Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[MadderDoc]

Yeah that's 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]

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]

<..>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]

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]

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-nrc-downsizes-deaths-nuclear-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]

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]

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]

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]

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]

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]

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]

Can you tell us more about that chart? Source of data, how many points were used, etc.?

The http://www.tepco.co.jp/en/nu/fukushima-np/f1/index-e.html" is Tepco.
In all, 3069 data points, i.e. every single published data point from the given period was plottted.

 

[MadderDoc]

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/400^oC inside an uncooled containment vessel with a design limit of about 0.5 MPa/150^oC 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]

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]

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]

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/400^oC inside an uncooled containment vessel with a design limit of about 0.5 MPa/150^oC 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]

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]

<..>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.

 

[rmattila]

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.

In addition to that, if there's fluctuation in the drywell pressure due to e.g. containment spraying or intermittent venting of steam in the drywell, the drywell pressure may occasionally fall below the S/C pressure, and this will let gases from the S/C flow back to the drywell through vacuum breaker check valves in the piping connecting S/C to the drywell.

 

[SteveElbows]

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:

Yes that certainly seems possible.

They used dust sampling from locations well off-site in order to come up with the release estimates. On the 15th when the highest magnitude release is thought to have happened, they could not do dust sampling due to rain.

The figures used are shown in a table on page 4 of this document:

http://www.nsc.go.jp/anzen/shidai/genan2011/genan031/siryo4-2.pdf

 

[SteveElbows]

Is there a reliable estimate of what percentage of Cs-137 inventory has been released so far at Fukushima?

I don't know how reliable they are, but the accident analysis documents that are public tended to estimate very percentage releases, often in the 0.6%-1% range, but I believe one of their scenarios lead to a broader range of something like 1%-6%. Its been a while since I looked at these documents, but I imagine this was for reactor 2 since their other estimates also have more stuff released from this reactor.

 

[SteveElbows]

Continuing the radioactive stack news, they have measured 3.6 Sv/h at the 'stack drain pipe', photos included in this document:

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110805_01-e.pdf

 

[NUCENG]

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?

Let me try. I will address the IC first as it is a very simple system. I will post a follow up on the more complicated possibilities with RCIC later.

The IC basically is a system to take reactor steam and condense it in a heat exchanger then route the condensate back to the vessel, removing heat in the process. The system runs on natural circulation. The steam rises to the condenser and is condensed. The condensate is cooler than the water being heated in the vessel so it flows back into the vessel. That is the theory. In operation all that is necessary is to open valves to allow the condensate to flow back into the vessel. The standpipe of condensate is kept filled by steam which is continuously available to the condenser. The condenser is basically a water tank that boils off and is vented to atmosphere. To keep it running all that is needed is to continue to add water to the tank. Since the tank is vented this can be done by a portable pump or fire truck.

Failure modes are azlso relatively straight forward. If the valve can't be opened the system won't work. At unit 1 the system was started, but apparently was stopped over concern about exceeding a design limit on cooldown rate. Later they tried to restart the IC, but it is not clear whether it worked. Power to the valve may have failed. The valve itself may have failed or the high temperatures in containment could have caused boiling in the condensate standpipe. This would have broken the driving force for natural circulation. Other possibilities are that the tank was damaged and leaked or boiled dry removing the coolant from the heat exchanger.

I have looked at the data dump from TEPCO from the first hour after the erathquake. It is clear that the IC was initiated and stopped after about 15 minutes. Following the tsunami there was no active instrumentation readings released so it is not clear what prevented reinitiation. The concern about cooldown rate was probably a mistake since the vessel was probably already on the way to core damage due to the extended SBO. I do think the mode of failure will be easy to identify when conditions permit examining the piping and valves.

Hope this helps. I will try to post on RCIC later tonight.

 

[nikkkom]

Let me try. I will address the IC first as it is a very simple system. I will post a follow up on the more complicated possibilities with RCIC later.

The IC basically is a system to take reactor steam and condense it in a heat exchanger then route the condensate back to the vessel, removing heat in the process. The system runs on natural circulation. The steam rises to the condenser and is condensed. The condensate is cooler than the water being heated in the vessel so it flows back into the vessel. That is the theory. In operation all that is necessary is to open valves to allow the condensate to flow back into the vessel. The standpipe of condensate is kept filled by steam which is continuously available to the condenser. The condenser is basically a water tank that boils off and is vented to atmosphere. To keep it running all that is needed is to continue to add water to the tank. Since the tank is vented this can be done by a portable pump or fire truck.

Failure modes are azlso relatively straight forward. If the valve can't be opened the system won't work. At unit 1 the system was started, but apparently was stopped over concern about exceeding a design limit on cooldown rate. Later they tried to restart the IC, but it is not clear whether it worked. Power to the valve may have failed. The valve itself may have failed or the high temperatures in containment could have caused boiling in the condensate standpipe. This would have broken the driving force for natural circulation. Other possibilities are that the tank was damaged and leaked or boiled dry removing the coolant from the heat exchanger.

I have looked at the data dump from TEPCO from the first hour after the erathquake. It is clear that the IC was initiated and stopped after about 15 minutes. Following the tsunami there was no active instrumentation readings released so it is not clear what prevented reinitiation. The concern about cooldown rate was probably a mistake since the vessel was probably already on the way to core damage due to the extended SBO. I do think the mode of failure will be easy to identify when conditions permit examining the piping and valves.

Hope this helps. I will try to post on RCIC later tonight.

Excellent post. I also track (wait for) information about IC of unit 1. They stopped IC prematurely, and didn't restart it later. So far it's not known why. There may be a valid reason (such as: IC was damaged (several possible failure modes) and simply wasn't working at all) or it may have been an error, and if they had not stopped it, unit 1 could have additional ~8 hours of life. Unfortunately, in this case ultimately it didn't matter, these additional 8 hours would only delay the meltdown. It looks like we don't have a "unit 1 meltdown could have been prevented" situation here.

But in general, IC seems like an excellent meltdown prevention mechanism - provided there is a way to passively replenish or condense water which boils off.

I'm curious why some newer designs replaced IC with more complicated cooling systems such as RCIC. I just don't see why designers replaced a simple passive system with just two valves by a system with more valves, some pumps, etc. They should have _augmented_ it, not _replace_.

 

[Dmytry]

I don't know how reliable they are, but the accident analysis documents that are public tended to estimate very percentage releases, often in the 0.6%-1% range, but I believe one of their scenarios lead to a broader range of something like 1%-6%. Its been a while since I looked at these documents, but I imagine this was for reactor 2 since their other estimates also have more stuff released from this reactor.

The Chernobyl released something like 40% of the Cs-137 inventory IIRC, and had IIRC comparable inventory to Fukushima reactors. ZAMG estimated first 4 days release of 50% of the Chernobyl's Cs-137, based on CTBT sensors picking up the stuff that was blown into ocean. So, >7% give or take, assuming nothing was released from spent fuel pools. Very inaccurate of course. You can look up the inventory for those reactors and compare to ZAMG source term estimate directly. The source term estimates for Fukushima based on measurements are all very inaccurate because the wind was blowing to the ocean.

The percentage release predictions are a very sensitive subject because a high estimate is expensive for the plant operators (forces them to implement filtered emergency venting, which costs money). So as per usual they seem to just make some sort of lower bound calculation that is quite low indeed.
If the lower bound is too high, you must implement safety features, of course. Via common fallacy of confusing if x then y with if not x then not y, though, when the lower bound is not too high, no filtered emergency venting.

For the source term estimates by Japanese researchers based on couple Japanese land measurement stations , the wind was blowing to the east almost all of the time, i.e. most of the fallout never reached the sensors, i.e. whatever you base on those sensors puts a lower bound on the release. I think it's the same thing as the 55% core damage estimate (100% actual) - someone plugs numbers into software he doesn't understand, presents the lower bounds as the estimates, etc.

 

[westfield]

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.

.

Ok, I misunderstood and was thinking we were talking total depressurization .

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.

Indeed, I gather that without some sort of Residual Heat Removal System in operation RCIC will, as you point out, eventually be useless once the S\C gets so hot it cannot function as a heatsink any longer.

There must be a compelling reason to have RCIC instead of IC's because it seems a big compromise to have RCIC that relies on several other systems to remain useful versus IC's which seem so simple and "stand alone". Not that having an IC system helped Unit #1's predicament.

I would be interested to know when the operators at Unit 1 saw the cooling rate was too fast why did they not just use one IC instead of either using both or none?
The cooling rate was too fast with two IC's running but the heating rate was too fast when they disabled the IC's. Surely a happy medium might have been acheived with just one of the IC's running? So many questions.

 

[joewein]

Goes to show how much trust we can put in TEPCO's cutely-colored site contamination maps.
I hope that worker got out of there in time

Talking about "cutely-colored site contamination maps", this seems to be the most recent one available still:
http://www.tepco.co.jp/en/nu/fukushima-np/f1/images/f1-sv-20110802-e.pdf

Tepco published it on Aug 2, 2011.

At the joint U1/2 stack near unit 1 the yellow label says "70~100" (mSv/h). On a large bubble off to the left it says "U1/2 SGTS >10,000".

I think the pictures were taken towards the west, as you see the slope behind. Also, there's a small damaged two storey structure behind the stack. This is visible inside the stack frame on the west on aerial shots from the south on Cryptome.

The vertical brown-stained pipe is probably connected to one of the two smaller pipes that run along the fat pipe from the Y-section leading to units 1 and 2. There's one at each side of the fat pipe.

Ian Goddard speculates on his site that the brownish colour is not rust but cesium. However, for that the pipe would have to be leaky, for you to see cesium condensate on the outside, not just the inside.

 

[joewein]

There must be a compelling reason to have RCIC instead of IC's because it seems a big compromise to have RCIC that relies on several other systems to remain useful versus IC's which seem so simple and "stand alone". Not that having an IC system helped Unit #1's predicament.

While the IC condenses steam and returns it back into the RPV as liquid water, the RCIC pumps water into the RPV using its steam turbine, while its steam gets condensed in the wet well.

Unlike the IC, the amount of liquid water the RCIC can feed back into the RPV is not limited to the exact amount of high pressure steam it receives from the core, as it pumps from the wet well.

Where this becomes significant is when pressure in the RPV rises such that steam has to be vented from there into the wet well. With the IC alone that steam could not be replaced under station blackout conditions. Consequently, the water level in the core would have to drop. With the RCIC water in the RPV can be topped up after venting as long as the wet well has not exhausted its heat sink capacity. I think this explains why melt down occurred much later in units 2 and 3 than 1.

 

[MadderDoc]

<..>
I would be interested to know when the operators at Unit 1 saw the cooling rate was too fast why did they not just use one IC instead of either using both or none?

I reckon you'll be interested in http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110618e15.pdf", on the logs and testimony of operator response during the first days after the earthquake.

From the available evidence and testimony, the operators do in fact appear to have opted for the use of just one of the IC systems (the 'A' system) for the control of reactor pressure, judging it to be sufficient to keep the vessel at 6-7MPa, while they initially relied on the HPCI system for the control of the reactor water level.

 

[tsutsuji]

Units 1 and 2 injection rates are unstable again:

[unit 1] Water injection was arranged at approx. 3.9 ㎥/h at 9:02 am on August 5
since we observed reduction of the water injection into the reactor.
[...]
[unit 2] Water injection was arranged at approx. 3.9 ㎥/h at 5:50 pm on August 4
since we observed reduction of the water injection into the reactor.
http://www.tepco.co.jp/en/press/corp-com/release/11080501-e.html

Various troubles at the water treatment facility:

At 5:32 am on August 4, we stopped Water Treatment Facility to improve the flow rate. After the work, we activated the facility at 3:30 pm and restarted operation of the water treatment system at 4:13 pm. When we adjusted the flow rate of the system at 6:55 pm, a pump of the decontamination instruments was stopped and the whole water treatment system was shut down. We confirmed soundness of the pump and reactivated the system at 8:30 pm, and operation of the water treatment system was restarted at 8:50 pm.

-At 2:12 am on August 5, a process alarm was activated and the water treatment system was shut down. At 4:03 am, the system was reactivated and the operation was restated at 4:21 am.

-Around 7 pm on 8:04, we discovered water leakage from a flange of transfer hose of filtered water which is used to clean up salt in a vessel of the cesium adsorption instruments in the site bunker building.

http://www.tepco.co.jp/en/press/corp-com/release/11080501-e.html

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110805_02-e.pdf map showing the location of the leak at the site bunker building.

http://sankei.jp.msn.com/affairs/news/110805/dst11080512450013-n1.htm A 700 l leak was found at the water treatment facility at 7 PM on 4 August. The water leaked from a pipe connection. It did not flow outside the building. When they are removed, spent adsorption towers are cleaned with freshwater to remove salt, which is a source of corrosion. It is water from this process that leaked.

http://www.yomiuri.co.jp/science/news/20110805-OYT1T00891.htm?from=main1 Until that leak occurred, Tepco had never measured the radiation from this water. The loose safety management is brought into sharp relief again. The leak is from the hose that takes the water back to the system after washing the towers. With 6,270,000 Bq/cm³ of CS-137, it is about the same radiation level as that of the water in units 3 and 4 turbine buildings basements.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110806/0630_shiunten.html The 700 l leak rang an alarm, after which the facility was stopped for more than 2 hours. Tepco has decided to delay the test run of SARRY initially planned to be performed for 2 days starting from 6 August. It is delayed to the middle decade of August or later. The reason is that this test requires to shut down the whole facility for 2 days, but Tepco cannot afford to do this as the water level in a waste treatment facility basement reached 30 cm below the maximum on 5 August.

http://www.47news.jp/CN/201108/CN2011080601000367.html At 12:50 PM on 4 August, there was a short blackout at the earthquake-isolated building. The electricity was restored within the first minute using a backup power source. It was found that a 2.5 m deep underground cable had been harmed during an excavation work performed as part of a ground water survey preparing the construction of the ground water shielding wall. The cable was changed and the electricity from that power line was restored within 3 and a half hours after the blackout. In the future, Tepco will make sure that underground cable maps are checked carefully enough before starting excavations.

SFP1 was being thirsty:

From 3:20 pm to 5:51 pm on August 5, we injected water to Unit 1 by using Fuel Pool Cooling and Filtering System.
http://www.tepco.co.jp/en/press/corp-com/release/11080602-e.html