For those who missed my previous diaries on this topic, I have a background in physics and worked at Oyster Creek Nuclear Generating Station providing computer support for the reactor core engineering group. For the entire qualifications spiel and/or some background on Boiling Water Reactor (BWR) containment structures, see
Everything You Never Wanted to Know about Nuclear Containments
That diary also contained a review of the then current status of each of the reactors at Fukushima. This diary is intended as an update reflecting what is known through today around 4 PM PDT. Anyone wishing to trace the evolution can look back through the previous update diaries 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.)
New Developments
It has been an interesting day to say the least. Not necessarily in a good way. It is now clear that water from the unit 3 reactor vessel is escaping from the system. The water which caused radiation burns to the feet of three workers at the plant yesterday contains very short-lived isotopes which are produced in the reactor. This rules out the leaking fuel pool as the source, or at least sole source, of this water.
There was a report on this in the New York Times in which a "nuclear industry executive with contacts in Japan" claimed that there was a vertical crack in the reactor vessel extending down below the water line. When I related this to Stan he was extremely skeptical to say the least. His basis for this skepticism is that this is not consistent with the events that have happened or with the nature of the reactor vessel materials. This would not be caused by thermal stresses, only by a strong internal explosion, something which there has been no evidence to support having happened.
I base my skepticism on a much more simple fact. That portion of the reactor vessel is not available for visual inspection. It is surrounded by shielding as seen in the diagram below. Even if they had working cameras in the primary containment that they could focus on this region of the reactor, they could not see the vessel itself because of this shielding material. There is simply no way that what this person claims to be fact could have been determined in the first place - especially to the level of detail he describes. This is also quite likely the same exact anonymous industry executive who was quoted yesterday claiming that one of the fuel pools must have been burning. This assertion was removed from later online editions of the Times.
In activating his network of contacts Stan found that there was a report of a crack on one of the down comers which feed steam into the torus suppression pool. This may have been the kernel of truth around which the reactor vessel crack legend grew.
And in following up this story, TEPCO is now saying that there are signs of reactor water in the turbine buildings of all three reactors. So, what is the path the water is taking to escape the reactor? Given the total situation the most likely culprit is corrosion of the sealing surfaces of various valves. Once a channel is etched across one of the faces of the seal, it is no longer a seal. This could result in water leakage in any number of locations. The basement of the turbine building is a likely catch basin for water draining in from parts of the reactor building as well.
A step they have taken today is to inject fresh water into units 1 and 3. They are also preparing to do the same at unit 2. This is all well and good, but with as much junk as they have pumped into these reactors, it will take several injections of fresh water to get concentrations of these contaminants down significantly. It helps to be not adding any more, and as they add water and blow steam - feed and bleed - the reduced salt content should help prevent loss of cooling efficiency. This is a concern because if too much salt plates out on the fuel rods and forms a crust layer that hinders heat transfer.
Here is what Japan's NISA said earlier, via Kyodo News
Early Friday, concern grew that the high-level radiation leak detected with the workers' exposure Thursday could indicate possible damage to the No. 3 reactor vessel, but the government's nuclear safety agency later denied the possibility, saying no data, such as on the pressure level, have suggested the reactor vessel has cracked or been damaged. The No. 3 reactor used plutonium-uranium mixed oxide fuel for so-called ''pluthermal'' power generation.
There were also some concerns raised last night about a data point in one of the reports from Japan's Nuclear and Industrial Safety Agency regarding the temperature of fuel pool 4. In the midst of reports of unreliable data, a value of 100 C was presented in one report. This is appearing likely to be a bad piece of data. I base this on the fact that there have been no significant updates on the status of that pool. They continue pumping in seawater there. They had switched to the plant's Fuel Pool Cooling System but according to NISA have returned to the use of firetrucks and cement boom trucks.
Also, there were reports earlier today that the INES level had been increased to 6. I have not found any confirmation of that from official sources - the people who would actually raise the level. The individual plants are all still listed as at Level 5. This may be related to the fact that French Authorities have declared this a Level 6 event for several days now. This may eventually be the case, but apparently not yet according to those who have the official say.
Current Status of Reactors and Containment Systems at units 1 - 6
Unit 1
The fuel pellets and fuel rods in the reactor have almost certainly experienced melting due to high temperatures resulting from the fuel being uncovered. (I only say almost certainly because we cannot look into the reactor at this point to verify. The same is true for units 2 and 3.)
As a point of clarification, as soon as the melting fuel, cladding, and control rod bits melt they drop through the water to the bottom of the reactor vessel where they very quickly lose their heat to the enormous heat sink of the lower half of the reactor vessel. In theory, a chunk of melted fuel slag could have a low enough concentration of control rod slag that it could achieve criticality and start a nuclear reaction on the floor of the vessel. This is one of the purposes of the boric acid that was being added to the seawater going into the vessels. Boron is a very efficient neutron absorber and being in solution it acts like liquid control rods to fully surround any iffy chunks of slag and keep them from reacting. Remember, the neutrons need to be slowed by water to interact at any appreciable level, but as soon as they hit the water they get sucked up by the boron.
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. 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.
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 have also been no clouds of steam coming from unit 1.- As noted above, there is steam at unit 1 as well though it appears to be the result of venting as opposed to a problem with containment.
The secondary containment also appears to be intact. (For those who question this evaluation in the aftermath of the hydrogen explosion there I refer you to my diary on containment structures linked in the intro. It has a discussion of my analysis of the explosion and why I don't believe either unit 1 or unit 3's secondary containment to be badly damaged.)
There also have been no reported problems with the fuel pool at unit 1 despite the explosion. According to the JAIF site, they are now considering adding water to this pool. There was no mention of how they intended to reach the pool under the collapsed roof of the reactor building. There were reports the other day of plans to use army tanks to help remove debris from the tops of units 1, 3, and 4 to make the fuel pools more accessible. They may also be able to use the same plant piping systems in use at units 2 and 4. As mentioned in yesterday's update, 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.
Unit 2
The fuel containment has been compromised as is made clear by the hydrogen explosion in the torus. It is also almost certain that there has been fuel melting due to the time that the upper parts of the fuel rods were uncovered.
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.
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. As of yesterday various reports have dialed back the claims of damage to the primary containment. This is both from JAIF and the NRC. They now say that it may be damaged or that damage is suspected. Given their apparent priority to get unit 2 power back it seems likely that there is still a problem there with the torus. And if they can isolate the torus from the drywell then they have a sealed containment. They just lack the cooling ability of the suppression pool, an ability that may have lost most of its efficiency already due to heating of the water.
They also were able to restore power to unit 2 on Sunday and discovered that a pair of key pumps were not working. Replacements have been ordered but may be a week or more. (I wonder whether there are identical pumps at unit 4 that could be used ... The plants are the same design and power rating.)
The secondary containment has been compromised by flying debris from the explosion at unit 3. It was reported that there was a hole in the wall of the reactor building. Whether any equipment was affected by this is not clear from reports. This, combined with the damage to the torus creates a situation where, when they need to vent, the steam is not cooled by the torus meaning that the pressure and temperature rise considerably in the drywell leading to a greater need for venting which now goes directly to the environment. And while the JAIF site lists this as slightly damaged as opposed to severely damaged for units 1 and 3, I believe the damage to the building at unit 2 has greater consequences because it is the secondary containment that is open. That does not appear to be the case with units 1 and 3.
There have been no reports of fuel pool issues at unit 2. They began using the new power line on Sunday and pumped thousands of gallons of water into the fuel pool at unit 2. They have been able to get the temperature there down to 47 C. I believe their normal operating temperature is 45 C / 81 F. OK, a little bit high, more like 20-25 C.
Unit 3
Clearly the fuel integrity has been compromised at unit 3 and there has almost certainly been fuel melting there as well.
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.
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. 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.
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 seawater pumped in via plant piping systems. There are still no reliable updates on temperature but it appears that the 100 C measurement was in error. It also appears that they consider the plant systems sufficient to provide the needed volume of flow.
Units 5 and 6
These reactors were both in cold shutdown at the time of the earthquake and did not have near the cooling needs as units 1-3. They have remained stable and there have been no reports of fuel breakdown or melting. They have now been able to use grid power to run their fuel pool cooling pumps. The temperatures in the pool have dropped considerably since they got the cooling pumps in operation and appear to be stabilizing. As a precautionary measure the other day TEPCO cut holes in the roof of the building at both 5 and 6. It now appears that while being a prudent measure, the holes will not be needed, either to vent hydrogen or to refill the pools from the air.
Common Fuel Pool
There is also another fuel pool at the Daiichi site which has fuel from each of the reactors. I had heard about this facility but this is the first solid information I've seen on it. From the IAEA web site:
In addition to pools in each of the plant's reactor buildings, there is another facility -- the Common Use Spent Fuel Pool -- where spent fuel is stored after cooling at least 18 months in the reactor buildings. This fuel is much cooler than the assemblies stored in the reactor buildings. Japanese authorities confirmed as of 18 March that fuel assemblies there were fully covered by water, and the temperature was 57 °C as of 20 March, 00:00 UTC. Workers sprayed water over the pool on 21 March for nearly five hours, and the temperature on 23 March was reported to be 57 °C.
Prognosis - The Good, the Bad, and the Ugly
The Good - Hard to find much good news, just that the bad news isn't as bad as the media claims. Starting freshwater injection is a positive and the fuel pool situation seems to be becoming more manageable at unit 4. That's about all I see that is positive in today's news. Oh, and the temperature measured at the bottom head of the reactor vessel in unit 2 is down to 104 C, and at unit 3 is down to 111 C. Once they get below 100 C that will help by removing the steam pressure. By way of contrast, at unit 1 it is still up at 148 C.
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.
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.
There is also the economic impact on the local farmers and watermen as the source of their income is now contaminated. There are also reports that the general radiation levels in Fukushima prefecture are hampering search and rescue operations, adding further misery to those in the immediate area.
Appeal for the people of Japan
Please remember the bigger picture here. There are millions of people still suffering in Japan at this hour. The aftermath of the earthquake and tsunami have created enormous problems and left many thousands dead, injured, or isolated and left unattended. This event is a natural disaster of unfathomable scope. The nuclear emergency, while certainly an important part of what is happening there, has still not had the effect on the population that the other events have to this date. If you can find it in your heart to contribute in some way to the humanitarian efforts for Japan you will truly make a difference in someone's life.
The Red Cross and Shelter Box USA
Also, my purpose with these diaries is to provide technical understanding of what is happening at Fukushima and discuss the likely ramifications of these events. It is not to engage in pie fights over the future or appropriateness of nuclear power. There are plenty of diaries for those discussions for people who choose to so engage. I also request that people kindly refrain from personal attacks and other forms of ad-hominem argument. This is an emotional topic for some folks but that does not excuse rudeness and personal insults.
BWR Mark I Containment
As per a suggestion from Jim P, This diary represents my take on the situation as of 23:00 on 3/25/11 UTC. (4:00 PDT 3/25)