Here is an update of things from the past couple of weeks and some thoughts on TEPCO's announced plans for the next several months. For those who are new, I have written a number of diaries regarding the situation at Fukushima. (Here is the most recent from 4/4. It has links to others.)
New Developments
A number of things have happened recently. The severity of the accident was raised to a 7 on the INES scale, the same level as Chernobyl. This was based on the levels of radioactive iodine released over the past month and ongoing releases to the environment. To date estimates of the total release of radioactive materials at Fukushima are 10% of those at Chernobyl. That amount will continue to increase for the next several months as efforts continue to contain leaks and cool the reactors and fuel pools. Provided that there are no major setbacks the ongoing rate of release should remain far lower than those of mid to late March, and should continue to decrease over time.
The use of robotics has increased on site. They are being used for reconnaissance in the reactor buildings at units 1 and 3 and recently reported high radiation levels - 57 mSv - in both units. There is also a remote controlled mini-helicopter being used to investigate the fuel pools and tops of the reactor buildings at units 1,3 and 4. In addition, remote controlled equipment is being used to remove some of the rubble from those buildings.
TEPCO has released their plan for the next several months. In addition to the information presented at the link there have been various news stories about their plans. Here is one from Kyodo News which summarizes the main points. They present their priorities and intended approach. (More on this later.)
On Friday the Atomic Energy Society of Japan released their analysis of the meltdowns at units 1-3. Their evaluation of the process is almost exactly what I presented in the New Speculation section of my Status Update for April 1st. The resulting status of the bottom of the reactor core also is in close agreement with what was discussed in the comments between myself and Celtic Pugilist.
Nuclear fuel inside the crippled reactors at the Fukushima Daiichi power plant has partially melted and settled in granular form at the bottom of pressure vessels, according to an analysis by the Atomic Energy Society of Japan made public by Friday.
...
The academic body's panel on nuclear energy safety has said the melted fuel at the Nos. 1 to 3 reactors has been kept at a relatively low temperature, discounting the possibility that a large amount of melted fuel has already built up at the bottom of the reactor vessels given the temperature readings there.
A large buildup of melted nuclear fuel could transform into a molten mass so hot that it could damage the critical containers and eventually leak huge amounts of radioactive materials.
The panel has also said that the fuel grains with a diameter of between several millimeters and 1 centimeter are believed to have settled evenly at the bottom of the vessels, leaving almost no possibility of a nuclear chain reaction called ''recriticality.''
BTW - this doesn't mean we are right, just in agreement.
Also, as noted in the above article, TEPCO has agreed to make an initial payment of roughly $12,000 to each of the 50,000 households affected by the evacuation orders. They are also spraying the site with a polymer compound used on construction sites to keep dust down. This has proven successful in tests and is being sprayed around the reactor buildings first, then the remaining areas at the site. There are also plans to install huge filtering curtains around the damaged buildings to help contain radioactive materials.
One of the primary steps mentioned in their plan is to flood the containment vessels of units 1 and 3 up to the level of the fuel to guarantee the fuel remains covered and to assist in cooling. They are also looking at building external heat exchangers to be used in conjunction with filtration systems to be able to recycle the water being pumped through the reactors and also cool the water in the containments. This step also helps to provide more effective cooling for the fuel sediment on the bottom of the reactors by conducting heat away from the bottom of the reactor vessel.
They also wish to do this for unit 2 but must first repair the damage to the containment vessel. There is mention of using "sticky cement" to make this repair, I'm not sure exactly what they mean by that. This is presumably the leak caused by the explosion in or near the torus during the first week of the emergency. Meanwhile, the leaks in the containment at unit 2 appear to be flowing directly to the service trench in the turbine building. They pumped thousands of gallons only to have the level rise back up again. They may end up needing to seal up the secondary leaks and treat the entire system - trench and all - as the "reactor vessel". Stop adding new water and instead process the water from the trench and inject it back into the reactor.
They are also injecting nitrogen into the containment at unit 1 and plan to do so at the other two reactors as well. While this initially concerned me regarding the possibility of new cladding breakdown, I recently read that the ongoing concern is accumulation of hydrogen due to a different process - radiolysis. This is an interaction between high energy (mostly alpha, sometimes beta) radiation and water molecules which results in free hydrogen. By injecting nitrogen into the drywells the amount of oxygen in the contained atmosphere can be kept below dangerous levels.
Regarding the spent fuel pools, their initial plan is to restore circulating cooling function for each. The longer term plan is to clear enough around the fuel pools, initially at unit 4 I believe, to be able to lower storage casks into the pool and transfer the fuel to another location. This will require large overhead cranes which will be moved into place in pieces by special US military helicopters. It is unclear whether they intend to ever try to remove the fuel from the reactors.
Also over the past week there were a few new aftershocks in the region. One of these caused the power to the site to be lost for about an hour. It is my understanding that a backup power line is currently being run to the site. They have also elevated the power distribution panel for the pumps feeding the reactors to prevent any possible disruption by a new tsunami.
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 been injecting fresh water into the core at a constant rate for the past few weeks. 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.
The primary containment structure appears to be structurally intact but is showing signs of air leakage. As the nitrogen was being injected the internal pressure initially rose but has now stabilized suggesting that the drywell can no longer hold its design pressure.
The secondary containment structure 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 previous diaries. 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 reported a temperature of 36 C. for this fuel pool on 4/14.
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. It also appears that water from the reactor is leaking to the turbine building service trench. This may be caused either directly or indirectly by the damage to the torus. Depending on the location of the leaks it may not be possible to repair them. Hopefully the sticky cement will work and that will be the only problem. Otherwise they're in the situation mentioned above of needing to treat the entire extended system as the current reactor vessel. It also does not appear that the unit 2 drywell can contain increased air pressure as both unit 2 and 3 have been at or very near atmospheric pressure for days.
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.
There have been no reports of fuel pool issues at unit 2. They began using the new power line and pumped thousands of gallons of water into the fuel pool at unit 2 using the spent fuel cooling line. The pool was confirmed to be full as of April 16th, and the temperature is 71 C as of 4/18.
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. And as with unit 2, several days of operation at atmospheric pressure suggest that air containment may no longer be possible. It may also be that they are purposefully leaving them open to prevent hydrogen buildup - but in the process allowing more radioactive internal atmosphere to escape.
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 may be a leak in the pool causing it to lose water faster than the heat load should cause. They are currently using plant systems to inject freshwater while also spraying freshwater externally via remote-controlled concrete pumper trucks. Thermography on April 14th put this temperature at 59 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 for 4/14 set this temperature at 63 C. They also took samples of this water on 4/12, activities for I-131, Cs-134 and Cs-137 were 220 Bq/cm3, 88 Bq/cm3 and 93 Bq/cm3 respectively.
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. Their fuel pools are at 37C and 31 C as of 4/18.
Common Fuel Pool
I've seen no updates on this facility. It was doing well at last report.
Prognosis - The Good, the Bad, and the Ugly
The Good - There is at least a roadmap however unclear some of the steps may be. High tech remote-controlled devices are making an impact and will play a large role in managing work in high radiation areas. Also, while the situation is still quite bad, it appears that there is no significant danger of things becoming dramatically worse. Another significant natural disaster in the immediate vicinity of the plants could change that, but absent that cause it appears that they are able to manage enough heat removal to keep things relatively stable. In time they should be able to increase their heat removal and bring the reactors to cold shutdown.
The Bad - The relative stability is only that - relative. It requires ongoing and pretty much continuous pumping of water into the cores of the reactors as heated water and/or steam escape through various leak paths. Some of this water is inevitably entering the ecosystem. This will continue indefinitely and there may be some leaks that simply cannot be fixed or isolated.
The Ugly - The best-case scenario for local residents being able to return home is measured in months. In some cases there will be much higher concentrations of radioactive materials than in others, all depending on wind direction and rainfall at the time of the major releases. Before residents should return, time will be required both to ensure the reactors are safely stabilized, and also to allow short lived isotopes to decay. Then places will need thorough surveys to determine habitability. Recovering the use of these properties will be time-consuming and costly. Some will be lost for longer periods of time than others, and their appropriate usages may be curtailed. Various methods may be used to rehabilitate the soil. But in the meantime, people need to live and rebuild their lives. And this is in addition to the rebuilding needed throughout the area due to the earthquake and tsunami.
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: I wish I could devote more time to this issue but I let myself get behind and need to catch up on projects, rent, etc. I'll try to jump back in as major developments occur, but cannot guarantee timeliness.
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Timestamp: This diary represents my take on the situation as of 18:00 on 4/18/11 UTC. (11:00 PDT 4/18)