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Japan Earthquake: nuclear plants

by gmax137
Tags: earthquake, japan, nuclear
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MadderDoc
#10783
Aug4-11, 08:50 AM
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Quote Quote by SteveElbows View Post
Yeah thats a fair point. Do we have any idea what happened on the 30th-31st? I don't remember hearing of anything specific in the past, is venting a possibility?
I suggest the basis for the data estimate for that period may be in error. If there had in fact been any striking change or elevation in emissions during March 30th-31st, it would be expected to have shown up in the measurements from the site monitoring posts, but there is nothing there to be seen:
nikkkom
#10784
Aug4-11, 11:36 AM
P: 610
Quote Quote by rmattila View Post
I'd say there is no such thing as a "wet meltdown": as long as at least half of the core is covered, the steam will provide some cooling to the top of the fuel to slow down the heat buildup, and cladding integrity is lost only after water will fall to -2m or so.

In the particular Finnish case, SAM procedures are entered when water level has been below +0.7 m for more than 30 minutes. At that time, the reactor pressure vessel is depressurized (to prevent high-pressure melt through, which the containment would not endure, and also to enable low-pressure core injection, if it would happen to be available) and the lower drywell is flooded in preparation for a melt-through.
ok...

However, there's no point in venting the containment at that time, since there's a relatively high probability that no venting will be needed at all - venting will only be needed, if the power outage lasts for several hours after the core has melted, and the decay heat removal from the wet well can not be started.
Now this doesn't sound good. What would be better - to vent a relatively cool (-> easier to scrub/filter) and relatively less radioactive steam earlier, or wait until after core melt and vent very hot, very radioactive steam/gas later?

In other words, venting is not a standard procedure to be applied in core melt situations, but rather an additional backup in case the containment heat removal has not been started after about 8 hours after the meltdown. If this can be done, then no venting will be needed to contain the core remains.
To me it looks that if core melt has started and progressed to a significant degree (say, 20% or more of core has melted), the reactor is not salvageable anyway, and attempts to reflood it actually may make the situation worse, not better: massive generation of VERY radioactive steam, possibly H2 from water/Zr reaction, explosions.
MadderDoc
#10785
Aug4-11, 12:33 PM
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Quote Quote by nikkkom View Post
<..>What would be better - to vent a relatively cool (-> easier to scrub/filter) and relatively less radioactive steam earlier, or wait until after core melt and vent very hot, very radioactive steam/gas later?<..>

It would seem to me as a lay man, that an early depressurization of the reactor pressure vessel into the S/C might have avoided much of the damage caused to the dry-wells by the excessive heat and pressure they must have been exposed to while the pressure and temperature were allowed to remain high in the RPV, while any cooling systems of the dry-wells became inoperable due to lack of power.

Naturally this would imply also an early need to do S/C venting, something one would have had to have the willingness as well as the technical ability to do in an efficient manner. Also it would have implied the assumption that the return of power to the plant would not be forthcoming in a relevant time frame.

As it worked out in reality, it appears to me, the RPV's were depressurized only with difficulty, and action to do so was postponed to become too late to avoid damage to the dry wells -- and in the end there proved to be major technical difficulties of doing any vents at all.
robinson
#10786
Aug4-11, 01:17 PM
P: 201
It's possible there actually are none, as in zero, plans in place for what to do when a meltdown is occurring. If there are plans, and drills done, what are they?

It seems like "run away" is the only actual response anybody is ready to do at a moments notice.
MJRacer
#10787
Aug4-11, 02:59 PM
P: 37
Quote Quote by Dmytry View Post
Regarding the filtered venting - just as Fukushima struck, NRC was in the final stage of preparation of a much lower impact estimate for core meltdowns:
http://www.physorg.com/news/2011-08-...-meltdown.html
, most interestingly, downsizing the Cs-137 release in a core melt from 60% of the inventory to 2% of the inventory.
My understanding is that it is the 60% estimate which justified Finnish/Swedish implementation of filtered high volume venting.
This reminds of Cockcroft's Folly...
Is there a reliable estimate of what percentage of Cs-137 inventory has been released so far at Fukushima?
NUCENG
#10788
Aug4-11, 04:36 PM
Sci Advisor
P: 916
Quote Quote by robinson View Post
It's possible there actually are none, as in zero, plans in place for what to do when a meltdown is occurring. If there are plans, and drills done, what are they?

It seems like "run away" is the only actual response anybody is ready to do at a moments notice.
Why would you say that when everything discussed here proves that isn't true?

US nuclear plants have Emergency Operating Procedures to preveny or limit core damage, These procedures are symptom based so it is directed at providing makeup to maintain water level, core cooling to remove decay heayt, rreactivity control to ensure shutdown, containment and presseure vessel pressure control and containment cooling to prevent containment failures. Once core damage is suspected (radioactivity increases, the Severe Accident management Guidelines are activated doing everything possible to protect containment and limit releases to the environment. These procedures are trained and drilled on simulators and are validated via walkthroughs in the actual plants.

Emergency drills are performed several times a year and exercise command control and communications for offsite response providers and regiulators. Evacuation plans are reviewed and approved for use and have been exercised on a limited basis.

After 9/11 a whole new set of requirements were included in US plants to address the effects of SBOs, aircraft attacks or large fires. These capabilitiies were reviewed and inspected at every US plant after March 11 to ensure they were operable and available.

Finally, TMI, Chernobyl, and Fukushima Daiichi have proven one thing - operators don't "run away." At TMI while they were trying to reduce the hydrogen bubble, they had to order people out of the control room becausae everybody wanted to help. Chernobyl operators died because they refused to abandon their responsibility. At Fukushima workers told us they were ready to die if necessary.

It is a legitimate point to say that the event at Fukushima did result in increased risk to the public and environmental and economic effects that will last for years. But we also know that more was involved in that failure that emergency procedures and planning. There was negligence on design basis, there were significant delays in venting containment, and there was deliberate understatement of risk after the event that contributed to the public risk.

Your flippant conclusion that there were no plans or drills is not helpful. They had plans and procedures at Fukushima. At one point there was a claim that they had to go look for them in another building. So my questions are:

What were the procedures?
Were they available?
Were Operators trained on their use?
Was everything available to execute the procedures (e.g., flashlight batteries, tools, etc.)?
Were they implemented?
Were they delayed?
Were they effective?
If they failed, why did they fail?

Look at a couple of specific points.

Operators at Unit 1 may have unintentionaly or inappropriately disabled the isolation condensers.
Uperator were not certain whether containment venting was successful at Unit 1.
Hardened Wetwell Venting may have been delayed by seeking approval until pressures exceeded even the design pressure for the hardened piping.
Fire trucks and pumps were used to inject water into the RPVs. Was the pressure within the capabilities of those pumps?

In short, it takes more than a one line zinger to add value to this discussion.
NUCENG
#10789
Aug4-11, 04:43 PM
Sci Advisor
P: 916
Quote Quote by MadderDoc View Post
I suggest the basis for the data estimate for that period may be in error. If there had in fact been any striking change or elevation in emissions during March 30th-31st, it would be expected to have shown up in the measurements from the site monitoring posts, but there is nothing there to be seen:
Can you tell us more about that chart? Source of data, how many points were used, etc.?
rmattila
#10790
Aug4-11, 05:00 PM
P: 242
Quote Quote by nikkkom View Post
To me it looks that if core melt has started and progressed to a significant degree (say, 20% or more of core has melted), the reactor is not salvageable anyway, and attempts to reflood it actually may make the situation worse, not better: massive generation of VERY radioactive steam, possibly H2 from water/Zr reaction, explosions.
Since there's no oxygen in the containment, there is no risk of H2 explosions as long as the stuff remains contained. I'd rather say the uncertainties and different opinions related to the severe accident mitigation strategies in older BWR:s revolve around the issue whether to catch the molten core in dry containment and face the risk of core-concrete interaction and failure of lower penetrations, or flood the lower drywell and face the risk of steam explosions. The optimal solution varies according to the plant design and the assumed failure mode of the reactor pressure vessel. The Nordic approach of flooding the drywell before melt-through is based mainly on two arguments: the depth of the flooded drywell is large, and the core melt is expected to come through one failed penetration rather than as a large slump. Therefore the risk of steam explosion is analyzed to be smaller in comparison to the risk of containment failure due to melt-through to a dry containment.

New plants can (and at least here: will have to) be equipped with better core catchers in order to minimize the risk of steam explosions further.

But concerning early venting: to me it would not seem a sensible approach to backfit the containment to endure the severe accident conditions without any release, and yet vent it to the atmosphere at the early stages of the accident, when there are no signs of containment not being able to contain the radioactivity as designed. The need to vent arises only, if there's the additional failure of losing the capability to remove heat from the containment for an extended period of time after the core has already melted - i,e. even though the core would not be salvable and there was a risk of getting very high radioactivity in the containment, there would still not be need to do any venting as long as the heat removal from containment can be started within about 8 hours. Early venting would buy some more time - at the cost of making a release - in a situation where additional time is not necessary even needed, and additional time can be obtained also by other means such as initiating the water filling of the containment, which is a standard procedure in severe accident situations.
robinson
#10791
Aug4-11, 06:03 PM
P: 201
I was watching a TV documentary on the Fukushima disaster again. They interviewed an American who was there at the time of the earthquake. He described the terror and the running. The hallways were pitch black. Because the power was out. Battery powered emergency lights seemed to be missing from the plant.

He also described how everybody ran, twice. And described the cracks he saw in the buildings.
Marita
#10792
Aug4-11, 06:45 PM
P: 3
robinson , did you ever think the "running" was part of the the drill to get the unnecessary people to a designated area ... also some of the running was people going to stations to man equipment.

Think General Quarters on a Navy ship ... people running all over the ship , to an outsider it would look like total chaos.

Also if it was Running in Panic I would have expected to see a few more bodies found in the wreckage of the plant.
robinson
#10793
Aug4-11, 09:08 PM
P: 201
They ran because everything in the buildings was falling on top of them. They even pried open the emergency doors to get out the control rooms. Which was a good idea, as anybody who stayed would have died.
MadderDoc
#10794
Aug4-11, 10:52 PM
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Quote Quote by NUCENG View Post
Can you tell us more about that chart? Source of data, how many points were used, etc.?
The data source is Tepco.
In all, 3069 data points, i.e. every single published data point from the given period was plottted.

MadderDoc
#10795
Aug5-11, 12:22 AM
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P: 698
Quote Quote by rmattila View Post
Since there's no oxygen in the containment, there is no risk of H2 explosions as long as the stuff remains contained. <..>
I can see how water/metal interactions should lead only to H2, but radiolysis of water would theoretically produce also O2, I have been wondering about this: Might a configuration with the RPV blowing out into the S/C be conducive to accumulation of radiolysis products there, including O2?

<..>concerning early venting: to me it would not seem a sensible approach to backfit the containment to endure the severe accident conditions without any release, and yet vent it to the atmosphere at the early stages of the accident, when there are no signs of containment not being able to contain the radioactivity as designed.<..>
I think in the case of Fukushima, leaving on an RPV at 7 MPa/400oC inside an uncooled containment vessel with a design limit of about 0.5 MPa/150oC should predictably lead to failure of the containment vessel, well before radioactivity containment eventually might be needed. If there is a rationale for early release of pressure and heat from the RPV, it would in turn provide a rationale for the following need for early venting. There is of course a line between the wish to contain and the risk of doing so until something gives in.
NUCENG
#10796
Aug5-11, 01:44 AM
Sci Advisor
P: 916
Quote Quote by robinson View Post
They ran because everything in the buildings was falling on top of them. They even pried open the emergency doors to get out the control rooms. Which was a good idea, as anybody who stayed would have died.
Sorry, you are either repeating nonsense or making it up. The films from the lower floors show very little debris, so what was falling on them? The explosions did result in a couple of evacuations, but the operators returned each time. I have seen reports of water leaking in the reactor buildings and about workers who were scared or had difficulties in getting out due to lighting failures.The control rooms did not evacuate until well after the explosions and increased radioactivity. They are still there in spite of poor "rest areas", doing hazardous work inbrutal conditions of heat and protective clothing. In your rrush to judgement you have NO RIGHT to question their courage. People did stay and emergency crews went to the plants, and did their best to protect the public. Some were injured in the explosions, all have been exposed to radiation, but they didn't die contrary to another of your claims.

I provided you with information about the emergency procedures in response to your claim there were none. You have claimed they all ran away without any source to support that claim. This is nothing more that the Dan Rather excuse : "My facts were wrong but my claims are true, because I want them to be true."

There are legitimate issues to be discussed, but it is necessary to get past the kind of uninformed, irresponsible, and simplistic claims being foisted by people who haven't got a clue about what they write.
westfield
#10797
Aug5-11, 02:52 AM
P: 145
Quote Quote by MadderDoc View Post
It would seem to me as a lay man, that an early depressurization of the reactor pressure vessel into the S/C might have avoided much of the damage caused to the dry-wells by the excessive heat and pressure they must have been exposed to while the pressure and temperature were allowed to remain high in the RPV, while any cooling systems of the dry-wells became inoperable due to lack of power.

snip.
I don't think its that black and white - I'm also speaking as a layman so someone please correct me if I'm mistaken, early depressurization to me would mean the first line of defence on a loss of power is gone, namely HPCI and RCIC both of which require high pressure steam to work as I understand it. Obvioulsy the operators would have wanted to try and keep the ability to use those systems so initially depressurization would be the last thing they want to do. The only other systems they had to control heat in the reactors were the LP ones which all require electricity (again, as I understand it).

Of course Unit#1 with its Isolation Condenser instead of RCIC is a different case. That should have worked fine without power, something else went wrong there, perhaps as simple as running out of water.

I guess my point is that the operators are trying to remove the heat from the core to prevent a meltdown, if the heat is under control there will be no pressure issues and therefore no need to vent AT ALL - From what I understand of the systems early depressurization and venting prematurely will not help the heat problem and will remove several critical systems from the picture.

From the sparse reports I've read the operators at Fukushima 1 could not keep the IC and RCIC systems running for some reason and this is one of the most important questions in my mind. If they had functioning IC and RCIC then we might not even be here on this forum now.

Perhaps someone here in the business could clear these aspects of ECCS up for us?
Is it a bad thing to lose those steam driven HP systems and try "fight" the fight with a depressureized reactor?
rmattila
#10798
Aug5-11, 03:39 AM
P: 242
Quote Quote by MadderDoc View Post
I can see how water/metal interactions should lead only to H2, but radiolysis of water would theoretically produce also O2, I have been wondering about this: Might a configuration with the RPV blowing out into the S/C be conducive to accumulation of radiolysis products there, including O2?
Radiolysis will produce oxygen, but this process is slow in comparison to the hydrogen production rate by Zr oxidation. Hydrogen deflagration will require O2 concentrations of >5 %, and detonation >9 %, and if there are recombiners (fixed / mobile) at the plant, they should be able to cope with that. The RPV is constantly "vented" to the S/C in any case after the shutdown (either through pressure control blowdown valves or safety valves) if the MSIVs are closed, so radiolysis O2 will also be vented to the containment and not accumulate in the RPV.


I think in the case of Fukushima, leaving on an RPV at 7 MPa/400oC inside an uncooled containment vessel with a design limit of about 0.5 MPa/150oC should predictably lead to failure of the containment vessel, well before radioactivity containment eventually might be needed. If there is a rationale for early release of pressure and heat from the RPV, it would in turn provide a rationale for the following need for early venting. There is of course a line between the wish to contain and the risk of doing so until something gives in.
A repeat my disclaimer of being familiar only with the Nordic BWR:s, i.e. I don't know about the details of Mark I containment. I have the impression that the heat sink capability of Mark I is rather low, and it may thus be that containment venting can not be avoided in the severe accident situations. In the ASEA BWR:s this is not the case, and without extra failures there should be no need to vent the containment at all unless the heat removal chain from the containment is completely lost for several hours.
MadderDoc
#10799
Aug5-11, 03:43 AM
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Quote Quote by westfield View Post
I don't think its that black and white - I'm also speaking as a layman so someone please correct me if I'm mistaken, early depressurization to me would mean the first line of defence on a loss of power is gone, namely HPCI and RCIC both of which require high pressure steam to work as I understand it. Obvioulsy the operators would have wanted to try and keep the ability to use those systems so initially depressurization would be the last thing they want to do. The only other systems they had to control heat in the reactors were the LP ones which all require electricity (again, as I understand it).
Ultimately HPCI and RCIC would fail due to loss of DC power, but until then the operator would of course want to use them to be able to inject water. I am not suggesting the RPV vessel might have been depressurized to atmospheric, only down to a level that would still keep those systems operable, i.e. to a pressure of about 1 MPa. It would seem to me to have provided a much better starting point, a colder and more water-filled reactor pressure vessel, once HPCI and RCIC failed. And in the meantime it would have protected the dry-well and associated systems from heat damage.

I guess my point is that the operators are trying to remove the heat from the core to prevent a meltdown, if the heat is under control there will be no pressure issues and therefore no need to vent AT ALL - From what I understand of the systems early depressurization and venting prematurely will not help the heat problem and will remove several critical systems from the picture.[<..>
The only respectable heat sink would seem to be the large body of water in the suppression pool. Once that had been filled, i.e. heated up, there would be no way to contain the heat produced by the core. From then on it would be either vent voluntarily, or build up excessive pressure and wait for something to give in.
MadderDoc
#10800
Aug5-11, 04:15 AM
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P: 698
Quote Quote by rmattila View Post
<..>The RPV is constantly "vented" to the S/C in any case after the shutdown (either through pressure control blowdown valves or safety valves) if the MSIVs are closed, so radiolysis O2 will also be vented to the containment and not accumulate in the RPV.
OK, that's also what I figured. That would mean non-compressible gases, including any hydrogen from metal/water interactions and any hydrogen and oxygen produced from radiolysis from the RPV would be transferred to and accumulate in the S/C.


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