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
That diary also contained a review of the then current status of each of the reactors at Fukushima. This review covered each of the levels of containment discussed in the diary plus the spent fuel pool. If this sounds somewhat foreign to you then you may wish to read the above diary for context. This diary is intended as an update reflecting what is known through Saturday evening around 8PM PDT. Anyone wishing to trace the evolution can look back through the previous update diaries here, here, 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.
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
In addition, from time to time I consult with a former colleague Stan who was a reactor core engineer and Site Technical Adviser at Oyster Creek. (BTW, the Oyster Creek experience is directly applicable in the sense that it is the same design plant as those at Fukushima 1-5, at 618 MWe it was more powerful than unit 1's 460 MWe but less than 2-5 which are all 784 MWe.)
I also make every effort to be clear when I'm writing about known fact versus theory, interpretation, and speculation. In those cases in which I speculate on possible causes of current conditions or what future events might be I provide the supporting evidence which causes me to arrive at these conclusions. I'm also not attached to being right. If you have a perspective that I have not considered please mention it in the comments and we can discuss the relative merits of how we see things. I have very few things that I believe beyond doubt, and even those I have my doubts about :) Seriously. What's so trumps theory, belief, interpretation, speculation, etc. When presented with reliable evidence that contradicts what I have held to be so, I change my beliefs.
Freshwater is now being pumped into all three reactors. There are plans to switch to the use of fresh water for the fuel pools as well. Also, TEPCO is in the process of removing the highly radioactive water from the basement of the turbine building of unit 1 to allow access for electrical runs. This will allow them to use electrical pumps instead of the fire pumps they have been using. Also, work has been suspended at unit 2 due to exceptionally high radiation levels in the water in the turbine building.
There have also been reports of significant radioactivity in the ocean near the plants. This has also included the kinds of isotopes which indicate that their source was the reactor and not merely wash down of contaminants from the plant or fuel pool. This is because they have elements that are only produced while the reactor is reacting and which also have very short half-lives. (It takes about 6 1/2 half lives to decay down to 1% of the original amount.) I believe I have a possible mechanism for how the water got to both the turbine building and also the ocean. This is obviously theory and speculation and there are some physical details of which I'm not completely clear. I'm not saying this is how it is happening, merely that it appears to be a possible mechanism. If you are not interested in reading my speculation on this please jump ahead to the status update section.
So how DID Reactor Vessel water get into the Pacific Ocean?
During normal operations steam flows out the top of the reactor in the main steam line which feeds the turbines. There are several devices involved and a number of turbine stages which use as much of the energy in the steam as it is practical to extract. The "dead" steam then passes through a condensing unit and heat exchange process to remove enough heat that it becomes water again. This is done by feeding the steam/condensate through a huge number of small tubes which are surrounded on their outside by cooling water drawn from the ocean. The ocean water absorbs the heat and carries it away and the condensate - water - is then pumped by the feedwater pumps back into the reactor.
When the plant(s) shutdown during the earthquake, their main steam line isolation valves would have shut immediately. (This theory applies to varying degrees to each plant.) The flow of water through the core would have switched to another cooling system which uses a heat exchanger to cool the reactor water. This and other similar systems were the ones which could not be used after the power failed.
As I've mentioned a number of times in diaries and comments, the most immediate threat I see from corrosion is on valves and seals. And in this case I believe it is backward flow through the feedwater line that is transporting reactor water to the turbine building and also to the ocean. The feedwater enters the reactor well above the level of the fuel rods. As such, that pathway has been constantly exposed to the reactor pressure and the steam that is present above the water line. This exerts backpressure on the feedwater line towards the pumps and heat exchanger. There may or may not be a cutoff valve on this line as well. If there was enough existing corrosion in the pump seals and cutoff valve to allow a little bit of flow, or if the cutoff valve did not properly close, it would allow backflow towards the heat exchanger.
[Update] Here is a diagram provided by FOYI in the comments. The blue path going backwards from the reactor and then mixing with the gray-blue coolant, (ocean) water is the process I'm describing here.
Now here is one of those dirty little secrets that may come into play in this circumstance. It is almost a certainty that some number of heat exchanger tubes have leaks. This is almost inevitable over time in an operating plant, nuclear or otherwise, especially one which uses saltwater as the cooling medium. It would not be noted under normal operating conditions because the radioactivity of the water would not be that great. Remember when they said that the water the workers stepped in was 10,000 times the radioactivity of normal coolant? That high level of contamination is because of all the fuel that was busted open when the water level was lost in the first hours of this emergency. Some of it melted into clumps and fell to the bottom of the reactor vessel, other bits just came out of their cladding and the ceramic pellets disintegrated in the heat. The clumps are contained fairly well. It is the pellets that broke up that are coming out in the water. The water at unit 2 is much hotter than at unit 3 which may be the result of the unit 3 reactor water being diluted with less radioactive fuel pool water.
They have gotten the operations of these reactors to the point where there is not much radioactivity at all in the coolant/steam/condensate during normal mode operations. It is because of this that they could get away with many leaks in their heat exchanger. There would be no appreciable radiation detected in the water, no regulatory flags would be waved, and they could go on operating longer without going through the costly process of repairing corroded heat exchanger tubes. My belief is that the constant steam pressure on the feedwater inlet line caused enough backpressure to generate flow out the bad heat exchanger tubes and directly to the ocean. At first this would have just meant emptying the relatively low radioactivity water that had stalled there when the reactor was shut down. But as more and more volume drained down the piping acted as a condensing surface causing the exceptionally radioactive steam to become water and mix with the remaining stalled water.
And after days of blowing this stuff out to the ocean, there may have finally been enough corrosion on one of the local components in the turbine building to cause it to leak into the basement. Or that water may be coming through an entirely different pathway. Anyway, those are my thoughts on how this is happening. I welcome your take on things.
I just read another theory being advanced regarding the possibility that the ruptured torus at unit 2 could be the source of the much higher levels of radiation in that water. I discount this because of how long ago it was that they popped the torus. The very short-lived isotopes would be much lower already in that water.
Current Status of Reactors and Containment Systems at units 1 - 6
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.) And as mentioned above, some fuel that did not melt has been released from its cladding and has broken up in the reactor water. They have begun injecting fresh water into the core to avoid adding even more salt.
While the reactor vessel itself appears to remain intact, there has been a loss of integrity in associated piping, valves and seals. among other things, this has allowed highly radioactive water to escape the reactor vessel and collect in the turbine building. There may also be other locations with this highly radioactive water and as such, site surveys for radiological conditions will be critical for worker safety. Also as noted above, there is evidence that reactor water is getting to the ocean. Temperature is still elevated at unit 1, 144 C, but continues to decrease slowly.
The primary containment structure appears to be intact. It was well shielded from the hydrogen blast and has not had any other reported troubles. There has been steam at unit 1 the past few days and 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. They may also be able to use the same plant piping systems in use at units 2 and 4. As mentioned recently, the heat load in this pool is significantly lower than any of the others as there are fewer than 300 spent fuel assemblies in pool 1. For comparison's sake there are over 1,300 in pool 4.
The fuel containment has been compromised as is made clear by the hydrogen explosion in the torus. Hydrogen would not have been present if fuel caldding had not oxidized. 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.
As with unit 1, while the reactor vessel itself appears to remain intact, there has been a loss of integrity in associated piping, valves and seals. This has allowed highly radioactive water to escape the reactor vessel and collect in the turbine building and also reach the ocean. The temperature at the bottom head of the reactor is 100 C.
The primary containment may have damage in the form of trouble with the torus. 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. 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. Their normal operating temperature is 20-25 C.
Clearly the fuel integrity has been compromised at unit 3 and there has almost certainly been fuel melting there as well. The same conditions apply here as in units 1 & 2 with a bit of a twist. There are 32 bundles of MOX fuel in the core of unit 3. This adds a greater amount of plutonium to the reactor inventory. Fortunately plutonium, like uranium, is quite dense and tends to be tough to transport for this reason.
And as with 1 & 2, while the reactor vessel itself appears to remain intact, there has been a loss of integrity in associated piping, valves and seals. This has allowed highly radioactive water to escape the reactor vessel and collect in the turbine building and also reach the ocean. Claims of a crack in the reactor vessel appear to have been nothing more than a rumor perpetrated by the New York Times. The temperature at the bottom head of the reactor is 102 C.
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. It appears that all MOX fuel on site has been loaded into the core. This was confirmed by both TEPCO and Areva - thanks to FOYI for staying on this subject. 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. There were reports yesterday of their use of plant piping for this but reports today are that they are back to external pumping methods.
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 is now having seawater pumped in via plant piping systems. There are still no reliable updates on temperature. It also appears that they consider the plant systems sufficient to provide the needed volume of flow. Plans are to switch to fresh water in the next few days. There was also water found in the turbine building at unit 4 and is presumed to be form the fuel pool.
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.
I've seen no recent updates on the common pool.
Prognosis - The Good, the Bad, and the Ugly
The Good - Work continues on site to power up equipment and find out what is working and what is not. This is a time consuming process for certain pumping systems because they must be properly bled and re-primed to avoid causing damage when they are started. The use of fresh water is also a positive sign. It may not make the situation better but it will stop making it progressively worse.
The Bad - The water leaks are indicative of a deteriorating situation with respect to the seals that are supposed to keep stuff in the reactor and out of the environment. If this were just happening at one reactor I might think it was a specific valve that had a problem. This is looking more systemic. This puts a much higher priority on getting the temperatures and pressures down to cold shutdown levels such that minimum additional stress is put on these components. It is also possible that the same kind of scenario I describe regarding the primary heat exchanger will exist with the core cooling systems. Actions which would normally result in minor releases may now end up leaking even more busted fuel pellet stew into the ocean water.
The Ugly - If the rate of water loss from the reactors becomes too great they will find it increasingly difficult to maintain reactor water level. This could require them to take even more extreme actions such as flooding the primary containments to ensure that fuel remains covered and cooled. Hopefully it will not come to this as it would result in an enormous cleanup effort and would inevitably result in larger releases to the environment. This would be preferable to the alternative of letting the core go uncovered, but still quite ugly.
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.
I was asked in a comment earlier today to provide what I believed to be the best case scenario for how this thing ends. In the best case the heat at unit 1 continues to drop and they are able to get electrical pumps running in the next few days with the core cooling system soon to follow. Also in the best case scenario the heat exchanger for this system is fairly well intact and free of corrosion. The other reactors having their core cooling systems working would follow shortly after.
Meanwhile, they would be able to get enough water in each of the fuel pools to be able sufficiently cover the fuel to be able to safely work on the refuel floor. This would allow at least some cleanup of the explosion debris to make efforts at the fuel pools more efficient and effective. It would also make it possible to take whatever action is chosen to resolve the fuel pool issues at the various units. Again, in the best case they would identify some material or method for sealing the leaks in 3 and 4, at least temporarily - long enough to allow shoring up from below and more permanent or semi-permanent repairs.
Also in this scenario it will be possible in the near future to process the contaminated water stored on site, remove fuel from the reactor cores and fuel pools, and decommission the sites including scraping the top half foot of ground up from all over the site and dumping it as fill into the containments and then entombing the whole thing in concrete.
Given the difficulties of the working environment and the problems with the leaking water I think it is optimistic to think they will have core cooling systems returned to service within the next week at any of the units. This will mean more feed and bleed to remove heat. Also, they will need to track down the exact pathway(s) water is taking to get to the turbine building and ocean. They may or may not be able to isolate the problem and stop the leaking. Next best would be to be able to contain the leak at its source and drain that container continuously. Obviously this doesn't work if there are heat exchanger leaks but it may be possible to isolate this system using locally controlled manual cutoff valves.
I'm really not sure what is likely to happen with the leaking fuel pools. I fear that concrete entombment is likely in the future for at least one of them, possibly both units 3 and 4.
I expect the releases will need to continue for the next week at least, likely more until they get alternate means of heat removal back online. I also fear the ocean releases will continue for that same period of time, perhaps longer depending on the condition of the heat exchangers.
Cleanup and decommissioning may not be done as thoroughly as possible due to dose rates all around the site. They may well end up covering everything with sand, gravel, or other more suitable materials to contain the surface contaminants. I expect the site to eventually be abandoned and reactors 5 and 6 to also be decommissioned.
You've heard this from any number of talking heads on TV. Noun-verb-Chernobyl. I seriously doubt the releases will be at the magnitude of Chernobyl, but if all three reactors fully melt down and break free of their containments and all 4 problem fuel pools go dry and burn then yes, it could get very ugly indeed. I don't see this happening given current conditions, but if any one of the problems were to escalate beyond control then it could cause the others to become unmanageable due to lack of site habitability. I presume that each of the individuals responsible for managing the reactors and fuel pools is well aware of the stakes of failure.
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, is just that, a part. 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.
[updated with new Red Cross link as per boatsie in the comments.]
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.
As per a suggestion from Jim P, This diary represents my take on the situation as of 07:00 on 3/27/11 UTC. (00:00 PDT 3/27)