Fukushima Have they located the melted fuel at Fukushima?

[Kutt]

Have the TEPCO workers found the precise location of the melted fuel at the affected Fukushima NPP nuclear reactors? If not, have they at least hypothesized where it might be?

Cameras have been inserted into the reactor pressure vessel, but the footage hasn't revealed very much in terms of the integrity and location of the core...

Has it been concluded whether or not the cores burned through the steel and concrete base of the reactor building and into the Earth in a "melt-through?"

 

[zapperzero]

Cameras have been inserted into the reactor pressure vessel

No, they have not. Cameras have been inserted into the PCVs alone (and not all the PCVs at that) with less than enlightening results.
There is a gigantic dedicated thread here
https://www.physicsforums.com/showthread.php?t=480200
that you may wish to peruse

 

[Kutt]

Are you talking about this?



The footage did not reveal the location of any of the melted fuel or core material.

 

[Astronuc]

Are you talking about this?



The footage did not reveal the location of any of the melted fuel or core material.

The video was uploaded on Jan 20, 2012 according to that page, so it is very old. Note the white noise in the video. This is attributed to the high radiation levels in the vicinity of the CCD in the camera.

 

[Kutt]

The video was uploaded on Jan 20, 2012 according to that page, so it is very old. Note the white noise in the video. This is attributed to the high radiation levels in the vicinity of the CCD in the camera.

Have they inserted a camera into the reactor pressure vessel itself yet?

 

[Astronuc]

Have they inserted a camera into the reactor pressure vessel itself yet?

Not yet in the RPV, or underneath it (I'm assuming that if Tepco has, they would share that information). It will be a BIG story when Tepco finally looks at the damaged core and fuel.

As far as I know, they have lowered cameras to the torus of one or more units.

Tepco is busily building a structure over unit 4 that will enable them to remove the fuel from the spent fuel pool.

 

[etudiant]

It seems to be a pretty peripheral issue.
The workers on the site are tweeting that the job will take decades.
Tepco is currently working on clearing the decks, removing spent fuel, enclosing the damaged reactors and dealing with issues such as the disposal of the decontaminated water.
It is not clear what knowledge of the melted fuel's status would add. There is no way to deal with it as yet.

 

[Astronuc]

It seems to be a pretty peripheral issue.
The workers on the site are tweeting that the job will take decades.
Tepco is currently working on clearing the decks, removing spent fuel, enclosing the damaged reactors and dealing with issues such as the disposal of the decontaminated water.
It is not clear what knowledge of the melted fuel's status would add. There is no way to deal with it as yet.

They will likely end up like TMI-2, which still has contaminated water in containment and is sealed off.

Aug. 1993 At TMI-2, the processing of accident-generated water was completed involving 2.23 million gallons. Accident was March 28, 1979. I was there during the early 90s for a project at TMI-1, and as IIRC, the water was still in containment of Unit 2.

Sept. 1993 NRC issued a possession-only license.

Dec. 1993 Monitored storage began.

Ref: http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html

Twenty years later, I expect it's still in monitored storage.

In 2010, the generator from TMI-2 was sold by FirstEnergy to Progress Energy for an upgrade at Shearon Harris.
http://www.world-nuclear.org/info/inf36.html

 

[etudiant]

They will likely end up like TMI-2, which still has contaminated water in containment and is sealed off.

Aug. 1993 At TMI-2, the processing of accident-generated water was completed involving 2.23 million gallons. Accident was March 28, 1979. I was there during the early 90s for a project at TMI-1, and as IIRC, the water was still in containment of Unit 2.

Sept. 1993 NRC issued a possession-only license.

Dec. 1993 Monitored storage began.

Ref: http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html

Twenty years later, I expect it's still in monitored storage.

In 2010, the generator from TMI-2 was sold by FirstEnergy to Progress Energy for an upgrade at Shearon Harris.
http://www.world-nuclear.org/info/inf36.html

Fascinating and vaguely disquieting.
I have no idea what the 'monitored storage' amounts to in practice.
Is it that a guy checks for drips once a year or is it something more substantial?
In a prior life in the aerospace industry, I did not get a good impression of government monitored storage, but maybe the nuclear industry is different.

 

[Kutt]

They will likely end up like TMI-2, which still has contaminated water in containment and is sealed off.

Aug. 1993 At TMI-2, the processing of accident-generated water was completed involving 2.23 million gallons. Accident was March 28, 1979. I was there during the early 90s for a project at TMI-1, and as IIRC, the water was still in containment of Unit 2.

Sept. 1993 NRC issued a possession-only license.

Dec. 1993 Monitored storage began.

Ref: http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html

Twenty years later, I expect it's still in monitored storage.

In 2010, the generator from TMI-2 was sold by FirstEnergy to Progress Energy for an upgrade at Shearon Harris.
http://www.world-nuclear.org/info/inf36.html

I thought that TMI reactor #2 was removed and replaced with a working reactor?

 

[Astronuc]

I thought that TMI reactor #2 was removed and replaced with a working reactor?

No. TMI-2 still exists in a condition known as 'post-defueled, monitored storage (PDMS). The older sibling unit continues to operate.

TMI 2 Placed in Monitored Storage

After cleaning up the damaged TMI 2 reactor, GPU Nuclear placed the plant in monitored storage in December 1993. In December 1999, GPU sold TMI 1 to AmerGen Energy Co., a joint venture of Exelon and British Energy Co. British Energy subsequently sold its interest in TMI 1 to Exelon. In 2008, AmerGen Energy Co. was integrated into Exelon Generation, and the AmerGen legal entity was dissolved.

Under the terms of the sale, GPU retained ownership of TMI 2. GPU subsequently merged with FirstEnergy, making First Energy financially responsible for the decommissioning of TMI 2. In-plant and off-site monitoring of TMI 2 will continue until it is fully decommissioned, with regular reports made to the U.S. Nuclear Regulatory Commission, the commonwealth of Pennsylvania and the public.

The two reactors will be decommissioned jointly when TMI 1 is taken out of service.

Ref: http://www.nei.org/filefolder/TMI_2_Accident_Aug_2010.pdf

TMI-1's license has been renewed for 20 years and will expire 04/19/2034.
http://www.nrc.gov/info-finder/reactor/tmi1.html

If TEPCO has keeped the generators and turbines in good condition, they could in theory be sold for other generation and the utility could recover some cost. However, maintaining a large turbine means that they have to keep the shaft rotating otherwise it will deform under its own weight. A warped shaft is scrap.

 

[nikkkom]

They will likely end up like TMI-2, which still has contaminated water in containment and is sealed off.

Aug. 1993 At TMI-2, the processing of accident-generated water was completed involving 2.23 million gallons. Accident was March 28, 1979. I was there during the early 90s for a project at TMI-1, and as IIRC, the water was still in containment of Unit 2.

Why not all water was pumped out?

 

[nikkkom]

However, maintaining a large turbine means that they have to keep the shaft rotating otherwise it will deform under its own weight. A warped shaft is scrap.

Due to these "anti-economy-of-scale" effects, why do power plants opt for using one huge turbine instead of a few smaller ones?

 

[jim hardy]

As Astronuc said the turbine must be rotated else the shaft will warp. That's because of uneven temperature in the casing as it cools down.
To that end there's a "turning gear" motor that rotates it very slowly. We had a backup DC turning gear motor in case of station blackout, and a place for a handcrank.

Once it's reached ambient temperature you can stop it.
Here's a photo of a small one apart for maintenance.
http://www.biztrademarket.com/User/8794/bb/200773014471292994.JPG
picture courtesy these folks.. http://www.biztrademarket.com/User/8794/bb/200773014471292994.JPG

and a bigger one from wikipedia http://en.wikipedia.org/wiki/Steam_turbine.

Due to these "anti-economy-of-scale" effects, why do power plants opt for using one huge turbine instead of a few smaller ones?

It takes no more people to operate a large one than a small one.
And as Lindbergh observed when choosing a single engine airplane to cross the Atlanic,
with just one there's fewer things to go wrong.

old jim

 

[Astronuc]

Due to these "anti-economy-of-scale" effects, why do power plants opt for using one huge turbine instead of a few smaller ones?

I only know of one PWR that has twin turbine trains, Sizewell B in the UK.

So that orders could be given to UK manufacturers, and to avoid project risk in dealing with what were at the time newly designed very large turbo-alternator sets, Sizewell B uses two full-speed, 3,000 RPM (50 Hz), nominal 660 MW turbo-alternator sets . . .

http://en.wikipedia.org/wiki/Sizewell_nuclear_power_stations#Design_2

Sizewell B is similar in design to Wolfcreek and Callaway units in the US, except, like US plants, they have one turbine set.

Why not all water was pumped out?

I don't know. I'll have to do some investigating.

 

[Kutt]

So TMI has two reactors but only one of them works?

I thought that the damaged reactor #2 had been completely removed and replaced with a working one.

Umm.. I assume that the energy production of the plant is halved?

 

[nikkkom]

It takes no more people to operate a large one than a small one.

Sure, I understand the basic idea of economies of scale.

However, scaling up things tends to bump into various obstacles at some point.

If you go from 1 ton to 2 ton piece of machinery, it's usually not a big deal, but when you go from 20 tons to 40 tons it sometimes is.

Just off the top of my head:

* larger objects are not road-transportable
* very heavy objects need specialized cranes
* disassembly and repair work becomes harder, because even individual parts need lifting equipment, they can't be handled just by hands.

So, why bother and torture yourself with one humongous turbine instead of having two smaller, but still quite large ones?

Also, this gives redundancy.

 

[a.ua.]

Reactor RBMK has 2 turbines.

 

[Kutt]

Sure, I understand the basic idea of economies of scale.

However, scaling up things tends to bump into various obstacles at some point.

If you go from 1 ton to 2 ton piece of machinery, it's usually not a big deal, but when you go from 20 tons to 40 tons it sometimes is.

Just off the top of my head:

* larger objects are not road-transportable
* very heavy objects need specialized cranes
* disassembly and repair work becomes harder, because even individual parts need lifting equipment, they can't be handled just by hands.

So, why bother and torture yourself with one humongous turbine instead of having two smaller, but still quite large ones?

Also, this gives redundancy.

You're right, having multiple smaller turbines is probably better than just one giant one for the reasons you stated.

 

[Astronuc]

Sure, I understand the basic idea of economies of scale.

However, scaling up things tends to bump into various obstacles at some point.

If you go from 1 ton to 2 ton piece of machinery, it's usually not a big deal, but when you go from 20 tons to 40 tons it sometimes is.

Just off the top of my head:

* larger objects are not road-transportable
* very heavy objects need specialized cranes
* disassembly and repair work becomes harder, because even individual parts need lifting equipment, they can't be handled just by hands.

So, why bother and torture yourself with one humongous turbine instead of having two smaller, but still quite large ones?

Also, this gives redundancy.

Large equipment is transportable by road. That's usually how it gets to (of from) the plant. Each steam generator at San Onofre was about ~400 tons, ~65 feet in height and about 17 feet at maximum width.
http://www.ocregister.com/articles/san-376670-onofre-generators.html
http://www.huffingtonpost.com/2012/11/05/san-onofre-nuclear-generator_n_2077732.html
(turn down the volume and ignore that advertisements)

Two 600 MWe turbines still need specialized cranes/equipment, and each turbine rotor and the casings cannot be lifted by hand. Most people cannot lift and carry an object of their body weight very well. There are usually limits on what people lift, <25 kg.

 

[jim hardy]

Sure, I understand the basic idea of economies of scale.

However, scaling up things tends to bump into various obstacles at some point.

If you go from 1 ton to 2 ton piece of machinery, it's usually not a big deal, but when you go from 20 tons to 40 tons it sometimes is.


So, why bother and torture yourself with one humongous turbine instead of having two smaller, but still quite large ones?

Also, this gives redundancy.

Well,, our generator stator weighed about 392 tons.
It arrived in town by rail the first time, and a special hundred wheeled trailer was supposed to haul it the last ten miles.
But on that road out to the plant the Earth beneath the pavement squished away and the generator tumbled into the swamp. So next time they barged it right to the plant.
Would a 200 ton generator on fifty wheels have squished the road? Anybody's guess...

It's really no more trouble to lift a 400 ton piece than a 200 ton piece just the crane is slightly bigger.

I think economy of scale applies - it's twice the complexity and twice the labor cost to build and maintain two half size machines instead of one full size one.
Dont forget the auxilliaries - a steam turbine needs a condenser, lubrication system, feedwater heaters, pipes, pumps , valves, etc.

Heed Thoreau - 'Simplify, Simplify"...

 

[LabratSR]

To the original question,

According to this report, TEPCO stuck a camera into the PCV "near" the pedestal room of Unit one but did not see anything that appeared to be corium.

From the summary of the report.

"Recently, within the October 2012 timeframe, TEPCO was able to insert a camera along with instrumentation through a penetration into the Unit 1 PCV [83]. Video within containment was obtained; however, the information has not been fully scrutinized and interpreted as of this report. The camera was able to view a small portion of the drywell floor [84] in a drywell location approximately 180 degrees opposite from the pedestal doorway. Core melt did not appear to be present in this view. Future analysis and data collection as to the debris location will provide insight into the accident progression."

I posted this in the big thread but it seems to fit here.

Enhanced Ex-Vessel Analysis for Fukushima Daiichi Unit 1:
Melt Spreading and Core-Concrete Interaction Analyses with MELTSPREAD and CORQUENCH

https://fukushima.inl.gov/PDF/MELTSPREAD%20CORQUENCH%20Analysis%201F1%20ORNL_ANL%20Feb2013.pdf

I went back and looked and found these associated reports.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_121015_05-e.pdf

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_121015_04-e.pdf

 

[Kutt]

Have they concluded whether or not the cores have burned their way through the concrete base of the reactor building and into the Earth beneath it?

 

[zapperzero]

Have they concluded whether or not the cores have burned their way through the concrete base of the reactor building and into the Earth beneath it?

There is no conclusion and there won't be for a long time. The modeling runs so far point to the fuel having eaten just a little into the basemat and stabilized there. Muon radiography is being considered

https://www.lanl.gov/newsroom/news-releases/2012/October/10.17-fukushimas-nuclear-scar.php

 

[nikkkom]

Have they concluded whether or not the cores have burned their way through the concrete base of the reactor building and into the Earth beneath it?

Thankfully, concrete base is about 10 meters thick.

Models so far say that corium almost reached the containment bottom (the light-bulb shaped thing), and if they are wrong, it may indeed reached it, but there are 7.5 more meters of concrete below it.

 

[Kutt]

How come giant containers at a steel mill which contain hundreds of tons worth of superheated white-hot molten steel do not have that molten mass burn through it's base? While a nuclear reactor pressure vessel cannot physically contain it's core if it melts?

 

[russ_watters]

How come giant containers at a steel mill which contain hundreds of tons worth of superheated white-hot molten steel do not have that molten mass burn through it's base? While a nuclear reactor pressure vessel cannot physically contain it's core if it melts?

Perhaps look up the melting temperature of steel vs the melting temperature of the container holding it?

 

[Kutt]

Perhaps look up the melting temperature of steel vs the melting temperature of the container holding it?

Why can't reactor vessels be made of a material that has a melting temperature greater than the temperature of molten corium?

 

[nikkkom]

Why can't reactor vessels be made of a material that has a melting temperature greater than the temperature of molten corium?

Corium has no defined temperature. It heats up due to decay heat, and without sufficient cooling will become hotter and hotter until it melts the vessel.

Tougher vessels theoretically can be built (say, using vanadium, molybdenum alloys and such), but they will cost astronomical sums and still won't be 100% safe wrt meltdown.

Safety versus meltdown can be achieved only by designing in very robust emergency cooling systems. In my "armchair engineer" view, something like "reactor sitting in a stainless steel lined pit with no drains, and with a set of large tanks beside it which can be manually drained into the pit, no electricity needed" should work.

 

[zapperzero]

with a set of large tanks beside it which can be manually drained into the pit, no electricity needed" should work.

Large tanks of what, please?

 

[nikkkom]

Large tanks of what, please?

Water.

 

[jim hardy]

Safety versus meltdown can be achieved only by designing in very robust emergency cooling systems. In my "armchair engineer" view, something like "reactor sitting in a stainless steel lined pit with no drains, and with a set of large tanks beside it which can be manually drained into the pit, no electricity needed" should work.

You just described PWR "Acumulators".

 

[Kutt]

There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.

More water can be brought in via truck or helicopter if needed.

Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

The roads leading to the Fukushima Daiichi NPP were blocked by debris from the Tsunami, making this impossible.

 

[nikkkom]

You just described PWR "Acumulators".

No. Accumulators are high pressure tanks (nitrogen pressurized) with water which are meant to inject this water into primary coolant loop.

They require RPV to be depressurized. As you know, in Fukushima depressurizing RPV and PCV proved difficult. Fail.

What I described are tanks which can, if all else fails, flood reactor pit and submerge the reactor, i.e. cool reactor from the outside.

 

[nikkkom]

Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

There should be enough fuel for weeks. And fuel can be delivered, by air if needed. But it's not of much use if your diesels or electrical switchboards are flooded, right?...

 

[jim hardy]

What I described are tanks which can, if all else fails, flood reactor pit and submerge the reactor, i.e. cool reactor from the outside.

Fair enough. You're suggesting a passive containment flood system.
Ours was active not passive and used pumps. It was intended to reduce containment pressure, it fed spray nozzles in upper containment near ceiling ..

Our accumulators were pressurized to about 1/3 reactor operating pressure so they'd flood vessel while pressure is on the way down following a break.

Fukushima operators might have been able to depressurize had they done it very early. It is suggested in the ORNL station blackout study to do that.
Hindsight is always 20/20........

we kept 30 days diesel fuel onsite. Yes, it was trucked in.

 

[Hiddencamper]

There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.

More water can be brought in via truck or helicopter if needed.

Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

The roads leading to the Fukushima Daiichi NPP were blocked by debris from the Tsunami, making this impossible.

Unit 2 and 3 have a RCIC system which uses reactor steam to pump water into the core. Unit 2 was cooled for 70 hours and unit 3 for 32 hours.

Unit 1 has a passive isolation condenser system, but operators were not properly trained on it and did not know if it was functioning properly. It likely wasn't

 

[Hiddencamper]

Fair enough. You're suggesting a passive containment flood system.
Ours was active not passive and used pumps. It was intended to reduce containment pressure, it fed spray nozzles in upper containment near ceiling ..

Our accumulators were pressurized to about 1/3 reactor operating pressure so they'd flood vessel while pressure is on the way down following a break.

Fukushima operators might have been able to depressurize had they done it very early. It is suggested in the ORNL station blackout study to do that.
Hindsight is always 20/20...


.....

we kept 30 days diesel fuel onsite. Yes, it was trucked in.

Per the ANS standard, 7 days of fuel PER GENERATOR OR the minimum amount of time required to get a resupply, whichever is greater, is the requirement. (I forget the US reg guide that endorses this).

With regards to depressurizing, you actually DONT want to depressurize the RPV in a Fukushima type situation. BWRs can utilize high pressure reactor steam to operate their RCIC, IC, or HPCI systems for extended core injection. While these systems do not provide for decay heat removal, you can achieve that function through early containment venting and portable pump injection. If you have no low pressure coolant injection pumps available, once you depressurize, you lower the water level remaining in the core (generally uncovering the fuel), and you lose ALL injection. Depressurization early in the event is good during short term station blackout, as depressurization gives you about 15-20 minutes of steam cooling after uncovery of the core and buys you some extra time to get portable pumps going, but ultimately containment venting and portable pump injection to the RPV and/or containment are REQUIRED to assure critical safety functions can be restored.

I've been involved with Fukushima responses for BWRs if anyone is interested on how the plants actually respond, and what we are doing and changing to respond to Fukushima-like situations.

 

[jim hardy]

As I've said i never spent any time around BWR's , just walked through one once.

I went by this report, which is an analysis and re-thinking of station blackout scenarios done around 1981. They used Brown's Ferry's design as their case study. I stumbled across it while following events at Fukushima on another forum. It's been posted here at PF back in 2011.
http://www.ornl.gov/info/reports/1981/3445600211884.pdf

The analysis shows, because of the loss of the drywell coolers, that it is necessary
for the operator to begin to reduce the reactor vessel pressure to about
0.791 MPa (100 psig) within one hour of the inception of the Station
Blackout. This depressurization reduces the temperature of the saturated
fluid within the reactor vessel and thereby decreases the driving potential
for heat transfer into the drywell, yet keeps the vessel pressure
high enough for continued operation of the RCIC system steam turbine.
With this action, the drywell average ambient temperature can be kept be
low 149°C (300°F) throughout the initial phase of a Station Blackout;
tests have shown that both the drywell structure and the equipment located
therein can be expected to survive temperatures of this magnitude.
The analysis also reveals an important second reason for operator action to
depressurize the reactor vesel early in the initial phase of a
Station Blackout. This depressurization removes a great deal of steam and
the associated stored energy from the reactor vessel at a time when the
RCIC system is available to inject replacement water from the condensate
storage tank and thereby maintain the reactor vessel level. Subsequently,
when water injection capability is lost for any reason, remote-manual re
lief valve operation would be terminated and there would be no further
water loss from the reactor vessel until the pressure has been restored to
the setpoint [7.72 MPa (1105 psig)] for automatic relief valve actuation.
Because of the large amount of water to be reheated and the reduced level
of decay heat, this repressurization would require a significant period of
time. In addition, the subsequent boiloff* would begin from a very high

vessel level because of the increase in the specific volume of the water
as it is heated and repressurized. Thus, an early depressurization will
provide a significant period of valuable additional time for preparative
and possible corrective action before core uncovery after injection capability is lost.

**The term "boiloff" is used to signify a monotonic decrease in reactor vessel water level due to intermittent loss of fluid through the
primary relief valves without replacement.
vxx

I'm not a BWR guy.
If you are one, you are certainly more versed than i in their station blackout approach .


old jim

 

[Hiddencamper]

As I've said i never spent any time around BWR's , just walked through one once.

I went by this report, which is an analysis and re-thinking of station blackout scenarios done around 1981. They used Brown's Ferry's design as their case study. I stumbled across it while following events at Fukushima on another forum. It's been posted here at PF back in 2011.
http://www.ornl.gov/info/reports/1981/3445600211884.pdf
I'm not a BWR guy.
If you are one, you are certainly more versed than i in their station blackout approach . old jim

First off, amazing reference. I'm always looking for gems like that to add to my little collection.

The problem is the station blackout scenario is only a 4~8 hour scenario. I fully agree with their approach of keeping RCIC online, as you can bypass RCIC interlocks and run it down to about 45 psig, and by keeping the reactor close to depressurized you have the ability to immediately transition to shutdown cooling the moment it becomes available again (typical SDC interlocks are around 145 psig).

Another quick note about the normal 4-8 hour station blackout scenario. The acceptance criteria is power being restored in that 4 or 8 hour timeframe, the reactor being brought to cold shutdown, and the suppression pool temperature not exceeding some arbitrary limit (in the range of 175F +/- 15). The other main reason you want to keep vessel pressure low, is that as the suppression pool heats up, you have less capability for the pool to absorb the energy during a reactor blowdown, which can result in containment failure if a LOCA occurred during the event. By keeping pressure low, you can have much higher suppression pool temperature limits, and also prevent containment failure in the event a blowdown or LOCA occurs towards the end of the event. (note: Mark 3 and possibly mark 2 containments don't have an issue here, as it is my understanding that they can survive the entire SBO coping time without violating their suppression pool temperature limits)

The issue though, is the relief valves for BWRs utilize air accumulators and DC solenoids. The total air accumulator supply for all SRVs is usually enough for about 30 lifts against design containment pressure. There are backup air bottles, however you can always have a situation where these are unable to restore pressure to the instrument air system due to damage (it is non-safety grade for some obscure reason), or worse, you don't have DC/AC power in the right places to open the valves and bypass the interlocks on the instrument air system.

If you are in an extended situation where you do not have DC power (Fukushima had no DC power at the start of the event in units 1 and 2, and limited in 3), or if you have to do more than a few dozen SRV lifts and deplete your accumulators, you will lose the ability to use SRVs to control vessel pressure. This is where the extended scenario departs from the normal station blackout scenario. The SRVs are manual staged valves, and are either open or shut. To maintain vessel pressure at 100 PSI requires operators to be opening and closing these valves manually, depleting the air supply in them and exhausting your DC power supply.

Further complicating the event is the fact that RCIC's operation is dependent on the suppression pool as a heat sink. If you are in an extended situation, and you blowdown early, you introduce a large volume of heat to the pool, and limit how long RCIC can operate. RCIC is self cooled, and once suppression pool water passes 200F you start to wear the bearings and can fail the pump. Furthermore, as vessel pressure decreases, and drywell/containment pressure increases, you get less dP over the RCIC turbine which results in less injection to the vessel. I should note that heatup of the suppression pool was likely one of the direct causes of the RCIC system failing at Fukushima unit 2 (ran for 70 hours).

The strategy the plant I'm at is taking, is recognizing the extended blackout event, stripping all non-essential loads and shutting down a full safety division of DC power, cross tieing the various DC power busses to the RCIC system to ensure continued injection and automatic flow control, keeping the vessel pressurized. We will actually declare 10CFR50.54(x) to violate our HCTL (heat capacity temperature limit) on the suppression pool in order to keep utilizing RCIC and keep injecting to the core. Once we either restore power, or get portable pumps going, we will cool the suppression pool via "feed and bleed". Once the pool is cooled below the HCTL again, we will do a reactor blowdown by taking portable batteries and directly wiring them up to the SRV terminal blocks and transition to cooling the reactor using portable pumps using the suppression pool -> reactor -> relief valves -> suppression pool loop (alternate shutdown cooling), and either feed and bleed or portable pump setups to cool the suppression pool.

The scenario changes if you are at an isolation condenser plant like dresden or oyster (it changes greatly at oyster as they don't even have a HPCI...ONLY IC and LP corespray). ICs buy you more time, and allow use of engine driven pumps, provided you can get it started properly and identify its function (one of the Fukushima unit 1 flaws, operators were not well trained on the system). Oyster, provided their ICs are in service and portable pumps or the on-site diesel driven pumps are available, is probably in the best shape for extended SBO, as you are essentially injecting against atmosphere and can keep cooling as long as there is enough decay heat for natural circulation (likely days).

 

[Kutt]

Dang, I forgot that the Fukushima reactors had to be depressurized before any water could to be injected. The safety relief valves located inside the PCV were stuck shut and could not be operated because of the crushing pressure (thousands of PSI) inside the primary containment vessel pushing against the exterior of the safety relief valves.

https://controls.engin.umich.edu/wiki/images/8/8e/Spring_loaded_safety_relief.jpg

 

[Hiddencamper]

Dang, I forgot that the Fukushima reactors had to be depressurized before any water could to be injected. The safety relief valves located inside the PCV were stuck shut and could not be operated because of the crushing pressure (thousands of PSI) inside the primary containment vessel pushing against the exterior of the safety relief valves.

Containment pressure in the Mark I containment system only goes up to about 125-140 PSI. Around that point you either have your rupture disks blow (for the Japanese containment systems), or your penetrations and seals start to fail, causing pressure leakoff (likely one mode through which hydrogen migrated into the secondary containment building).

And not all of the relief valves issues were due to containment differential pressure. Between loss of DC power allowing solenoids to go back to the closed position during the event, some unique behaviors of the SRVs in general, and difficulties establishing portable air supplies to the SRVs, there were a number of reasons the SRVs had trouble.

The SRVs are designed to lift against the design containment pressure several times without being recharged, and should be capable of doing at least 1-2 lifts against double containment design pressure (which is what the Fukushima containments were getting up to). Leakage of the system, constant use of the SRVs, and inability to restore the Instrument Air Supply to recharge the SRVs was really what prevented the valves from opening/staying open. A higher containment pressure does require higher accumulator pressure to lift the SRVs, but it doesn't prevent the SRVs from lifting.

And a final note, this isn't just the "Fukushima reactors", of which you are referring to the BWR series of reactors, but this is an attribute of virtually all light water commercial power reactors, including PWRs.

 

[Kutt]

Containment pressure in the Mark I containment system only goes up to about 125-140 PSI. Around that point you either have your rupture disks blow (for the Japanese containment systems), or your penetrations and seals start to fail, causing pressure leakoff (likely one mode through which hydrogen migrated into the secondary containment building).

And not all of the relief valves issues were due to containment differential pressure. Between loss of DC power allowing solenoids to go back to the closed position during the event, some unique behaviors of the SRVs in general, and difficulties establishing portable air supplies to the SRVs, there were a number of reasons the SRVs had trouble.

The SRVs are designed to lift against the design containment pressure several times without being recharged, and should be capable of doing at least 1-2 lifts against double containment design pressure (which is what the Fukushima containments were getting up to). Leakage of the system, constant use of the SRVs, and inability to restore the Instrument Air Supply to recharge the SRVs was really what prevented the valves from opening/staying open. A higher containment pressure does require higher accumulator pressure to lift the SRVs, but it doesn't prevent the SRVs from lifting.

And a final note, this isn't just the "Fukushima reactors", of which you are referring to the BWR series of reactors, but this is an attribute of virtually all light water commercial power reactors, including PWRs.

The pipework which supplied emergency pressurized air to the SRV's were severely damaged in the earthquake and rendered useless.

 

[russ_watters]

There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.

Perhaps you should try calculating how many thousands of people cranking by hand would be required to pump enough water to cool a nuclear reactor...

 

[Kutt]

Perhaps you should try calculating how many thousands of people cranking by hand would be required to pump enough water to cool a nuclear reactor...

What about a massive water tank that feeds water into the reactor (using gravity) which can activated by turning some valves?

 

[Hiddencamper]

The pipework which supplied emergency pressurized air to the SRV's were severely damaged in the earthquake and rendered useless.

I assume we are talking about the instrument air system. Do you have a link to this, I do not recall seeing any absolute data suggesting this was the case, especially because SRVs were used in units 2 and 3 to support blowdown and transition to portable pumping equipment, which implies the IA system was not damaged.

Some info about IA, it is a non-safety system. It automatically isolates (shuts/locks out) on loss of power, and requires specific manual interlock overrides in order to restore it. I'm not sure if the IA system actually failed or if there were more complicated events and I would appreciate a source. Thanks.

 

[Hiddencamper]

What about a massive water tank that feeds water into the reactor (using gravity) which can activated by turning some valves?

Basically the AP1000 or ESBWR designs. Both of those plants will automatically depressurize and utilize gravity type feeds for reactor cooling.

 

[jim hardy]

Kutt you might enjoy these links

http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

http://pbadupws.nrc.gov/docs/ML0228/ML022830867.pdf


.................

Camper here's a couple more for your collection.. from that 2011 thread that was so long..

http://www.osti.gov/bridge/servlets/purl/6980202-feK1wp/6980202.pdf

http://www.osti.gov/bridge/servlets/purl/6124656-R8y05j/6124656.pdf

http://www.osti.gov/bridge/servlets/purl/6124656-R8y05j/6124656.pdf

http://pbadupws.nrc.gov/docs/ML0210/ML021080117.pdf

there's some higher resolution photos available here than I've seen before..
http://photo.tepco.co.jp/en/date/2013/201302-e/130201-01e.html

 

[Kutt]

I assume we are talking about the instrument air system. Do you have a link to this, I do not recall seeing any absolute data suggesting this was the case, especially because SRVs were used in units 2 and 3 to support blowdown and transition to portable pumping equipment, which implies the IA system was not damaged.

Some info about IA, it is a non-safety system. It automatically isolates (shuts/locks out) on loss of power, and requires specific manual interlock overrides in order to restore it. I'm not sure if the IA system actually failed or if there were more complicated events and I would appreciate a source. Thanks.

I remember seeing a two-hour long docu-drama about the Fukushima incident with actors re-creating the events. It also went into technical detail to how the pipework which supplied the SRV's with enough pressurized air to open, were damaged and rendered inoperable by the earthquake.

I watched this documentary on youtube, but so far, I haven't been able to find it again.

I'll keep looking.

 

[Hiddencamper]

I remember seeing a two-hour long docu-drama about the Fukushima incident with actors re-creating the events. It also went into technical detail to how the pipework which supplied the SRV's with enough pressurized air to open, were damaged and rendered inoperable by the earthquake.

I watched this documentary on youtube, but so far, I haven't been able to find it again.

I'll keep looking.

I wouldn't trust a youtube documentary.

Looking at the official report

http://www.cas.go.jp/jp/seisaku/icanps/eng/02Attachment1.pdf

it looks like loss of power caused an isolation of the instrument air system, which was not recoverable due to AC power being lost, and that the accumulators were depleted.

And there were two types of accumulators for the SRV, one for the relief valve (85L) and the other for the ADS (250L). Both of them were installed inside the PCV. Normally, nitrogen for the relief valve accumulator was supplied from the liquid nitrogen system (AC) line, and nitrogen for the ADS accumulator was supplied from the nitrogen cylinder. The pressure gauge set at each supply line was scheduled to be checked daily by the shift team who were on duty between 15:00 and 21:00. However, because the isolation valve of the supply line to the accumulator had closed due to external power loss after the tsunami, the supply to the accumulator had not been carried out as they were unable to do so. For this reason, the residual pressure that allowed the pressure to be applied to the SRV was the relief valve accumulator, ADS accumulator, and pressure remaining in the piping between the isolation valve (which had closed) and the accumulator side

In other words, the earthquake did NOT cause the recharging system to fail. Instead, the inability to open the isolation valves caused the issue. Even if the system did fail, it is not safety-grade and is not expected or required to operate post accident (although it probably should be).