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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.  In this diary I will be updating that status information and discussing the various possibilities moving forward from here and their relative likelihood.  This update is based on information from a number of sources including the UN's International Atomic Energy Agency, IAEA, 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?

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.)

The reactor vessel and associated piping appear to be fully intact.  Were this not so the reactor would not be able to retain the water being added and there would be unexplained pressure drops.

The primary containment also 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

The secondary containment also appears to be intact.  There also have been no reported problems with the fuel pool at unit 1 despite the explosion.  It is, however, getting near the time frame when this fuel pool will start uncovering fuel if water is not added.  I have read no reports on this.  I suspect the utility was informed of the temperature there by the thermal imaging done the other day.  The only saving grace I see here is the possibility that in the cold weather the collapsed roof is acting as a condensing surface for water that is evaporating from the pool and that at least some of that condensed water is making its way back into the pool.

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.

The reactor vessel and associated piping seem to be intact here as well.  There was a brief time when they had trouble maintaining water level increases but that was apparently resolved as a valving issue.

The primary containment has damage.  Initial reports from TEPCO stated that the damage did not breach the primary containment boundary and instead is in an internal section of the torus chamber.  This report turned out to be inaccurate.  They are now reporting that the torus has damage in its lower portion.  It is not clear if this was intentional withholding of information by TEPCO or whether it represented their evolution in understanding the situation.  I suspect the former.  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.  It is hopeful that they have isolated the torus from the drywell as a result of this.  I do not have information to either confirm or deny this however.

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.

There have been no reports of fuel pool issues at unit 2.  Again though, we are getting into the time frame where the pools need water and cooling flow.  The pool at unit 2 is also still covered by the roof of the refueling level of the reactor building.  This is not necessarily a good thing because if fuel is uncovered then there could be a hydrogen explosion there as well.  The fact that they did not cut holes in the roof at unit 2 may be a sign that they have a better handle on that pool than on those at 5 and 6.  Or, it may mean that the work is too dangerous to do at unit 2 now, either because of radiation levels or the danger of the work itself setting off an explosion.

Unit 3

Clearly the fuel integrity has been compromised at unit 3 and there has almost certainly been fuel melting there as well.

The reactor vessel and associated systems appear to be intact and reactor water level is able to be maintained.

The primary containment at unit 3 was reported to be intact by TEPCO in the aftermath of the hydrogen explosion.  Over the weekend they have admitted that there appears to be damage.  (They should know better than to do this kind of stonewalling.  Very frustrating.)  Based on the events at unit 3 and observations of steam coming from the roof of the secondary containment, it is likely that the damage was to the drywell dome plug.  If  you look at the BWR diagram below you'll see the cross section of the three layers to this seal directly above the dome of the reactor vessel.  The sealing surface is a compressed o-ring which may have been disrupted by the explosion in the space above.  This would mean that when they vent steam, any that is not recondensed by the torus - which is a larger amount now because the torus water has become hot over time and less effective at suppression - becomes free steam in the drywell and can escape through this breach in the primary containment.

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 incude 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.  

The fuel pool at unit 3 is a major 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.  However, that only began recently and from what I have read, there are just 30 fuel rods in the pool that contain MOX fuel.  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.  Because the pool is recessed into the floor, just getting the water close is what counts, gravity can do the rest.

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 may be the biggest concern at the moment.  There have been reports that it had run completely dry but they appear now to have been inaccurate.  There was also a rumor the other night about the pool starting to go critical - meaning that a nuclear reaction had started there.  Again this was not borne out by later reports.  But regardless, it is losing water faster than it should and refill operations are crucial.  Again, one of the saving graces here is the fact that they don't need to get the water directly into the pools.  

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 recently connected portable generators and been able to run their fuel pool cooling pumps.  The temperatures in the pool have dropped considerably in the past day due to this operation.  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.

Prognosis - The Good, the Bad, and the Ugly

The Good - Each of the reactors at Fukushima appear to be on a path to cold shutdown.  Thermal imaging reports suggest that the vessels at units 1-3 are each close to 100 C.  Cold shutdown would bring them down to around 95 C.  There also appears to be headway in getting water into the fuel pools at units 3 and 4, though these efforts will need to continue for the foreseeable future to keep the fuel covered until a more permanent solution can be devised.  

The Bad - Having lost the integrity of the primary containment at both unit 2 and unit 3 is quite significant.  It means that more radioactive material has been released than would otherwise have been necessary.  At least with the containments intact it was possible for them to let some materials settle out before releasing to the environment.  Now this buffer space/time is lost.  It also means that if something were to happen to take the reactors back into unstable conditions, the design level of protection is no longer available.  These containments can no longer hold their pressure or gaseous contents.  They may be able to survive an internal steam blast simply because they now have escape routes for the expanding gases.

The condition of the spent fuel pools is also an ongoing concern.  While they appear to have made progress, this will be a long fight and there may be more hurdles ahead.  And with the structural integrity of the pools at 3 and 4 being highly in question, having significant aftershocks in the region cannot be helpful as they only add stress to these fractures and potentially make them worse.

Also bad is the fact that food and water are now being affected in the area around the plants.  This is adding to the already enormous humanitarian crisis in Japan.

The Ugly - regardless of how the next few weeks unfold, there will be an enormous cleanup and mitigation job to be done at Fukushima.  Many, many workers will be "burned out" - have their occupational exposure limits met for either the quarter or the year - in the process of recovering from this event.  There will also be many recriminations against upper management of TEPCO for their lack of complete transparency over the past week.

Personal note:  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 not had near 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.

From frisco in the comments: links for 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.

BWR Mark I Containment

PhotobucketUpdated by kbman at Sun Mar 20, 2011 at 02:14 PM PDT

FishOutofWater has a diary up that has good information about the affect of anticipated weather conditions in Japan along with more information on the food and water situation ...


http://www.dailykos.com/...


Updated by kbman at Sun Mar 20, 2011 at 03:05 PM PDT

This situation changes quickly as new information becomes available.  An update from Kyodo news service relates that there was a pressure increase measured in the containment vessel at unit 3 but that it has stabilized and no action is planned at the present.  It is unclear from their statement whether they meant the reactor vessel or the primary containment.  It seems likely that they meant reactor vessel because if the primary containment is damaged, as has been reported by TEPCO, then it cannot hold pressure anymore.  It seems much more likely that they have an increase in reactor pressure but are reluctant to vent because of the condition of the containment structure.

Updated 4:15 to add links for Red Cross and Shelter Box USA.

Updated by kbman at Sun Mar 20, 2011 at 05:03 PM PDT

A check of the IAEA site shows that they have also begun adding seawater to the fuel pool at unit 2 ...

"On 20 March, workers began pumping 40 tonnes of seawater into the spent fuel pool."

http://www.iaea.org/...

Updated by kbman at Sun Mar 20, 2011 at 10:15 PM PDT

From BlueSue in the comments -

Tokyo Electric Power Co. proceeded Monday with work to lay power cables to the two remaining reactors still without electricity at the crisis-hit Fukushima Daiichi nuclear power plant after fire trucks sprayed water earlier in the day to help cool fuel pools at two other reactors.

External power reached the power-receiving facilities of the No. 2 and No. 5 reactors on Sunday, paving the way for the plant operator to restore their systems to monitor radiation and other data, light the control rooms and cool down the reactors and their spent-fuel storage pools.

But it may take a few more days before the vital cooling system is restored at the No. 2 reactor, whose containment vessel suffered damage in its pressure-suppression chamber, as some parts replacements are needed in the electrical system, according to the government's Nuclear and Industrial Safety Agency.

Earlier Monday, Self-Defense Forces fire trucks sprayed water at the No. 4 reactor's spent-fuel pool for the second day, after firefighters poured water at the No. 3 unit, dousing it with over 3,700 tons of water in total since the unprecedented effort to lower the temperature in its fuel tank from outside its damaged building began Thursday.

from:

http://mdn.mainichi.jp/...

Originally posted to 2011 Japan Earthquake and Tsunami Information and Assistance on Sun Mar 20, 2011 at 02:03 PM PDT.

Also republished by Nuclear dkos and Japan Nuclear Incident Liveblogs.

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