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Posted: Sun Mar 13, 2011 2:14 pm
*Assuming* they don't lose containment, it sure seems like you could make a strong pro-nuclear argument out of this (even though I'm sure that won't end up being its lasting legacy... which will certainly be "meltdown").
Japan's nuclear plants held up to one of the very worst earthquakes ever... and on top of that, at least one survived the nightmare scenario losing its backup power as well. All with a very small overall radiation leak.
On the critical side of things, though, one thing I would like to know is was a tsunami really not planned for? Or did the design to deal with a tsunami not function as expected? At least, I bet the next nuclear plant hit by a tsunami from a megaquake will have backup power that can survive it.
Posted: Sun Mar 13, 2011 2:40 pm
Cherynobl was a different plant design. They were an open containment graphite moderated core. In essence, a really big open top box, filled with the core assy, and graphite packing. They also were using essentially a large warehouse building as the surrounding structure. It was a really dangerous design. No containment in any sense that we consider in our designs.
The failure there revovled around the graphite moderator catching fire, which is the single greatest risk in that type of plant. Theirs did, and then it did not go well at all. Complete destruction is an understatement as well all know. Think of the plant as an extremely large highly radioactive BBQ Bricket, that self ignited.
In Japan, the plant design is completely different, and thus very resistant to exposure of the fuel to atmosphere. The boundary layers start with and actual Fuel Assy. It is a "Pop Tart" made from a Fuel Matrix filling, then encased in a Zircalloy shell. These pop tarts are in trun assembled into fuel rod assys. The Pop Tarts are shaped and arranged to allow water to flow in channels all through the Fuel Rod, thus drawing off the heat. Multiple Fuel Rods are placed into a holding "cage" or Frame that makes up the actual Core Assy. The core structure contains spaces for the Control Rods (the Neutron Sponges) that control the reaction. In a "normal" state, all rods are fully inserted into the core, and there are thus not enough Neutrons to sustain the reaction. This is the Core Geometry part, it determines balancing and control of fuel burn. We use very large safety margins in our designs regarding Geometry.
The entire core assy resides in a very large very thick solid alloy container we call the Reactor Vessel. This vessel is designed, and tested to extremes to ensure that it can contain the core in the event of an accident. This is the second major boundary for the fuel. The vessel designs have been rigorously tested through the years to include what is called live Prompt Criticality core failures. This is an abrupt an instantaneous cook off of the core. For the record, TMI-2's core completely melted. It ended up as a puddle in the bottom of the vessel, under the crusted melt shell, which looked like a defalted basketball. Imagine placing a planet in a bucket, a poking a hole in it so the mloten interior leaks out, deflating the shell. A western designed (US centric) Reactor vessel has never failed under operation or in casualty (tests or actual). The vessel is the heart of the matter.
We then place the Reactor Vessel and the entire Primary system in a hardened building (structure) which is called the Containment Building. It is the final Major Boundary. The building is designed to withstand many issues, both from the inside and out. My money says that some desgin reviews and changes will take place, because the introduction of H2 via cycling Primary Relief Valves from compromised Fuel Assys has demonstrated itself to be an issue. They will seek to introduce a method to control the H2 buildup for sure. As I recall, TMI-2 also had a H2 problem that ignited, but not to the scale of Japan.
It is extremely unlikely that the failed core will "melt" itself out of the Reactor vessel. That said, it already is a large expensive permanent failure. Unit 1 will not be recovered. The core compromise, and subsequent Boron Kill, plus the release of fuel matrix components into the vessel, have trashed not only the vessel, but also the primary loop components used for the emergency cooling efforts. I imagine they will either eventually remove the entire Reactor Vessel for permanent disposal as we did at TMI-2, or they will entomb it in place, similar to, but not to the incredible extent the Russians did at Cherynobl. Probably they will end up removing the Vessel and relocating it to a waste storage site.
In order to have a Cherynobl scale disaster, you need to physically and violently explode the flaming reactor core over a square mile or so while the remaining melting and flaming core burns its way down into the foundation structure of the power plant building. This is not going to happen in Japan.
Posted: Sun Mar 13, 2011 3:42 pm
Ladajo, from what I understand, even if the core melted through the reactor vessel it would still be contained by the contaiment vessel which is specifically designed to deal with such a situation.
Posted: Sun Mar 13, 2011 4:24 pm
The Reactor Vessel is the Containment for the Core.
The Vessel normally sits in a shield tank, which is a large steel box filled with water. This acts as the primary Neutron shield. This shield tank then in turn rests on a re-enforced steel and concrete foundation, and sub-foundation. This entire assembly resides inside the "Containment Building". Which for Unit 1, is now just a skeleton, as the concrete blocks were blown away by the H2 explosion. If you could get close enough with a Helocopter, you could see the top of the Reactor Vessel structure and its associated components.
The ability of the core to actually melt through the vessel is more a concept of media driven hysteria. I would be really impressed it if happened, and so would a lot of design engineers.
If it did, then the slag would have to boil off the shield tank, (not so much a problem given that it had enough juice to get through the vessel), and then eat its way down into the re-enforced foundation. All of this would dilute the slag as well as bleed off heat. Cherynobl was able to burn down into its foundation. However, it did not get through the subfoundation and supporting cooling infrastructure. As I recall, the soviets lost a couple of guys in investigating that, as then again during subsequent emergency forced concrete fill to re-enforce the subfoundation and contain the breaching.
edit: my usual atrocious spelling
Posted: Sun Mar 13, 2011 6:10 pm
Don't know if you've covered this, but this appears to be interesting insight from another expert.
http://www.cringely.com/2011/03/flea-po ... -in-japan/
Posted: Sun Mar 13, 2011 6:22 pm
I do not think they have lost all 11 plants. Right now, I think they have lost 4. Two burns, and two sea water injections. The cost to recover the sea water injections is going to be extreme, and probably not worth it.
I did talk about Boron Kill above, and that you don't recover once driven to that.
They may eventually recover the plants, but my guess is they will pull the Reactor Vessels and ship them off for analysis and subsequent disposal storage, as we did at TMI-2. And the remaining plants will become permanent monuments to design considerations, just like TMI-2.
For those interested:
http://www.wikimapia.org/#lat=37.422157 ... 19&l=0&m=b
http://en.wikipedia.org/wiki/File:2011- ... n_shot.jpg
Also, please note that I have grealty simplified plant descriptions for several reasons.
Fuel Matrix resides inside Fuel Plate "Pop Tart" which makes up sub-assys and assys to make up a "Fuel Rod" which resides with others in the Core Assy (the mounting frame), which resides inside the Reactor Vessel, which resides inside the Primary Shield tank, which resides inside the Containment building.
Hope this all helps those without backgrounds in this stuff understand better.
Posted: Sun Mar 13, 2011 6:24 pm
So there is no containment vessel?
I am pretty sure that I heard talk about a containment vessel. The reactor building obviously is not the containment vessel.
The shield tank = containment vessel?
Posted: Sun Mar 13, 2011 6:36 pm
The primary "Containment Vessel" is the "Reactor Vessel". In actuality, the "Primary" containment is the Fuel Plate itself. Some would say the primary boundary is the Fuel Pellets themselves in the Matrix, but for simplicity, let's say not.
The idea of "Containment" is tied directly to "Boundaries". Ie. How many boundaries must the Fuel cross to get to atmosphere/environment.
The three main points of "Containment" are the Fuel Plate, The Reactor Vessel, and the Containment Building.
The Shield Tank is just a large water box to keep neutrons from cooking off the operating staff in the secondary plant. It does however play a limited role as another "layer" to the boundaries, just not a robust one.
Note the the top of the Sheild Tank does not cover the top of the Reactor Vessel. Think of the Reactor Vessel as a large pressure vessel that is sitting inside a big bathtub of water (Shield Tank), with the top sticking out.
The Containment Building IS the designed structure to prevent the spread of core material in the event of Reactor Vessel failure. However, as noted in Japan, the Containment Building has been compromised. THis is why they will visit the H2 issue in depth. This is something we knew from TMI-2, and other accidents and testing, that Zircalloy can interact in overtemp conditions to make H2 gas. However, the H2 "pop" in TMI-2 was much smaller, and the Containment Building much more robust, and not damaged. US Containment Buildings are traditionally more robust I think, but follow VERY similar design guidelines. Ours are actually designed to withstand aircraft impacts for example.
Posted: Sun Mar 13, 2011 7:04 pm
Are you sure this is the same with the japanese reactors?
Got any references to that?
Posted: Sun Mar 13, 2011 7:56 pm
You are referring to "Containment in a Containment" setup. This is a "room" in the building that the Reactor Vessel and associated Primary Components are located in.
The location of the Primary Relief Valves will determine the after blast integrity of this structure. The probable outcome was that the re-fueling access at the top was compromised in the blast.
http://www.nrc.gov/reading-rm/doc-colle ... cr6906.pdf
The Japanese Unit 1, is a Mark I based design, but called a Type 3.
See Figure II on Page 4.
Try this for failure modes:
NRC also has an Idiots Guide to power plants:
Posted: Sun Mar 13, 2011 8:01 pm
And for the record:
Radioactive materials are released into the high temperature and high pressure coolant when a
fuel failure occurs. Therefore, a reactor containment is provided so that the coolant would not
discharge to the outside (Figure 7). All BWR containments are pressure suppression (pressure
suppression pool) type, and the steam discharged into the containment is led to the water pool
of the pressure suppression chamber, cooled and condensed, and the pressure rise within the
containment is suppressed as a result. Moreover, as the temperature and pressure of the
containment rise due to the fuel decay heat in a long term after an accident, it is necessary to
cool the inside of the containment. Furthermore, it is also necessary to remove radioactive
materials such as iodine within the containment. For such purposes, the containment spray
system is provided within the containment (drywell spray, pressure suppression chamber
spray). Furthermore, the standby gas treatment system is provided in the reactor building so
that the radioactive materials will not be released to the outside of the containment.
In addition, following a loss of coolant accident, the temperature of fuel cladding could rise
and hydrogen could be generated by a water-metal reaction, which could impair the
containment integrity due to hydrogen gas combustion. In order to prevent such a case, BWR
containments are kept inert with nitrogen gas (Mark-III type containment is designed not to
use the nitrogen gas, but it is not adopted in Japan) during normal operation, and the
flammability control system to prevent hydrogen combustion by recombining the generated
hydrogen gas with oxygen gas.
From page 10,
Posted: Sun Mar 13, 2011 8:02 pm
Skipjack wrote:Ok, so please correct me if I am wrong, but a meltdown would not mean another Tschernobyl.
If the reactor melted down, it would form a puddle on the bottom of the reactor vessel. If it still kept getting hotter, it would melt through the reactor vessel into the containment vessel, which is built to deal with a situation like that.
So a meltdown would be an economic catastrophy, but not harming the environment or human life. That is what I understand. Please anyone correct me if I am wrong.
My impression from recent reports is that a complete meltdown implies that the extremely radioactive core has breached containment. The molten slag within the primary containment vessel would be considered a partial meltdown, until it melted through the steel and concrete floors and reached the enviornment. This is a simple description. The radiation can escape confinement through venting without the core itself escaping. This is magnitudes less serious, but still potentially deadly. The Russion reactor blew the lid off of the primary (and only) containment vessel- in a sense it was a LARGE venting. Also, chunks of the core was thrown out, so the problem was compounded locally. But the radiation in the form of gasses or fine dust would be the major concern as this can travel far from the source. Also, I don't know if the slag in the base of the fractured containment structure managed to melt all the way through the floor and contaminate the ground water.
The Japanese reactor is an interesting reversal. The concrete and steel building designed as a backup containment barrier if the primary fails, instead failed first. This makes one wonder how well the the designs have been thought out. This reinforces what I have ignorantly speculated on after the TMI accident. There is two levels of confinement , but pressure, venting concerns would seem to suggest a need for radioactive, dangerous gas venting reservoirs that would mitigate and at least partially contain dangerous gasses so that venting would not be delayed until even more dangerous conditions developed. Essentially, a few large swimming pools of doped water in series to hopefully limit the radioactive gasses finally escaping into the air. This wouldn't completely eliminate venting concerns, but it would ease the burden on the decision makers and the environment.
Posted: Sun Mar 13, 2011 8:03 pm
Also, give you a better idea graphics for reference:
and don't forget to scroll down.
Posted: Sun Mar 13, 2011 8:18 pm
And if you want to take the time to read details on plant safety:
http://www.ansn-jp.org/jneslibrary/BWR_ ... Design.pdf
Please note that I have tried to simplfy a lot of this as a courtesy, and hit on the key points. Sorry if that has caused any confusion.
The key point here is that the H2 Blast compromised the plant containment. The Reactor Vessel itself is going to be OK, but the surrounding structure is breached. So there is still containment, just fewer "boundary layers".
This happened because the Primary Reliefs are inside the structure. Cycling the reliefs accumulated H2 in the structure. The most proable failure point in the structure is the re-fueling access at the top. As I noted previously, the main going forward point for plant safety is going to be a hard look at the H2 gas control. The existing approach did not work. To be fair, newer designs seek to mitigate this risk in a better fashion. ABWR's are more robust in this regard.
Posted: Sun Mar 13, 2011 8:29 pm
Good simplification. Thanks.
The Cherynobl plant did breach down, but it did not get thorugh the sub-foundation and cooling system. The Russians spent several lives, including one of the principle engineers in an rushed emergency concrete fill under the plant to keep it from further breach. I also can not remember off the top of my head the wieght of the cover slabs on the core, but I do remember they was purdy darn heavy, and when she popped, it just went away. Literally.
I have also wondered periodically about the design concept regarding primary venting. But, I guess now they will take a harder look at it. It has been basically sound in concept, vent into the pools and spray down for contaminant control. But, knowing about the H2 pop at TMI-2, I have always had that little nagging in the back of my head. I guess I know why now. The drama of Primary Relief venting paths has always been around.