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, 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 a number of updates to the situation. The most significant news I believe was the announcement of various modeling results in an article in the New York Times today. This included at statement from Dr. Steven Chu regarding the DOE's best guess estimates regarding the condition of the cores at units 1-3. He described unit 1 as having an estimated 70% of the core damaged with 33% damage at unit 2 and some unspecified amount of damage at unit 3. I will discuss this more below in New Speculation.
There was also the arrival of US Navy water tankers. The transfer of fresh water from the first ship was interrupted shortly after it began by a connector failure. There are also 4 more concrete pumpers fitted for water spraying. Two from Germany have 62 meter booms, and two from the US have 70 meter booms. There are also rumors that these are planned for later concrete use on site. These pumpers can all be operated by remote control which allows them to be used for fuel pool spraying without significant additional human radiation exposure. They have also used thermography to determine the water temperatures for the spent fuel pools at units 1, 3 and 4. Unit 2 has intact temperature measuring instrumentation and was also the hottest at 61 C.
Also in infrastructure updates, the temporary pumps being used for water injection at units 1-3 have all been switched over to pumps running on the new external power line. It is unclear whether they mean that existing reactor equipment was connected to the external power though that is somewhat implied. There has also been lighting restored to parts of the turbine buildings at units 1-4. Also, the bodies of two workers who had been missing since the earthquake were discovered in the basement of the unit 4 turbine building.
They have completed emptying the condenser tank at unit 1 to enable that volume to be used to contain water pumped form the unit 1 turbine building. The IAEA report also stated that the condenser tank at unit 2 was being emptied in preparation for pumping from unit 1. I'm not sure if this was a typo or if they intend to use both condensers for the water in the unit 1 turbine building basement. And if so, then it is not clear what they plan to do for the water at unit 2. Longer term plans for dealing with the tons of contaminated water may include the use of a floating island donated by a local city, the digging of temporary pools, and the use of tankers, possibly even those which delivered fresh water. In time a decontamination plant will need to be constructed on site to process the water before its release to the environment.
Meanwhile, there was a 20 cm (8 inch) crack discovered in the trench at unit 2 allowing highly radioactive water to escape into the Pacific. TEPCO attempted to repair this using concrete but failed. Their newest approach was to use a polymer which absorbs 50 times its weight in water. This material is apparently used in diapers, thus making the "Nuclear Boy" video ironically prescient. According to the NISA update, this also was not effective. Perhaps just throwing the diapers in might have worked better.
Old Speculation
A few points about theories raised in my last diary. I mentioned a conversation with Joieau which caused me to rethink the process by which the melting occurred. I did not mean to imply that she was in agreement with the conclusions reached. Also, in discussions with billmosby he explained that criticality was going to be required somewhere to be able to get the measurable concentrations of new fission products. This would rule out sub-critical fissions in the torus or balance of plant component as a possible source for these isotopes.
New Speculation
The numbers cited by Dr. Chu were quite surprising, as were some of the results of the computer simulations mentioned in the Times article. The estimate of 70% damage of the core at unit 1 is a good bit higher than any I'd read earlier. I'm wondering if there was additional fuel damage during the temperature rise back on March 22nd. This could also account for TEPCO announcing plans to inject nitrogen into the unit 1 containment this week to help guard against the possibility of another hydrogen explosion.
This also represents an enormous amount of stuff in the bottom of the reactor vessel - melted and disintegrated fuel pellets, fuel cladding, control rod parts, assembly structural steel. And as noted in a discussion with CelticPugilist, the situation is worse in a number of ways if less fuel melts than breaks apart in the water. It makes it worse where there are leaks because the water has more radioactive material in suspension and/or solution. It also makes it tougher to properly cool the lower regions of the vessel. There is still decay heat being produced by the fuel and it needs water circulation to carry that heat away. If there is a large slag pile of melted bits and broken and crumbled bits all being filled in with smaller particulates from broken up fuel pellets, it could create a stagnant zone where heat can build up. It also makes it tougher for the boron in solution to do its job as a neutron poison, making new criticality somewhere in the mess more of a possibility.
I'm also wondering about the current status of heat removal from the three reactors. The reactors at unit 2 and 3 appear to be running at atmospheric pressure and are having water continuously pumped into the core. The question is how much heat is leaving via steam release through the leaking top of the reactor and how heat is leaving via leaking pipes below the water line? In each of these cases, the leaking water appears to be making its way to the service trench. In the case of unit 1, if there is a water leak it would appear to be into the primary containment and being contained. There is some evidence to support this as the radiation levels in the drywell at unit 1 are a bit higher than the other two units.
Dr. Chu also mentioned unit 2 core as having 33% damage and some lesser estimate for unit 3. Hopefully the leaks will be all that need to be managed at those units. Unit 1 seems to have a ways to go before it can be declared stable.
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. The DOE estimates that 70% of the core has been damaged.
It appears that the reactor pressure vessel itself is intact. However, the associated piping, valves, etc at unit 1 may be starting to lose their integrity. The reported pressures are lower without the corresponding decreases in temperature, and it was reported that pressure readings were unstable. 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. Thermography reports a temperature of 25 C. for this fuel pool.
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 cladding 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. According to DOE estimates, about 33% of the core was damaged.
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 somewhat to 61 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. The DOE estimates for core damage at unit 3 are that there is significant damage but not to the extent of unit 1 or 2.
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. Thermography on April 3rd put this temperature at 56 C.
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. Thermography results set this temperature at 42 C.
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 both just below 30 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.
And from Detlef translating from a German site, the common pool was measured at 32 C on 3/30.
Prognosis - The Good, the Bad, and the Ugly
The Good - The estimates of core damage for units 2 and 3 are positive signs in terms of their manageability and the complexity of cleaning them up. Also, it appears they are making progress in getting back at work in the turbine buildings, getting closer to bringing circulating cooling heat exchangers online.
The Bad - The situation at unit 1 is not good. The amount of fuel that is damaged and sitting in the bottom of the vessel is quite significant. It will also need to be considered in any plans to restart cooling flow. Quite a bit of crud could get stirred up in the process. I hope their modeling has included dry runs of how to move forward from here.
The Ugly - The continued flow of highly radioactive water into the ocean is just one part of the contamination from this episode that will continue. Even after the major emergency is resolved there will be an enormous task of cleaning up this mess.
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.
Also, there are many, many people who have contributed information, updates, new perspectives to consider and more. Some I have mentioned above or in previous diaries. I will try to get a more comprehensive list together for the next diary as I wish for people's contributions to be recognized and appreciated. I will need to cut back on my coverage of this situation as I need to do real paid work in the world and have a pressing project that is calling me. I will get to comments when I can though it may not be for a while today.
---
Timestamp: This diary represents my take on the situation as of 06:00 on 4/4/11 UTC. (23:00 PDT 4/3)