Japanese nuclear accident FAQ 7

[MintJulep]

I'm sure no one is thinking of bringing anything back online in anything close to its design capacity.

Most likely the goal is to get some pump of some sort running, and pump water onto things that are hot.

If memory serves, the spent fuel pool water circ pumps are one or two decks below the top of the pool. It's very possible that they are undamaged by flooding, explosions or fire.

 

[rcchap]

trottiey: Nice job on the Q&A. It makes be sick to watch the mainstream media interview a UCS physicist or a politician as a nuclear industry "expert". It is important to get the facts out there.

Printing724: Keep in mind that sub plants are PWRs, so you have the capability to remove heat with natural circulation using the steam generators. Some PWRs in the US also have this capability. One that I worked at could be cooled as long as you had a few feet of water in the steam generator. BWRs (like Fukushima) are different. They have recirculation pumps and supression pools. They may not be able to be cooled as easily using natural circulation. (Rickover knew what he was doing, eh?)

 

[trottiey]

Thanks rcchap! I find that most nuclear types tend to avoid interviews about safety issues, or at least avoid speaking openly about them. So it's not wonder the journalists resort to the second-rate guys for answers. This doesn't help anyone.

 

[rmw]

I'd appreciate it if they would scrounge up some second rate guys. I don't think we've had that quality yet.

rmw

 

[msquared48]

Trottiey:

You might add saving face to your 18:44 list posting of communication problems. Unfortunately this is a reality that is frequently ignored and factored into the communication equation.

And could you please address the role that the presence or lack of oxygen played in the explosions and fires? I have heard rumors that seem ridiculous. Thanks.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Sorry, but I meant NOT factored.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[trottiey]

Wow, I hadn't heard anything about the oxygen controversy until you pointed it out. I can confirm that ridiculous rumours are rampant out there, and I can't afford the time to disprove them all. But here's some basic reality checks that should help:

1) Radiolysis routinely produces both hydrogen and oxygen in normal reactor operation. Recombiners dispose of the radiolysis products by turning them back into water. Radiolysis will continue at a reduced rate when the reactor is shut down, but recombiners may not work (or lose efficiency) if they don't have electrical power.

2) If the core overheats, the zirconium-water reaction may produce hydrogen and zirconium oxide, (and maybe some form of hydroxide?) but no oxygen. The recombiners are no help if they don't have enough oxygen to react with the hydrogen.

3) In a meltdown scenario, you could have hydrogen production from both of the above sources. The flammability and explosive bands for hydrogen and oxygen are impressively wide, buy the energy output tends to be weaker at the edges of the bands than for a perfect stochiometric mix.

4) You can't rule out an explosion inside the reactor pressure vessel. It didn't happen in Three Mile Island, but I know of two other reactors where it did. Those cases were relatively innocuous pops relative to the vessel strength. They bent some attached piping and cracked supports, but the vessel itself didn't seem to notice.

The next three points are educated guesswork, not be trusted.

5) The venting procedures are unclear to me, but it appears that some RPV coolant was vented to primary containment. I'm told that there are some pneumatic actuators inside their primary containment, and I'm guessing they run on air. So the pneumatic exhaust becomes an additional source of oxygen, raising the risk and potential power of the explosion, inside a significantly weaker vessel.

6) I can't make much sense of the secondary containment design, i.e. the reactor building, except as a collector of fugitive emissions and holding tank for the filters and exhaust stack. Therefore I presume that any vent to atmosphere, either from the RPV or from primary containment, is supposed to go through secondary containment. That would reduce the flow rate through the filters and increase their effectiveness, but it means mixing the hydrogen with a lot of air. Going out on a limb, I would guess that the refuelling deck was meant to be frangible in the event of overpressure.

7) Someone eventually got the idea to cut vent holes in secondary containment of unit 2, 5, and 6 to improve hydrogen venting. You can see these square holes in the walls on some pictures. Those three units now look much better from the outside than 1, 2, and 4, so somebody deserves a raise.

The last three points are not necessarily true, but they at least give a plausible alternative to some of zany conspiracies you can Google.

Having said all that, the hydrogen explosion are still puzzling to the best nuclear chemists I work with. As best as we can figure out, any plant should normally have enough recombiner and igniter capacity within containment to handle this accident scenario. So why didn't they work? Were they broken or blocked? Was the hydrogen production or venting far higher than our estimates? Did someone plug them into a regular outlet instead of one that was protected by the back-up battery system? Was this just another effect of the extended station blackout? We'll have to wait for the lessons learned report years hence to get complete answers.

 

[printing724]

@rcchap

Your comments here (and in the hindsight thread) are right on. And you are right, decay heat removal in a submarine PWR vs a land based BWR are completely different. I used the submarine PWR example only because (a) I was personally familiar with it and (b) it shows that decay heat removal without electrical power is a practical proposition. Whether it is cost-effective in a given situation is a matter for regulators and business managers, not engineers.

Fukushima physically survived a greater than design basis seismic event only to lose control of the plant because comparatively fragile electrical systems failed.

I think a relevant question (that will be heard frequently) is, "What will your plant conditions be after a long term site blackout?" The list of correct answers will not include statements that start out, "A long term SBO couldn't happen here because..."

Good or bad, the long term SBO has just made the list of foreseeable events, at least in the public eye. Doesn't matter what kind of inconceivable circumstances cause it. The genie is out of the bottle. IMHO, the nuclear industry must acknowledge that if it is to overcome the loss of public trust resulting from this tragic event.

And yeah, Rickover had it right. (I just read an interesting first-hand account about the first full power runs that were done on the Nautilis prototype. When you consider the limited prior art knowledge base they were building on (none), how successful they were, and how much it influenced reactor designs that followed, it was an amazing achievement.)

Kevin Snyder
SW2010 x64 SP3
Win 7 Pro
Core2 Quad Q6600 2.4Ghz 8Gb
NVIDIA Quadro FX570
3D Connexion SpacePilot Pro

 

[printing724]

@rcchap

I just re-read your post above. The steam generators had nothing to do with the decay heat removal system (XC system, for emergency cooling). It was a standalone heat exchanger that tapped into the primary loop inside the main coolant cutout valve boundary as I recall. A thermal siphon of primary coolant flowed through it, and it was cooled by a seawater thermal siphon (under loss of all power conditions, with power available it was cooled by the RPFW fresh water system). The thermal siphons are only possible because the heat sink (ocean) is at a level above the heat source (reactor core).



Kevin Snyder
SW2010 x64 SP3
Win 7 Pro
Core2 Quad Q6600 2.4Ghz 8Gb
NVIDIA Quadro FX570
3D Connexion SpacePilot Pro

 

[rcchap]

printing724:

I agree completely. A long term SBO is no longer beyond design basis. This will our affect our entire industry.

Your XC system sounds unique to a sub plant. Land based PWRs (at least the ones I have worked at) don't have that. It makes sense to have this on a sub.


Trottiey:

Most of the hydrogen production is a result of the zirconium-steam chemical reaction.

After they lost power from their batteries, the RCS pressure would be instreasing from the continued core decay heat and exothermic zirconium-steam chemical reactions. They vented the RCS into the primary containment. Then, to protect the primary containment from overpressurization, they vented it through a series of ducts with filters to remove radioactivity. The vent path would have discharged this at an elevated release point. The hydrogen detonation occurred during venting shortly after an aftershock. The prevailing thought is that a spark ignited the hydrogen during the venting process.

Venting of H2 was of great concern during TMI. I remember reading somewhere that the containment pressure spike did occur, so it is possible that there was an explosion inside containment. But their containment held (a PWR only has one containment, not two like a BWR) and thus there was no significant radiation release.

 

[rmw]

Printing724,

Is what you read about the Nautilus prototype testing a publicly available book or document?

rmw

 

[Zogzog]

Here is a link to the article (I assume) he was refering to. I tourded this plant when I was stationed at A1W, it was shut down and if I recall they were removing the vessel at the time.

http://www.theatlantic.com/technolo...e-of-the-worlds-first-nuclear-reactors/72870/

 

[owg]

Gillespie asked the following a few days ago. I did not see an answer.
"I have a question about the seawater they are using as a coolant.How is it being brought in?How does it cool the reactor? Does it just flood the whole reactor?Does it just turn to steam?"

I have been reviewing the reactor circuit and I note that the primary cooling water is normally radioactive. It leaves the containment building as steam, drives the turbine, gets condensed, and goes back to pick up more heat from the reactor. What circuit is used for the emergency seawater? Is it also radioactive? Does it just vent radioactivity to atmosphere after it goes through the reactor? Thanks.

HAZOP at

http://www.curryhydrocarbons.ca

 

[trottiey]

The primary cooling circuit you describe is normally driven by feedwater pumps. Those pumps need megawatts of electricity to run, and they may have been damaged by the tsunami since they reside in the turbine hall. Or the condenser pumps might not be running, depriving the condensers of cold seawater. There may be additional back-up pumps, or the system may have some ability to thermosiphon, but basically the cooling system hasn't been running adequately for the last couple of weeks.

The primary coolant is still boiling or evaporating, and that endothermic process still cools the fuel just like it does during normal operation. But with the cooling circuit handicapped, the condensers can't keep up and the water level starts to fall. So to keep pressures down and keep the core covered with water, it sounds like they have been venting steam from that primary coolant circuit and replacing it with seawater.

Any water that flows over the fuel is going to pick up some radioactivity, much more so if the fuel is damaged. At least some of that radioactive steam has been vented to primary containment and condensed to water there. Excess gases and steam could normally be piped through filters that capture most of the contaminants and then released through a high smokestack to dilute any remaining fallout before it reaches the workers. In the case of Fukushima, the filters have evidently been overwhelmed or damaged.

Yesterday's newspapers indicate that they have released a large amount of radioactive water in liquid form. That could be a result of primary containment breach or leakage, or it might have been an intentional release of the condensate from primary containment, which would be consistent with Fermi2's earlier analysis here:

http://warships1discussionboards.yuku.com/topic/15893/8-9-Earthquake-Tsumani-in-Japan?page=3

I don't think we'll know much more specific details of which pipes and pumps they used until much later. There's lots of plumbing in a nuclear reactor, there's a recirculation circuit you might be interested in, there's water in the torus that also needs to be kept cool, etc. And we can presume that they also have a lot of broken pipes, stuck valves, and temporary bypasses at this point.

 

[zlindauer]

What's the worst case scenario? What would be the likely outcome if everyone just walked away from the plant?

 

[owg]

Thanks very much Trottier. What you suggest sounds reasonable. The only phrase I did not understand was "vented to primary containment and condensed to water there". I thought the primary coolant was already in primary containment. Wouldn't venting it put it into some other containment or just to atmosphere? However I appreciate that we don't have access to the full process flow scheme. Keep up the good work.

HAZOP at

http://www.curryhydrocarbons.ca

 

[owg]

After a couple of passes at Fermi2's notes, I think the steam gets vented to the Tora. It goes in underwater so that is how it gets condensed. There is some thermosyphon during normal cooling but not if the condensers after the turbogenerators have no cooling water flow. I am also wondering how Fukushima normally gets rid of low grade heat, I don't see any cooling tower in the pictures. Could they be warming the sea?

HAZOP at

http://www.curryhydrocarbons.ca

 

[itsmoked]

owg; You have it. NORMALLY the cooling loop is closed and none of it is released to the atmosphere. Now since there is no functioning condenser system the only thing left to remove the steam, really the pressure it builds, is to directly vent it to the atmosphere. Normally that isn't particularly bad as the normal nucleotides that would be in that are fairly short lived. However as the core becomes damaged the resulting products get substantially worse/different and more concentrated.

zlindauer; Worse case? Walk away? Think Chernobyl. All the spent full pools would empty. The fuel in them would melt and probably burn off their jackets.

Probably 3 reactor vessels would eventually rupture. There would be fires. They would carry radioactive dusts having half lives up to thousands of years. All this carried away on the smoke particles.

The smoke would be going in any weather driven direction and probably dangerously contaminate anywhere and everywhere with-in something like 200 miles. Out to perhaps a thousand miles you would have areas of serious contamination with islands of notable contamination depending on local air and wind patterns and shadows.

Ultimately approaching the site would be extremely hazardous.

That's why this fantasy will not be happening.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[trottiey]

The primary coolant is normally kept at very high pressures (say 100 atmospheres) inside the reactor pressure vessel. (RPV) That's the blue pill-shaped thing in the picture. The RPV resides inside the light-bulb shaped primary containment, which is not normally pressurized and is only supposed to take 2-3 atmospheres in an accident scenario. The torus (also called "wetwell") is connected to the primary containment, so overpressure is supposed bubble through that water. That traps particulates and some gases, and condenses the steam. I believe primary containment also has its own cooling systems, although they're not shown on this diagram. If these cooling systems are overwhelmed, the torus water can eventually get so hot that it evaporates away, which might have happened intermittently in Fukushima.

The low-grade heat is ditched to sea, which is represented by that green pool at the bottom right-hand of the diagram.

Defining worst-case scenarios at this level of complexity is a sucker's game, because it depends on where you want to draw the line on low-probability events. Itsmoked's assessment feels somewhat too bleak to me, but I don't feel I can make more precise predictions.

 

[786392]

Hello/Good
Afternoon Although many very much learned &knowledgeable people are around.
I have mentioned in another forum seawater's with very high NaCl contents has severe corrosive and imaginable consequential impacts all around the reactor primary containment,secondary containment and even on to main core itself(although may not be exactly estimated very correctly as per close vicinity radioactivity exposure threat).
It has already done great harm and now being planned for replacing with fresh water which seems too late.

A safer activity right now is to start one by one covering the reactors at risk(starting from most damaged two reactors) with targeted concrete & sand mixtures covering till the radio activity subsides to very safe levels.
This is the safest for Japan and the whole world as otherwise the "Grave Consequences are really foreseeable"

Best Regards
Qalander(Chem)