Cold shutdown that doesn't require coolant circulation?

  • #26
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The SWR1000 (later renamed Kerena) has (had?) an IC inside the containment and without any valves, activated by reactor level drop.
 
  • #27
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The premise of this thread was whether there are designs that can survive an extended time period without power or generators whether recently scrambled or simply shut down and whether it is even possible or feasible.

Let's call that period of time 3 months without any factors other than not having electricity to run pumps or fuel supply to run generators.
Generation 3+ plants (AP1000, ESBWR) get 72 hours with no operator action or AC power. AP1000 in particular gets 1 week with a diesel engine pump and some battery power (simply to fire squib valve charges for depressurization) in the first 72 hours.

Generator 4 plants should be weeks or months or indefinite w/out electrical power.

No gen 2 or early gen 3 design can go more than a 1/2 a day without active cooling. Conservative analysis show that uncovery happens in 3-6 hours, fuel damage in another 2, vessel in another 2-4, containment in another 4-8, in generally near worst case conditions.
 
  • #28


The premise of this thread was whether there are designs that can survive an extended time period without power or generators whether recently scrambled or simply shut down and whether it is even possible or feasible.
Re feasibility: it is certainly feasible. Isolation condenser design can be augmented to provide much longer, potentially unlimited, passive cooling.

Remember that decay heat power does drop off. In one day after scram, it drops to ~0.5% of full power. That will be 15 MWt for a 3 GWt plant.

The passive cooling system can be designed to sacrificially (e.g. boiling water in IC) absorb the initial high decay heat power, and to be able to dump the prolonged, but lower-power heat output _without_ the need to consume water: 15 megawatts thermal can be dissipated by passive air cooling.

A passive air cooler of this size would be a rather large device, though. I'm afraid the obstacle is that NPP industry doesn't want to spend money on it, since it will stand idle.
 
  • #29


Another note: it should be possible to rig a quite simple "poor man's passive cooling system" in a BWR reactor. Before fuel rods start to crack and melt, depressurize RPV to 1 atm and keep it that way, then gravity-feed it with fresh water, letting water boil in the RPV itself.

This requires some designing: gravity-fed water must be available. It, and steam dump must be possible to activate w/o AC or DC. Otherwise, the system won't be fully passive.

This method of cooling would release some radioactive steam (unless initial steam dump is quenched), but again, compared to what actually happened at Fukushima, it'd be a *much* milder accident.
 
  • #30
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Another note: it should be possible to rig a quite simple "poor man's passive cooling system" in a BWR reactor. Before fuel rods start to crack and melt, depressurize RPV to 1 atm and keep it that way, then gravity-feed it with fresh water, letting water boil in the RPV itself.

This requires some designing: gravity-fed water must be available. It, and steam dump must be possible to activate w/o AC or DC. Otherwise, the system won't be fully passive.

This method of cooling would release some radioactive steam (unless initial steam dump is quenched), but again, compared to what actually happened at Fukushima, it'd be a *much* milder accident.
This model would also require that the containment be opened/vented, as you need a way to remove that pressure from the containment.

The issue is there really isn't a way to gravity feed in a BWR. (not counting the ESBWR, but none of those have been built or even purchased yet). Just about all lines into the vessel go up from a lower elevation (the lines are all going down to lower elevations to help with NPSH on the pump suctions).
 
  • #31
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Re feasibility: it is certainly feasible. Isolation condenser design can be augmented to provide much longer, potentially unlimited, passive cooling.

Remember that decay heat power does drop off. In one day after scram, it drops to ~0.5% of full power. That will be 15 MWt for a 3 GWt plant.

The passive cooling system can be designed to sacrificially (e.g. boiling water in IC) absorb the initial high decay heat power, and to be able to dump the prolonged, but lower-power heat output _without_ the need to consume water: 15 megawatts thermal can be dissipated by passive air cooling.

A passive air cooler of this size would be a rather large device, though. I'm afraid the obstacle is that NPP industry doesn't want to spend money on it, since it will stand idle.
Well remember, if I build a passive air cooler, I have the following engineering challenges:

I need to to not be in service when the plant is normally operating, but I need it to come in service when called upon under all conditions (requires DC power for squib valves or MOVs).

If the reactor is not boiling (natural circulation), I need the water level above the steam dryer (at least at the separator skirt), and enough decay heat to drive natural convection.

I need to have penetrations and piping which allow reactor coolant to go outside the containment to be cooled (in a situation where I potentially dont have power to close those valves upon a pipe rupture).

I need the structure (as it is safety related), to be protected from all external hazards, so it needs to have a shield structure around it. But that same shield structure is going to impinge upon air flow. Because of this I need natural chimney effect and other means to promote cooling.

It needs to be very large, but it also needs to be seismically qualified.

I need at least 2 of them for redundancy.

Ideally, they should be able to handle a wide range of events as not to be cost excessive (which was your reason why the industry didnt want to do it).

When you look at all of the above, the AP1000 design makes a lot of sense, but it also sheds some insights on why you can't apply this to a BWR design easily, and why you still need evaporation flow for an extended period of time for the AP1000 design. The biggest thing is the fact that you would need something outside of containment (again AP1000 being the exception as the heat transfer surface IS the containment), but the consequences of a line break would be very severe outside of containment.

I'm just throwing some thoughts out there. I agree it's physically possible and feasible, and it is an even better idea for molten salt or high temperature reactors where you can get much more bang for your buck in terms of heat transfer, but it is a very difficult thing to build and justify when you have things which already provide more than the necessary amount of safety which can also perform other functions. AP1000's PCS is a containment and a decay heat removal structure. PXS can be used for all DBAs. Combined, you find out that the AP1000 does not have safety related diesel generators, and that all AC powered ECCS systems are considered non-safety in the AP1000 (they are now "asset protection" systems and are given augmented quality, even though they are not safety related).
 
  • #32


Well remember, if I build a passive air cooler, I have the following engineering challenges:

...

If the reactor is not boiling (natural circulation), I need the water level above the steam dryer (at least at the separator skirt), and enough decay heat to drive natural convection.
I don't understand why that should be difficult. If RPV is cooled by an IC or a theorized passive air cooler, and RPV not damaged, you aren't losing water from it.

I need to have penetrations and piping which allow reactor coolant to go outside the containment to be cooled (in a situation where I potentially dont have power to close those valves upon a pipe rupture).
These penetrations already exist for routing steam from RPV to IC.

I need the structure (as it is safety related), to be protected from all external hazards, so it needs to have a shield structure around it. But that same shield structure is going to impinge upon air flow. Because of this I need natural chimney effect and other means to promote cooling.
I don't see why you absolutely must encase the air cooler in some sort of massive shielding. This will drive costs up and make it even more likely that utilities wouldn't want to build it at all. Ergo, making it seemingly "better" in fact drives us to a situation where we don't have a passive, indefinitely-operating cooling.

I agree it's physically possible and feasible, and it is an even better idea for molten salt or high temperature reactors where you can get much more bang for your buck in terms of heat transfer, but it is a very difficult thing to build and justify when you have things which already provide more than the necessary amount of safety which can also perform other functions. AP1000's PCS is a containment and a decay heat removal structure. PXS can be used for all DBAs. Combined, you find out that the AP1000 does not have safety related diesel generators, and that all AC powered ECCS systems are considered non-safety in the AP1000 (they are now "asset protection" systems and are given augmented quality, even though they are not safety related).
Good to hear that :)


Okay, here's my design which is very likely isn't going to work because I'm just a software engineer, but I'd like to hear where I'm wrong :)

Let's take an IC design similar to Fuku Unit 1. Its tanks are somewhere up in the reactor building. Modify it by adding many (a dozen or two) independent heat pipes whose lower ends are submerged into the IC tanks and then they are routed up to the roof and have a significant length of heat pipes all over it. (And, of course, I assume that operators know how to activate IC in the event of SBO. Post-Fukushima, it's kind of an obvious fix, right?)

Heat pipes are efficient at heat transfer starting from a few degrees above melting point of the internal liquid they use. In this case, water will do nicely. In this design they should be working to keep IC tank water cooled down to the outside ambient temperature, unless it is below ~+2 C in which case they will stop conducting heat (IOW: they won't try to freeze IC tank during winter).

During an accident, when IC tank water goes to 100 C and starts boiling, heat pipes will transfer and dissipate some of this heat to the outside air on the roof.

I wonder is it practical to size IC tanks so that, as decay heat rate goes down, they will stop boiling vigorously and start transferring most of the heat via heat pipes before they boil dry?

This design has redundancy (heat pipes are independent, leak and resulting loss of heat transfer ability in one of them doesn't impair others) and does not increase radiological hazards (since heat pipes' water is physically separated from IC water, not to mention reactor steam).
 
  • #33
Astronuc
Staff Emeritus
Science Advisor
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This might help the discussion: Boiling Water Reactor, GE BWR/4 Technology Advanced Manual, BWR Differences
http://pbadupws.nrc.gov/docs/ML0230/ML023010606.pdf

Standard Technical Specifications — General Electric Plants (BWR/4): Bases (NUREG-1433, Revision 3, Volume 2)
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/ [Broken]
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/sr1433r3v2.pdf [Broken]


http://www.oecd-nea.org/press/2011/BWR-basics_Fukushima.pdf
 
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  • #34
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I don't understand why that should be difficult. If RPV is cooled by an IC or a theorized passive air cooler, and RPV not damaged, you aren't losing water from it.
The reactor does lose water, mostly through wet seals and small known leaks (valve packings and the like) and does need makeup. What I was specifically referring to was the fact that you cannot bring a reactor to cold shutdown using IC alone. Once water stops boiling, you need to have the water level raised above the steam separator to have ANY natural cooling circulation in the reactor, and optimally you would have it flooded through the steam separator for natural convection. Once boiling stops I need a way to get water, not steam, to the external cooling source and I'll need a large water injection to do that.


These penetrations already exist for routing steam from RPV to IC.
Not true. the IC is inside the secondary containment boundary. If the IC heat exchange tubes were to break, monitors will detect it and isolate the leak, and any released radiation will be captured by the secondary containment standby gas treatment system. The IC does not actually put reactor coolant outside the containment boundary.

I don't see why you absolutely must encase the air cooler in some sort of massive shielding. This will drive costs up and make it even more likely that utilities wouldn't want to build it at all. Ergo, making it seemingly "better" in fact drives us to a situation where we don't have a passive, indefinitely-operating cooling.
You cant take any credit for a passive air cooler if it cannot withstand all of the environmental effects it could be faced with. Since you have to run reactor coolant through it, and it will likely need to be outside or in some type of chimney structure, if it is hit by a missile or an airplane (which US nuclear plants are required to be designed for now) it no longer will be a functional system, and in fact, can create a credible leak path for reactor coolant and fission product release. This is why a shield building is required, and is part of the reason that the AP1000 heat transfer structure is its primary containment.

Another thing to remember, if I cant take credit for my passive air cooler in all cases, that means I now need active systems which are safety grade to back it up, and thats expensive.

Okay, here's my design which is very likely isn't going to work because I'm just a software engineer, but I'd like to hear where I'm wrong :)

Let's take an IC design similar to Fuku Unit 1. Its tanks are somewhere up in the reactor building. Modify it by adding many (a dozen or two) independent heat pipes whose lower ends are submerged into the IC tanks and then they are routed up to the roof and have a significant length of heat pipes all over it. (And, of course, I assume that operators know how to activate IC in the event of SBO. Post-Fukushima, it's kind of an obvious fix, right?)

Heat pipes are efficient at heat transfer starting from a few degrees above melting point of the internal liquid they use. In this case, water will do nicely. In this design they should be working to keep IC tank water cooled down to the outside ambient temperature, unless it is below ~+2 C in which case they will stop conducting heat (IOW: they won't try to freeze IC tank during winter).

During an accident, when IC tank water goes to 100 C and starts boiling, heat pipes will transfer and dissipate some of this heat to the outside air on the roof.

I wonder is it practical to size IC tanks so that, as decay heat rate goes down, they will stop boiling vigorously and start transferring most of the heat via heat pipes before they boil dry?

This design has redundancy (heat pipes are independent, leak and resulting loss of heat transfer ability in one of them doesn't impair others) and does not increase radiological hazards (since heat pipes' water is physically separated from IC water, not to mention reactor steam).
The decay heat rate will still be on the order of 10s of MW for days and on the order of MWs for years. I don't think the heat pipes will provide any real benefit in this case. When water boils in the IC, the majority of the heat being removed is due to the latent heat to bring liquid water to boiling. Sticking some heat pipes in the IC will still require a large heat transfer surface, and some way to force air to pass over them (As simple radiative heat transfer is not sufficient for large reactor sizes). It's really a matter of scales here. You wouldnt be able to remove large enough amounts of heat prior to the IC tanks going dry because the decay heat loads remain very large for quite a while.

Sorry I'm being negative here. In concept/theory if you make it large enough you can have a system that works (again see the AP1000 containment), but in terms of a realistic integrated design theres a lot more to it then that. To make it worth using it needs to be credited for just about all accident scenarios and environmental hazards it could be subject to, and to make it cost effective it needs to be able to replace active safety systems.
 
  • #35
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This might help the discussion: Boiling Water Reactor, GE BWR/4 Technology Advanced Manual, BWR Differences
http://pbadupws.nrc.gov/docs/ML0230/ML023010606.pdf

Standard Technical Specifications — General Electric Plants (BWR/4): Bases (NUREG-1433, Revision 3, Volume 2)
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/sr1433r3v2.pdf


http://www.oecd-nea.org/press/2011/BWR-basics_Fukushima.pdf
I really appreciate the first link. I've worked in 5s and 6s but dont have the same kind of experience with 3s and 4s. my knowledge of them is limited to reading their procedures, design documents, and SARs
 
  • #36
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I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.
 
  • #37
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I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.
I think I'll be able to find out oyster's actual IC capacity.

That differences document states a 20 minute IC capacity (which is in line with Dresden's IC). I do know that IC capacity of the shell itself in some plants (like dresden) is only about 20-30 minutes per IC (i found this chapter of their FSAR online), but it can be extended to several hours with water pumped in via a diesel driven or electric driven motor from a water tank sitting outside the plant which refills the IC.

part of the reason for the smaller time is the greater decay heat load. Dresden is a very high power plant for the core size, so it has much more decay heat to deal with.

I've read claims that Fuku #1 has a 6 hour IC capacity in the heat exchanger shell....I'm a little skeptical (because I know some BWRs have much smaller ICs)...and there could be a translation issue here, but I have no verified evidence that they don't have a 6 hour IC capacity. I doubt we will ever get Fukushima's specifics for its IC size (in the form of a design document), but Oyster's should be in chapter 5 or 6 of their FSAR (they have a different FSAR layout so I'm not positive which chapter), and that's semi-publicly available. I know someone at oyster so I'll see if I can get a rough number.
 
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  • #38
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I have very limited knowledge of the GE BWRs. I know that the only ASEA BWR with IC, Oskarshamn 1, has 6 hour shell side capacity and capability to gravity-fill from the SFP (see http://www.ensreg.eu/sites/default/files/Swedish national report EU stress tests 111230.pdf , page 160), but that the capacity in SBO is limited to 2 hours due to the battery capacity needed to keep the valves open.

EDIT: The Spanish 466 MWe Santa Maria de Garona NPP, which AFAIK is close to FK1/1, apparently only has 1 hour worth of water on the shell side: http://www.ensreg.eu/sites/default/files/Spain_Stress-Tests.pdf , p. 157.
 
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  • #39
Astronuc
Staff Emeritus
Science Advisor
18,825
2,054


This might also be of interest in understanding BWRs.

https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Boiling%20Water%20Reactors.pdf [Broken]

Oyster Creek and Nine Mile Point 1 are BWR/2 units without jetpumps, but with direct cycle.


FYI - Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants
IAEA TECDOC 1624 - http://www-pub.iaea.org/books/IAEAB...culation-in-Water-Cooled-Nuclear-Power-Plants
 
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  • #40
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I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.
Thanks to a colleague at Oyster, I've seen copies of Oyster's safety analysis report and their operations training manuals. The IC at Oyster can operate for 45 minutes each without a makeup water supply. so 2 ICs in service is 90 minutes. It also takes about 100k gallons to actually do a full cool down on the oyster creek reactor. This "several hours capacity" is not correct. The IC is only sized for a little over 90 minutes w/out makeup.
 
  • #41


You cant take any credit for a passive air cooler if it cannot withstand all of the environmental effects it could be faced with.
Interesting. Your position boils down to "we can't build an air-cooled heat exchanger which can withstand 9M earthquake, therefore let's not have it at all".

Since you have to run reactor coolant through it
Wrong.

if it is hit by a missile or an airplane (which US nuclear plants are required to be designed for now) it no longer will be a functional system, and in fact, can create a credible leak path for reactor coolant and fission product release.
You are criticizing something different from my proposal, because in my proposal reactor water does NOT go directly thru air cooler; but anyway:
I am 100.00% sure Fukushima refugees would take a small, TMI-like transient radiation leak instead of a massive Cs-134/137 plume and ensuing permanent evacuation any day, thank you very much!
 
  • #42


What I was specifically referring to was the fact that you cannot bring a reactor to cold shutdown using IC alone.
In a SBO, your primary concern is not to bring reactor to cold shutdown. In a SBO, your goal is to not let it melt down. If IC would be able to stabilize RPV at 120C for days without any power, I am a happy camper.
 
  • #43
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Interesting. Your position boils down to "we can't build an air-cooled heat exchanger which can withstand 9M earthquake, therefore let's not have it at all".



Wrong.



You are criticizing something different from my proposal, because in my proposal reactor water does NOT go directly thru air cooler; but anyway:
I am 100.00% sure Fukushima refugees would take a small, TMI-like transient radiation leak instead of a massive Cs-134/137 plume and ensuing permanent evacuation any day, thank you very much!
Ok so are you talking about SBO or are you talking about design basis accidents?

In either case, why are you going to install something that isn't capable of functioning in ALL environmental and accident conditions. You cant even accredit it as safety. There is absolutely no purpose in nuclear to install a piece of equipment with a safety function if it cannot handle the design basis earthquake, floods, weather events (wind snow tornado), plus any effects from design basis accidents including jet impingment/pipe whip due to High energy line breaks, LOCA, LOOP, etc. So yes, if you cannot build a structure that can withstand all of that, then it is not worth it to built it at all in nuclear.

As for not having to run reactor coolant through it, I'm curious... how are you going to transfer heat from one loop to another? So you are going to use reactor natural circulation combined with gravity for a primary loop heat removal...but how are you going to get the secondary loop to do the same. It is a large challenge, but not an insurmountable one, but I have a feeling (based off of experience) that adding in another loop to a natural air cooled heat exchanger for LWRs is not going to be effective without AC electrical power or some other motive force.
 
  • #44
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In a SBO, your primary concern is not to bring reactor to cold shutdown. In a SBO, your goal is to not let it melt down. If IC would be able to stabilize RPV at 120C for days without any power, I am a happy camper.
There is still operational leakage even during SBO, so makeup is an issue as well (albiet a long term one). If you cannot bring the system to cold shutdown, it is very difficult to makeup to the vessel using external pumps, and is an issue we saw at Fukushima, when they couldn't get RPV and PCS pressures low enough to allow injection.

The IC on its own extracts too much heat from the reactor, a detailed analysis would need to be performed, but its possible it would cool the system down so rapidly that if would lose natural circulation for a period of time. You need motive force to somehow turn the IC "on" and "off". Additionally you still need motive force plus a makeup source to have the IC maintain the vessel in a hot standby condition, although that force does not necessarily need to be AC power.
 
  • #45


Ok so are you talking about SBO or are you talking about design basis accidents?
SBO must be a design basis accident. Otherwise we will have more Fukushimas.
 
  • #46


If you cannot bring the system to cold shutdown, it is very difficult to makeup to the vessel using external pumps, and is an issue we saw at Fukushima, when they couldn't get RPV and PCS pressures low enough to allow injection.
They couldn't bring RPVs to atmospheric pressure because some idiot decided that prolonged SBO "can't happen" and therefore accident planning and personnel training regime does not need to include instructions and drills for venting RPVs in SBO conditions.

This corporate/regulatory blunder has nothing to do with technical merits of ICs.

If anything, if IC maintains RPV internals at near 100 C and pressure just a tad above 1 atm, that makes injection easier, not harder.
 
  • #47
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SBO must be a design basis accident. Otherwise we will have more Fukushimas.
SBO cant be a design basis accident. Under plant design you can never have that many failures happen to put you in prolonged SBO. It wasn't a decision, its the fact thats how your design is of your facility. SBO is like any other extensive damage or severe accident, the fact that you got there in the first place meant that circumstances occurred which could not be prevented by design, and as such, you cannot plan for it like you would plan for any normal accident scenario.

Also, there are accident scenarios and procedures for venting the RPV, even in SBO conditions. The issues involved were due to the Japanese position to not vent until double the maximum containment design pressure. There were a lot of things that occurred as a result of this decision, such as not enough SRV accumulator pressure to actuate the SRVs in ADS or relief mode, failure of penetrations and seals in the PCV (and a potential breach in unit 2 PCV), etc.
 
  • #48
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The term "design basis" is internationally a bit vague, since in some countries it refers to the original US definition, whereas in some countries additional more extensive conditions ("design extension conditions") and even severe accidents are in fact within the design basis. For example, the Finnish event classification is described in that post: https://www.physicsforums.com/showpost.php?p=3671859&postcount=566 and a SBO falls under the DEC B category, where systems are only required to withstand external conditions with frequency once per 1000 years, not once per 100 000 years as the DBC4 systems.
 
  • #49
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As for not having to run reactor coolant through it, I'm curious... how are you going to transfer heat from one loop to another? So you are going to use reactor natural circulation combined with gravity for a primary loop heat removal...but how are you going to get the secondary loop to do the same. It is a large challenge, but not an insurmountable one, but I have a feeling (based off of experience) that adding in another loop to a natural air cooled heat exchanger for LWRs is not going to be effective without AC electrical power or some other motive force.
They're doing something like that at the Kudankulam VVER being built by Russia in India: http://www.frontlineonnet.com/fl2824/stories/20111202282403300.htm [Broken]

http://www.frontlineonnet.com/fl2824/images/20111202282403305.jpg [Broken]
 
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  • #50


SBO cant be a design basis accident. Under plant design you can never have that many failures happen to put you in prolonged SBO. It wasn't a decision, its the fact thats how your design is of your facility. SBO is like any other extensive damage or severe accident, the fact that you got there in the first place meant that circumstances occurred which could not be prevented by design, and as such, you cannot plan for it like you would plan for any normal accident scenario.

Also, there are accident scenarios and procedures for venting the RPV, even in SBO conditions.
Glad that you are sure there are.

From where I sit, empirical evidence (Fuku) says that those procedures are not known to people operating NPPs, and when SBO occurred, they had no idea what to do.
 

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