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 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 Thursday evening around 8PM PDT. Anyone wishing to trace the evolution can look back through the previous update diaries here, here, here, 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
What, exactly, IS a nuclear meltdown?
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.) It was fairly easy to track him down via Google and a hit on his profile at LinkedIn. I reconnected with him via a networking request through LinkedIn.
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.
New Developments
There are various minor developments and updates provided over the past day. High levels of iodine-131 are still being detected in the water near the plant. Efforts are also ongoing to get water pumped from the turbine buildings at each unit to allow access to equipment needed to eventually get the plants on circulating cooling through heat exchangers. I say eventually because once they gain access there is still much that needs to be done to test components and prepare them to be returned to use. This is especially true with pumps which must be properly primed to avoid causing damage when started.
For a few days it had been believed that they were riding a tough balance between providing enough cooling water to the cores to manage temperatures while trying to limit their flow rates to avoid leaking even more water into the problem areas. They are now reporting that TEPCO believes the bulk of the water in the trenches was caused by the tsunami, not leaks from the reactors or fuel pools. As such it does not appear that the reactor cooling flow is competing with the effort to pump the buildings clear. (Thanks to FOYI in yesterday's comments.)
There were also plans to spray some type of resin compound around the site in an attempt to prevent radioactive materials on the ground from being raised up into the air. This was delayed due to rain which would have apparently interfered with the resin's ability to do its job. They were also busy transferring lightly radioactive water from various containers in the plants to create available volume for what is pumped from the turbine buildings.
New Speculation
There has been a lot out in the last few days regarding possible vessel damage, leaks of water from the bottom of unit 2, leaks of molten fuel from either of the reactors, even molten fuel in the unit 1 reactor building that could spontaneously go critical and kill workers. There were news stories on these, diaries on Daily Kos, and much confusion and concern were created. There weren't a whole lot of actual facts in play, just speculation on what might be happening. Discussions in yesterday's comments with Joieau regarding the nature of the meltdown and the present status of the insides of the reactor cores caused me to rethink the process. I think there may be many people making inaccurate assumptions about the degree of fuel melt.
The only familiar examples we have of large scale melted nuclear cores are TMI and Chernobyl. In each of these accidents large amounts of power/heat were being generated and the melting of fuel, cladding, structural materials and control rods all happened within a very short period of time. In each instance this led to large molten globs of melted fuel. At TMI where there was still water in the lower part of the vessel, this molten glob crusted over as the surface layer was quickly cooled by the water. This greatly slowed the release of the remaining heat from the molten glob as the crust tended to insulate the heat from the water.
At Fukushima the circumstances were significantly different. This is partly because they are all Boiling Water Reactors and are therefore designed a bit differently from TMI. It all has to do with the relative efficiency of heat removal from the fuel rods during plant operations. In a Pressurized Water Reactor the water never boils as it is kept under very high pressure. (You may remember a demonstration in science class in which a light vacuum is drawn on a beaker of water. At this lower pressure, just the heat from your hand was enough to cause the contents to bubble and boil. Just as lowering the pressure lowers the boiling point, raising the pressure raises it.) Because the water never boils in a PWR the fuel is arranged in a fairly tight packing to optimize space and power production.
In a Boiling Water Reactor the water is boiled to steam in the reactor vessel. This results in a mixture of water and steam bubbles throughout the region of the core where the fuel is located. Because this mixture is not as efficient at conducting heat away from the fuel, the fuel rods are less densely packed and the flow channels for coolant are larger. This provides some margin in the event of dropping coolant levels. At the beginning of this incident, even before the first hydrogen explosion, a Kossack who I remembered as someone with nuclear plant operations experience posted a comment about this. He stated that a BWR could uncover up to 1/3 of its fuel for a period on the order of an hour prior to fuel melting. This is because the rate of heat removal purely by circulating steam was enough to slow the temperature rise caused by the decay heat. There reactors also had the advantage of having cooled for several hours prior to their loss of water level. The rate of decay heat production had dropped as had the temperature and pressure in the vessel.
By uncovering up to half of the fuel they passed the 1/3 point by a bit. That would have increased the rate of temperature increase and significantly decreased the time they had to recover before fuel damage. But still, the overall rate of heating should have been far less than during the TMI accident. What is the point of all of this? The amount of fuel melted versus dissolved, and the size and status of the pieces of melted core are fairly dependent on how rapidly the melting happened. How rapidly the melting happened depends on the heat balance between generation and steam cooling.
Up until the point of cladding oxidation the source of heat generation was decay fission by various radioactive elements in the fuel. Once the temperature reached somewhere in the 1200 C range there began to be small local oxidation reactions between zirconium and water - the source of the hydrogen that later exploded. As the temperature continued to increase there was a point where the fuel rod cladding material began to burn along its axis - sort of like a sparkler on the 4th of July. Meanwhile, pressure differences caused by the overheated decay gases in the fuel have also caused the cladding to bulge and split releasing disintegrating fuel pellets into the reactor vessel and the water below.
Eventually, (and this may be two minutes or twenty minutes, I don't have the information to make this determination), the combination of heat from decay plus heat from the cladding oxidation reaction, minus the heat removal via circulating steam raises the temperature enough to cause melting of fuel. If the heat were being generated fast enough there would be large scale melting of rods, fuel, structural metal, and control rods all together forming a large molten mass that could crust over in the water and remain molten inside - the TMI scenario.
It is my contention that the melting at Fukushima would have happened at a far slower rate than at TMI. And as a result, much more of the fuel could have escaped its cladding prior to reaching melting temperatures. Also as a result, instead of being one or two large masses of partly molten "corium" I see many smaller pieces being created by this process. These pieces would be small enough to cool much more effectively than large chunks. I also see the pieces in the vessel as not being as highly mixed in composition as the material at TMI. Instead I envision melted fuel, melted control rods and melted structural metals, not a melted "alloy" with significant amounts of each. I'm sure there is some of this, but not near as much as at TMI.
The significance of this is that I also do not envision slugs of molten corium looking for holes in the bottom of the reactor, or worse yet, creating them. I also believe that the very high radiation readings being found in leaked water and steam releases is related to the relatively large amount of fuel that dissolved instead of melting. The melted masses do a fairly good job of locking the radioactive materials in the vessel. Some minor amounts may leave the surface but most is inside the hardened outer layer. This could also help explain the indications that fission has happened recently.
As I understand from Stan, the torus water is treated with anti-corrosive chemicals but is not borated. As more and more steam was dumped from the reactors to this suppression pool, more and more radioactive particulates were scrubbed from the steam and fell out in the bottom of the torus. As this material accumulated it may be that low level fission started to occur. There would not have been near enough neutron production to create a sustained chain reaction - to go critical - but rather to generate spurts of decay chain reactions in several locations. This same process could have happened as a result of backflow into the Balance-of-Plant, (non-nuclear or turbine & condensers) part of the system via leaking isolation valves and pump seals. These kinds of reactions would be stopped in the reactor vessel itself by the boron that has been continually added during water pumping. It is also possible that the high levels of radioactivity observed in the trench at unit 2 are the result of this contaminated and possibly sub-critical water from the damaged torus draining there via the turbine building basement.
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.) 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.
It appears that the reactor pressure vessel itself is intact and that the associated piping, valves, etc at unit 1 are also substantially holding. Earlier reports of high radiation and leaks at this plant are not being borne out by the data. The temperature and pressure are decreasing and the pressure is significantly above atmospheric which means that the system has retained its ability to hold pressure. But just as units 2 and 3 eventually lost this ability, it is predictable that unit 1 will also do so within a relatively short time frame. There has apparently been consideration given to flooding the drywells at all three units. This comes only from notations on the JAIF status summary forms from the past few days. There has been no confirmation or further mention of this in other reports.
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 have begun spraying the pool with fresh water.
Unit 2
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.
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. In addition, the Reactor Vessel Containment System is no longer able to hold pressure. I suspect the most likely cause is with one or more valves that were unable to fully close after opening for steam venting. The cause for this would be the buildup of seawater contaminants in the vessel. There are other possible causes but this seems the most likely given the events to date.
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. And as mentioned above, this could also be the source of the highly contaminated water in the unit 2 service trench.
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. Injection of freshwater continues there and the temperature has risen slightly to 48 C.
Unit 3
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. Small amounts of plutonium from the reactor have been found on site. It is quite doubtful that it has spread far beyond the plant boundaries just because it is so damn heavy that it will fall out quickly.
And as with unit 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. Also like unit 2, unit 3 has lost its ability to maintain significant pressure. It has measured a few pounds above atmospheric but not enough to signify that it is intact.
The status of the primary containment has been reported as compromised, and as not compromised. It apparently began holding pressure again after failing to do so in the immediate aftermath of the hydrogen explosion. I have my doubts regarding its ability to withstand the pressures of a serious incident. The problems should not affect their ability to flood the containment as the problem appeared to be related to seals at the top of the structure.
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) 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. They are currently using plant systems to inject freshwater while also spraying freshwater externally.
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 is now having freshwater injected via plant piping systems in addition to external spraying. There are still no reliable updates on temperature.
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. The latest report listed their temperatures as 37 C and 26 C.
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 still seen no recent updates on the common pool.
Prognosis - The Good, the Bad, and the Ugly
The Good - It is not easy to find good in this situation. I guess the plan for resin spraying qualifies. There are also international experts showing up in Japan to lend their efforts to getting this situation resolved without catastrophic results.
The Bad - The reactors are becoming more leaky as time goes on and may eventually require drywell flooding to maintain water over the existing fuel line. They also are still not able to make progress on the technical aspects of restoring cooling circulation due to the contaminated water issue. And even when it is eventually drained it will leave behind a higher than normal radiation area in which workers time will be limited due to dosage concerns.
The Ugly - Even if things go as well as could possibly be expected over the coming weeks, months and years, there will be an enormous collective radiation dose resulting from this incident. The effort to get things stabilized is critical and is resulting in many people absorbing doses well beyond what is recommended for occupational exposure. In addition, many, many workers will be needed at the site to replace those being "burned out" and the need will continue for the foreseeable future. Finding these workers should not be too difficult because of the worldwide recession. Sad to note, but just as with the US imperial wars, finding fresh bodies to absorb their limit of zoomies at Fukushima will be a booming business for the various contracting bodyshops in the worldwide nuclear industry. Radiation technicians will be in high demand and many new ones will be trained. Likewise, many skilled trades workers will be recruited to collect premium pay for plying their trade in radiation zones. And while this is good for the pocketbooks of those who go and those who pimp them, it adds to the enormous costs which this incident will inflict on the already heavily afflicted Japanese people.
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, 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.
The Japanese Red Cross Society and Shelter Box USA
Personal Note:
My purpose with these diaries is to help provide technical understanding of what is happening at Fukushima and provide a forum for discussing various theories and 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 have repeatedly requested 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.
Unfortunately, I am confronted with a quandary. There is a Kossack named Jake Johnson who has apparently become obsessed with attacking me and doing everything he can to discredit me. This has included relentless mocking, claims that my old colleague Stan is my "imaginary friend", disingenuous arguments trying to "prove" inconsistencies in what I've written by taking comments completely out of context, and also a healthy dose of insults, untrue accusations, and a very disturbing post in which he fantasized about torturing a small animal. I have tried ignoring him but the problem is that in his posts in which he challenges me on X, Y, or Z, if I don't respond to him it leaves the impression among those unfamiliar with his game that he is correct and that I have no answer. OTOH, if I DO answer then I'm sucked back into his dishonest arguments which go on and on and on. I also then open myself up to even more insults, accusations and mockery. My request to readers here is to ignore this man. It is inevitable that he will show up with more of his nastiness. Him calling me a whiner for writing this paragraph is almost guaranteed. I really have no other idea of how to handle this situation. To get confirmation of what I'm talking about just go into any of my previous diaries and do a search on his name.
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Timestamp: This diary represents my take on the situation as of 03:00 on 4/1/11 UTC. (20:00 PDT 3/31)
Updated by kbman at Fri Apr 01, 2011 at 01:09 PM PDT
As per Detlef in the comments,
http://www.dailykos.com/...
The common fuel pool was measured at 32 C on 3/30