of the spent fuel is MOX because they just started using it this fuel cycle. I will update this with more certainty when/if I am able to get confirmation one way or the other. If you have any additional information on this please leave a comment and include a link to your source - Thanks!
Capacity Irradiated fuel assemblies Unirradiated fuel assemblies Most recent additions of irradiated fuel
Unit 1 900 292 100 March 2010
Unit 2 1,240 587 28 Sept 2010
Unit 3 1,220 514 52 June 2010
Unit 4 1,590 1,331 204 Nov 2010
Unit 5 1,590 946 48 Jan 2011
Unit 6 1,770 876 64 Aug 2010
Also, it may sound alarming that there is unirradiated, (unused in the reactor to date), fuel in the fuel pools. In fact, the opposite is more the case. Before being bombarded with neutrons in a reactor - neutrons that were slowed by water to enable fission interactions - Uranium oxide fuel is relatively harmless. You can hold a new fuel pellet in your hand without receiving a significant does. It does not have any of the fission products such as iodine, cesium, strontium or plutonium.
The numbers also make it more clear why they have not placed as high a priority on dealing with the fuel pool at unit 1. This had concerned me over the past day or so because of how long it has been since the hydrogen explosion. Looking at the numbers it is clear that there is much less heat load to be managed at unit 1. There are fewer irradiated assemblies and they have been sitting for a year already. Compare this with unit 4 which has more than four times as much spent fuel, is near capacity, and has only been sitting for four months. This explains a lot about the problems there, both in terms of heat load, and also water displacement - leaving less volume of water available in the region of the fuel rods and hence less cooling efficiency.
Also, something may be going on at the unit 4 fuel pool ... new update in that section below ...
Also, a bit of explanation regarding reported level of severity of this emergency. There are many who are of the opinion that this should be higher than a level 5 rating, the equivalent of a TMI type accident. And BTW, I certainly agree that the environmental impact of this accident far exceeds TMI and will likely cause a higher rating before this is all done. Something to keep in mind in this is that there have been nine different reports based on the events at Fukushima 1 & 2. Both the Daiini and Daiichi plants have had site reports in addition to individual reports on units 1-4 at Daiichi and 1,2 and 4 at Daiini. Unit 4 at Daiichi along with the three units at Daiini have all been assigned a level of 3 at the present. Units 1, 2, and 3 at Daiichi have been assigned levels of 5. I have not seen an update on the overall site level assignments. So they're not actually calling it a TMI scale accident so much as three TMI scale accidents plus a number of less severe but still significant problems.
Current Status of Reactors and Containment Systems at units 1 - 6
Unit 1
The fuel pellets and fuel rods in the reactor have almost certainly experienced melting due to high temperatures resulting from the fuel being uncovered. (I only say almost certainly because we cannot look into the reactor at this point to verify. The same is true for units 2 and 3.)
As a point of clarification, as soon as the melting fuel, cladding, and control rod bits melt they drop through the water to the bottom of the reactor vessel where they very quickly lose their heat to the enormous heat sink of the lower half of the reactor vessel. In theory, a chunk of melted fuel slag could have a low enough concentration of control rod slag that it could achieve criticality and start a nuclear reaction on the floor of the vessel. This is one of the purposes of the boric acid that was being added to the seawater going into the vessels. Boron is a very efficient neutron absorber and being in solution it acts like liquid control rods to fully surround any iffy chunks of slag and keep them from reacting. Remember, the neutrons need to be slowed by water to interact at any appreciable level, but as soon as they hit the water they get sucked up by the boron.
The reactor vessel and associated piping appear to be fully intact. Were this not so the reactor would not be able to retain the water being added and there would be no significant pressure buildups requiring venting. You may note that the status chart kept by JAIF shows the reactor vessel Integrity to be unknown. Looking into their methodology it becomes clear that they evaluate this parameter based on reactor water level and reactor pressure. Accurate pressure measurements have not been available for either reactor for several days. Hence they list the reactor pressure vessel integrity as unknown, this does not mean that the pressure vessel itself is not intact. This does not necessarily mean that things will remain that way indefinitely which may be the basis for JAIF's rating.
As time passes with the seawater stew inside it is possible that valve seats - the faces that meet to form a tight seal - may be affected by corrosion. Hopefully they have taken electro-chemical means to prevent or retard this process but that is not clear. This makes it a priority to get freshwater pumping systems reestablished so they can begin to flush the saltwater. Another high priority if their ability to use their cooling condensers so that venting is no longer needed and the reactors can be brought down to sub-boiling temperature and atmospheric pressure.
The primary containment also appears to be intact. It was well shielded from the hydrogen blast and has not had any other reported troubles. -There have also been no clouds of steam coming from unit 1.- As noted above, there is steam at unit 1 as well though it appears to be the result of venting as opposed to a problem with containment.
The secondary containment also appears to be intact. (For those who question this evaluation in the aftermath of the hydrogen explosion there I refer you to my diary on containment structures linked in the intro. It has a discussion of my analysis of the explosion and why I don't believe either unit 1 or unit 3's secondary containment to be badly damaged.)
There also have been no reported problems with the fuel pool at unit 1 despite the explosion. According to the JAIF site, they are now considering adding water to this pool. There was no mention of how they intended to reach the pool under the collapsed roof of the reactor building. There were reports the other day of plans to use army tanks to help remove debris from the tops of units 1, 3, and 4 to make the fuel pools more accessible. As noted above, the heat load from the pool at unit 1 is significantly lower than from the other units.
Unit 2
The fuel containment has been compromised as is made clear by the hydrogen explosion in the torus. It is also almost certain that there has been fuel melting due to the time that the upper parts of the fuel rods were uncovered.
The reactor vessel and associated piping seem to be intact here as well. There was a brief time when they had trouble maintaining water level increases but that was apparently resolved as a valving issue. There was also a steam release on Monday. It is not clear whether this was a planned release or what the mechanism for release might have been. There have been reports of a high radiation zone in the turbine building. This may have been related to the initial valving problem which prevented them from maintaining reactor vessel water level.
The primary containment may have damage. If so, this is a serious issue because it means that when they need to release steam from the reactor to reduce pressure, they no longer can use the large volume of water in the suppression pool to cool the steam. As of yesterday various reports have dialed back the claims of damage to the primary containment. This is both from JAIF and the NRC. They now say that it may be damaged or that damage is suspected. Given their apparent priority to get unit 2 power back it seems likely that there is still a problem there with the torus. And if they can isolate the torus from the drywell then they have a sealed containment. They just lack the cooling ability of the suppression pool, an ability that may have lost most of its efficiency already due to heating of the water.
They also were able to restore power to unit 2 on Sunday and discovered that a pair of key pumps were not working. Replacements have been ordered but may be a week or more. (I wonder whether there are identical pumps at unit 4 that could be used ... The plants are the same design and power rating.)
The secondary containment has been compromised by flying debris from the explosion at unit 3. It was reported that there was a hole in the wall of the reactor building. Whether any equipment was affected by this is not clear from reports. This, combined with the damage to the torus creates a situation where, when they need to vent, the steam is not cooled by the torus meaning that the pressure and temperature rise considerably in the drywell leading to a greater need for venting which now goes directly to the environment. And while the JAIF site lists this as slightly damaged as opposed to severely damaged for units 1 and 3, I believe the damage to the building at unit 2 has greater consequences because it is the secondary containment that is open. That does not appear to be the case with units 1 and 3.
There have been no reports of fuel pool issues at unit 2. They began using the new power line on Sunday and pumped thousands of gallons of water into the fuel pool at unit 2. They have been able to get the temperature there down to 47 C. I believe their normal operating temperature is 45 C / 81 F.
Unit 3
Clearly the fuel integrity has been compromised at unit 3 and there has almost certainly been fuel melting there as well.
The reactor vessel and associated systems appear to be intact and reactor water level is able to be maintained. Venting is continuing to release steam and remove heat from the reactor.
The status of the primary containment has been reported as compromised, and as not compromised. It apparently began holding pressure again in the past two days after failing to do so in the immediate aftermath of the hydrogen explosion. I believe I may have an explanation for what has happened. If you refer to the BWR diagram below you will see the drywell dome plug in the center of the top floor of the secondary containment, under the roof on the refuel level. The seal for this plug is an O-ring which gets compressed between the slabs of this plug. When the explosion happened, I believe the blast forced the top slabs to smash the O-ring momentarily before rebounding into place. I also doubt that this pressure would have been evenly spread over the surface of the plug. This would result in a deformed O-ring, leaving a gap where it had been most severely squashed and allowing reactor gases to escape at will.
But after a few days of steam venting through that gap, the rubber has had a chance to recover and re-expand to fill the gap. Thus the containment is holding pressure once again and caused them to dial back the damage assessment. And as I pointed out in the comments the other day, the ambient temperatures there were cold enough for it to snow so it is plausible that it would take days for the steam coming through to warm the rubber sufficiently to reseal the gap. This would also mean that the containment cannot be counted on to perform up to its design specifications as the condition of that O-ring seal and its capacity to withstand higher pressures would still be marginal. Again, this is a theory. I base this on the structure of the building, the location of the explosion, and the changing reports regarding containment status.
This is a serious enough issue that TEPCO would not have released the information that the containment had been compromised unless it were absolutely so. There were also reports in the first days after the explosion of white steam escaping from the remaining top of the reactor building. That happened from the 14th and it was noticed decreasing on the 19th - as per IAEA summaries. Then a few days later they reported that it was holding pressure, and today the JAIF site has the primary containment listed as not damaged. I do not share their optimism on this point.
The secondary containment at unit 3 took a hell of a hit from the hydrogen explosion, much more energetic than the others. (Units 2 & 3 are roughly 1-1/2 times as powerful as unit 1) From the various photos I've seen it still appears that the damage was largely confined to the refuel level - this would include the plug for the primary containment mentioned above. I do not discount the possibility of further damage within the reactor building as a result of this blast but have not seen any evidence that would confirm this. It may have something to do with their delay in connecting electrical power beyond the control room, but that remains to be seen.
The fuel pool at unit 3 is still a concern. It appears that there is a leak in the pool causing it to lose water faster than the heat load should cause. Many have raised concerns about plutonium due to the use of MOX fuel at unit 3. As noted above I am still tracking down the details about this. There would be plutonium in the pool anyway as a result of it being a fission product - this means that it is created in the reactor as a result of the nuclear fission process. Efforts have been ongoing to dump tons of water onto the refuel floor in an effort to refill this pool. They have also updated their method of getting water into this pool, as per IAEA:
Starting 23 March, seawater was injected into the spent fuel using the cooling and purification line. By midday 24 March, 4-5 tonnes of seawater had been injected through this line.
Unit 4
The condition of the reactor and containment at unit 4 is relatively unimportant given that all of the fuel was offloaded to the fuel pool to do maintenance on the reactor shroud, (the wall immediately surrounding the reactor cylinder).
-The fuel pool at unit 4 remains a large concern but appears to be stabilizing.- I may have changed this status too soon ... from rja in the comments, one of the updates today to Japan's Nuclear and Industrial Safety Agency listed a temperature of 100 C for the unit 4 fuel pool. Their previous and following reports listed the temperature as unavailable. If this value is accurate then that indicates that they are falling behind badly in their water additions and that is not a good sign at all. Given that they have improved their methods of water delivery this would indicate that the leaks may have gotten worse. OR, the temperature reading was not an accurate reading, possibly an artifact of their attempts to re-energize equipment and sensors. If this is an accurate temperature then I would expect to see stepped up efforts at water delivery while also consideration being given to immediate concrete entombment - an option I've read that has been discussed as a possible last ditch method to prevent a fuel pool fire. I think the reliability of this temperature reading can be evaluated based on the level of attention being given to this by operations people on site. I have looked for other stories on this or confirmation and found none.
Units 5 and 6
These reactors were both in cold shutdown at the time of the earthquake and did not have near the cooling needs as units 1-3. They have remained stable and there have been no reports of fuel breakdown or melting. They have now been able to use grid power to run their fuel pool cooling pumps. The temperatures in the pool have dropped considerably since they got the cooling pumps in operation and appear to be stabilizing. As a precautionary measure the other day TEPCO cut holes in the roof of the building at both 5 and 6. It now appears that while being a prudent measure, the holes will not be needed, either to vent hydrogen or to refill the pools from the air.
Common Fuel Pool
There is also another fuel pool at the Daiichi site which has fuel from each of the reactors. I had heard about this facility but this is the first solid information I've seen on it. From the IAEA web site:
In addition to pools in each of the plant's reactor buildings, there is another facility -- the Common Use Spent Fuel Pool -- where spent fuel is stored after cooling at least 18 months in the reactor buildings. This fuel is much cooler than the assemblies stored in the reactor buildings. Japanese authorities confirmed as of 18 March that fuel assemblies there were fully covered by water, and the temperature was 57 °C as of 20 March, 00:00 UTC. Workers sprayed water over the pool on 21 March for nearly five hours, and the temperature on 23 March was reported to be 57 °C.
Prognosis - The Good, the Bad, and the Ugly
The Good - The reactors continue to slowly cool and have responded as expected to corrective actions taken for heat removal. Also, efforts at each of the spent fuel pools seem to be gaining ground in assuring that the fuel is covered and cooling continues. They are also continuing in efforts to test and restart equipment using the new grid power line. Over the next few days it is anticipated that more plant equipment will be able to be used to help manage conditions.
The Bad - Having lost the integrity of the primary containment at both unit 2 and unit 3 is quite significant. It means that more radioactive material has been released than would otherwise have been necessary. At least with the containments intact it was possible for them to let some materials settle out before releasing to the environment. Now this buffer space/time is lost. It also means that if something were to happen to take the reactors back into unstable conditions, the design level of protection is no longer available. These containments may no longer be able to hold their gaseous contents at higher pressures.
The condition of the spent fuel pools is also an ongoing concern. While they appear to have made progress, this will be a long fight and there may be more hurdles ahead. And with the structural integrity of the pools at 3 and 4 being highly in question, having significant aftershocks in the region cannot be helpful as they only add stress to these fractures and potentially make them worse.
Also bad is the increasing degree to which food and water are being affected in an expanding area around the plants. This is adding to the already enormous humanitarian crisis in Japan. I will leave the discussion of radiological health effects to others with more background in that field.
The Ugly - There is still a large amount of seawater inside these reactors. Seawater is corrosive and provides a very complex chemistry inside the reactor given the number of elements present. Where this will have an effect sooner rather than later is on the metal surfaces that meet to form seals in valves, (the valve seat), and also on the internal moving surfaces/seals that maintain pumping pressure. This will not necessarily be a safety concern as the reactors are now closing in on being under boiling temperature and should never again expose these surfaces to the kinds of operating pressures for which they were originally designed. It may add costs and delays however if pumps fail and need replacement due to this cause.
It also means that the plant needs to be cleaned up and decommissioned as soon as possible. The longer it sits with its seawater nuclear stew, the more opportunities it has to cause new problems due to systems degradation and failures.
There is also the economic impact on the local farmers and watermen as the source of their income is now contaminated. There are also reports that the general radiation levels in Fukushima prefecture are hampering search and rescue operations, adding further misery to those in the immediate area.
Appeal for the people of Japan
Please remember the bigger picture here. There are millions of people still suffering in Japan at this hour. The aftermath of the earthquake and tsunami have created enormous problems and left many thousands dead, injured, or isolated and left unattended. This event is a natural disaster of unfathomable scope. The nuclear emergency, while certainly an important part of what is happening there, has still not had the effect on the population that the other events have to this date. If you can find it in your heart to contribute in some way to the humanitarian efforts for Japan you will truly make a difference in someone's life.
The Red Cross and Shelter Box USA
Also, my purpose with these diaries is to provide technical understanding of what is happening at Fukushima and discuss the likely ramifications of these events. It is not to engage in pie fights over the future or appropriateness of nuclear power. There are plenty of diaries for those discussions for people who choose to so engage. I also request that people kindly refrain from personal attacks and other forms of ad-hominem argument. This is an emotional topic for some folks but that does not excuse rudeness and personal insults.
(I have projects I have been ignoring that need attention. I will get back for questions and comments off and on as I am able for the next few hours, then more after dinner.)
BWR Mark I Containment
As per a suggestion from Jim P, This diary represents my take on the situation as of 19:00 on 3/24/11 UTC. (Noon PDT 3/24)
Updated by kbman at Thu Mar 24, 2011 at 05:36 PM PDT
Latest update from IAEA on reactors status ...
Reactor pressure is increasing in Unit 1, pressure readings are unreliable in Unit 2, and stable in Unit 3 as water continues to be injected through their feed-water pipes. The temperature at the feed-water nozzle of the Reactor Pressure Vessel (RPV) is decreasing at Units 1 (243 °C) and 3 (about 185 °C), and stable at Unit 2 (about 102 °C).
[Title edited as per recommendation from Meteor Blades in comments.]