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PostPosted: Jul 22, 2017 7:49 pm 
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I have a bit of a strange question about the AP600/AP1000 Passive Containment Cooling system.
As can be seen in the attached photo, air is drawn in at the top of the building, pulled down to the bottom of the building before being allowed to flow back up, against the actual containment barrier that is heating it.

Why would you do this?
Doesn't this drastically increase your pressure drop and reduce your hydraulic head compared to simply drawing in air at the bottom of the building through appropriate intakes?

Additionally if we were to try and operate without water, the reactor will overheat up until about 250 hours after reactor shutdown.
Would it be possible to increase passive heat transfer by simply adding fins to the outside of the containment vessel to increase its surface area, or would the increased presssure drop overwhelm that effect?
[And how could you fabricate a steel barrier 0.5-2" thick with fins on the outside?]


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PostPosted: Jul 23, 2017 3:06 am 
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The downflow over the outside annulus with the baffle would reduce temperatures of the silo and other structural components, as well as shielding concrete. This is important for higher temperature reactor such as a PRISM RVACS or the IMSR IRVACS type systems. The ap1000 containment cooling can probably do without the baffle, since temperatures would be lower.

Fins will work fine, but it only deals with one heat transfer bottleneck. So overall effect on total heat removed will be ok but not drastic. The fins, if properly designed, would add strength though, esp. buckling strength and that is quite nice.

Personally I would have used the water moat concept instead of the sprayer tank. Eliminates the valve failure mode (not to mention eliminating the safety grade valves themselves). Can fit a lot more water as a bonus. But here, you do need a baffle, and elevated air intake, obviously...

Should point out that if PCCS water makeup fails, but there is no core damage (250 hours to determine if this is the case!) then you could simply vent the containment and add water to containment instead.


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PostPosted: Jul 23, 2017 5:27 am 
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Apparently the limiting factor in a SBLOCA is the transfer of heat away from the outside edge of the containment wall to the atmosphere.
Analysis indicates that if water is expended or fails for whatever reason, natural circulation of air will remove at peak 12MWt (from this paper) from the containment, with the containment at its rated temperature and pressure conditions.
I am pondering adding thermal mass to the inside of the containment (pressure suppression with ice or water) which will absorb the initial high heat peak, but if fins can significantly increase heat transfer / air flow then we would be in business and we could dispense with the huge safety critical water tank on the roof of the building.

For example apparently exhaust temperatures from the stack are only 75C when the containment wall is much hotter than that, so fins could significantly increase heat tranwfer, even if pressure drop reduces air flow.


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PostPosted: Jul 23, 2017 4:05 pm 
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Cooling a PWR LOCA passively without air for a 3400 MWth reactor, probably not going to work. 12 MW is not much, 0.35%. With just the heat sink of the primary loop, you'd want multiples of that to be safe. 20% or 30% increase in total heat transfer is pretty easy with fins but not 200%.

If you have the water handy for boiloff and then makeup fails, that should be a more doable design situation with air only cooling. 12 MW is enough with a reasonable water supply in the annulus. If the annulus is the PCCS tank, then tank valve failure is not a failure mode.


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PostPosted: Jul 23, 2017 5:34 pm 
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Cyril R wrote:
Cooling a PWR LOCA passively without air for a 3400 MWth reactor, probably not going to work. 12 MW is not much, 0.35%. With just the heat sink of the primary loop, you'd want multiples of that to be safe. 20% or 30% increase in total heat transfer is pretty easy with fins but not 200%.


I was proposing to swamp the initial pressure spike from a LOCA with an ice bed in the style of DC Cook, which adds roughly 660GJ at least of heat sink before we reach ~120C degrees.
I am also pondering a PCIV pressure vessel with an integral system, which would probably exclude most large-break accidents. Currently I project the biggest break being a fuel channel breach at something like 300kg/s. Which is a lot but not like a main cooling system line break. [This is, in case you are wondering, for another one of my knockoffs of Atucha - this being a derivative of the original Siemens concept of an integral PCPV circuit]
Cyril R wrote:
If you have the water handy for boiloff and then makeup fails, that should be a more doable design situation with air only cooling. 12 MW is enough with a reasonable water supply in the annulus. If the annulus is the PCCS tank, then tank valve failure is not a failure mode.


I like the idea of water in the annulus, that can absorb ~2GJ/t of water, and I suppose if you fill the downcomer and vent to the intake level then you can get quite a lot without compromising the post-water operation in the form of a natural circulation system.

AFter 250 hours of shutdown reactor power will be down to ~12MWt, now if only ice melted at 100C instead of 0, we could hold the temperature at the rated value for hours whilst it melted [and dumped heat from the annulus], instead of melting it all at the start of the accident


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PostPosted: Jul 24, 2017 12:22 am 
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Boiling away water is much more effective than melting ice. 2.5 GJ/ton water. Vs. 0.34 GJ/ton for melting ice.

1 meter thick water annulus, 40 m dia, 40 m tall is about 5000 m3. More than a week of decay heat for AP1000. Air cooling should do it after this.

The annulus could be vertically compartmentalized so if one leaks it has little consequence for heat sink availability. The compartments would add strength to the structure too, in addition to not needing that big tank of water at the top that is not attractive from a seismic loading viewpoint.


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PostPosted: Jul 24, 2017 6:12 am 
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Possibility of LOCA is the only shortcoming of water as coolant.
There have been experiments using pebbles as fuel containing moderator and molten salt as coolant. FNaBe is a promising coolant. Designing everything inside the reactor vessel for high temperature could be a good design idea. High pressure could be confined to generating section as in thermal plants.


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PostPosted: Jul 24, 2017 10:17 am 
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Cyril R wrote:
Boiling away water is much more effective than melting ice. 2.5 GJ/ton water. Vs. 0.34 GJ/ton for melting ice.

1 meter thick water annulus, 40 m dia, 40 m tall is about 5000 m3. More than a week of decay heat for AP1000. Air cooling should do it after this.

5000 tonnes comes out at a decay energy of 12,500GJ of total energy dispersed by boiling.
That is 3,680 seconds of full power equivalent.
If we assume 5,000 hours of pre-shutdown operation (in other words, a continuously fueled reactor at equilibrium).
I put that at something like the first three hundred hours of decay heat.
At which point the decay power of the reactor would be something like 6.8MWt.
Which means the equilibrium temperature of the AP1000 under natural convection is something like 105 degrees celsius.

Honestly if we increased the tank size slightly then we could probably shrink the containment height drastically and still reduce its design pressure. DC Cook has half the containment volume of an AP1000 with a peak design pressure of under one bar (gauge).
Cyril R wrote:
The annulus could be vertically compartmentalized so if one leaks it has little consequence for heat sink availability. The compartments would add strength to the structure too, in addition to not needing that big tank of water at the top that is not attractive from a seismic loading viewpoint.


The limiting factor will likely be the pressure on the base of the containment wall/dividing walls, it coudl be over 4 bar if one cell is empty and the next one is full.


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PostPosted: Jul 24, 2017 11:05 am 
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What decay heat curve are you using? I'm using the ans curve for inf operation on U235, with actinide heat plus some allowances for activated materials. Bit on the conservative side.

Quote:
Honestly if we increased the tank size slightly then we could probably shrink the containment height drastically and still reduce its design pressure. DC Cook has half the containment volume of an AP1000 with a peak design pressure of under one bar (gauge).


Could probably double the annulus thickness to 2 meters of water. Reducing design pressure reduces air cooling ability though, as does reducing the height. Less height is less bouyancy and less area for cooling.

Quote:
The limiting factor will likely be the pressure on the base of the containment wall/dividing walls, it coudl be over 4 bar if one cell is empty and the next one is full.


It's less than 10 MPa stress in the shield wall, so pretty small even for normal concrete.

Compartment walls would preferably be designed to be flexible so they decouple thermal or seismic strains in a transient, in terms of the containment vs. the external event wall. Likely some sort of curved metal membrane. It would be nice to have the compartments connect at the top (think ice cube tray) so only one fill line is needed. Perhaps compartment walls can act as additional cooling fins too.


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PostPosted: Jul 24, 2017 12:34 pm 
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Cyril R wrote:
What decay heat curve are you using? I'm using the ans curve for inf operation on U235, with actinide heat plus some allowances for activated materials. Bit on the conservative side.

This one, I am not sure how accurate it is, but 5,000 hours seems reasonable to me if I am really doing on load refuelling.

Cyril R wrote:
Could probably double the annulus thickness to 2 meters of water. Reducing design pressure reduces air cooling ability though, as does reducing the height. Less height is less bouyancy and less area for cooling.

Indeed, but if we have ten thousand tonnes of water that is 25000GJ of heat absorption, which takes us out to one thousand hours of cooling.
With a decay power at water exhaustion of only 3MWt.

So we can probably tolerate some loss of passive cooling power.
I wonder how practical it would also be to provide a natural circulation coolant loop between the tank and the pressure vessel that could be initiated without AC power.
I suppose you could alwys react to an SBO accident by using a squib valve to deliberately turn it into an SBLOCA, but that would pressure-cook all the equipment in your containment for days.

Cyril R wrote:
It's less than 10 MPa stress in the shield wall, so pretty small even for normal concrete.
Compartment walls would preferably be designed to be flexible so they decouple thermal or seismic strains in a transient, in terms of the containment vs. the external event wall. Likely some sort of curved metal membrane. It would be nice to have the compartments connect at the top (think ice cube tray) so only one fill line is needed. Perhaps compartment walls can act as additional cooling fins too.


Indeed, ideally we want the entire thing to flex so you don't break the cell dividers open if the outer wall flexes under an impact like a plane smashing into it.
Luckily the tanks are clearly open topped so we don't have to worry about them bursting from contained overpressure.


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PostPosted: Jul 25, 2017 3:14 am 
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Yeah 25 TJ is a lot of heat sink! With this much heat sink, you probably won't worry about air cooling efficiency; likely you'll get rid of the baffle to simplify things. A giant metal cylinder this size at >100 Celsius is going to lose 3 MW very easily even without engineered air cooling path.

The air cooling would start to help as soon as the water level drops, so that's a significant heat transfer bonus, especially over 100s of hours. This will extend the heat sink water supply even further.

Quote:
I wonder how practical it would also be to provide a natural circulation coolant loop between the tank and the pressure vessel that could be initiated without AC power.
I suppose you could alwys react to an SBO accident by using a squib valve to deliberately turn it into an SBLOCA, but that would pressure-cook all the equipment in your containment for days.


Not entirely sure but I think the ap1000 has a passive c tube heat exchanger for the core with a valve that (IIUC) opens on loss of power. This transfers heat from the reactor loop to the in containment refueling water tank. This water then boils and goes into containment, where it condenses on the walls and is collected by gutters and troughs and drained back to the refueling water tank. This can go on almost indefinately in a SBO, as long as the containment cooling has a heat sink.

Quote:
Indeed, ideally we want the entire thing to flex so you don't break the cell dividers open if the outer wall flexes under an impact like a plane smashing into it.
Luckily the tanks are clearly open topped so we don't have to worry about them bursting from contained overpressure.


Welding in some stainless steel C shaped or S shaped plates would be a fine and flexible enough barrier, and quite strong even if very thin.


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PostPosted: Jul 25, 2017 8:51 am 
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Cyril R wrote:
Yeah 25 TJ is a lot of heat sink! With this much heat sink, you probably won't worry about air cooling efficiency; likely you'll get rid of the baffle to simplify things. A giant metal cylinder this size at >100 Celsius is going to lose 3 MW very easily even without engineered air cooling path.

The air cooling would start to help as soon as the water level drops, so that's a significant heat transfer bonus, especially over 100s of hours. This will extend the heat sink water supply even further.


Dropping the baffle will certainly make things simpler, although we might need heat resistant concrete that can survive prolonged contact with ~100C water and steam.... do such things exist?
Cyril R wrote:
Not entirely sure but I think the ap1000 has a passive c tube heat exchanger for the core with a valve that (IIUC) opens on loss of power. This transfers heat from the reactor loop to the in containment refueling water tank. This water then boils and goes into containment, where it condenses on the walls and is collected by gutters and troughs and drained back to the refueling water tank. This can go on almost indefinately in a SBO, as long as the containment cooling has a heat sink.


If we have access to a 40m tall column of water and the reactor vessel (likely with integral steam generators) is at the bottom, could we perhaps not use the column of water itself as an alternative feedwater supply to one or both of the steam generators?
If we assume the reactor will natural circulate at decay power (And even CANDUs do that) or will require sufficiently low circulating power that it will natural circulate after a few hours [and ~50 kilowatts for a few hours is easily doable even with NiFe batteries] could we provide a valve that will blow down the steam generator to near atmospheric pressure, then supply it feedwater through a one way valve from the containment?

Steam could be exhausted out of a chimney built into the side of the shield wall with an outlet above the level of the containment water, so water could never use it to drain out of the annulus.
An appropriate set pressure valve would be provided at the top of the chimney to ensure the steam leaves with a reasonable enthalpy.

It has the advantage that the containment stays cold (cooled by direct contact with the cooling water annulus) and thus delays equipment damage et al until as late as possible in the accident.
Even though a single one way valve would only be able to provide water from a single "segment" of the annulus, you could either provide more valves or simply accept it, even a thousand tonnes of water boiled in the SGs gives a couple of hours to restore site power, and you can always vent the Reactor Vessel and go to an SBLOCA with direct conduction through the containment if you have to.

You would need a couple of powered valves (one in the intake and one on the steam vent line), a one way valve on the intake and the set pressure valve per steam generator. You want to have direct control over it so it does not activate in a steam generator tube break that could vent radiation out of the containment through the pipework, or allow water to drain from the containment through a broken feedwater line. But such things will be detectable immediately by a high containment or steam cycle radiation count signal, so a computer logic circuit could be used to make the initial decision on whether to use it.

But it keeps the containment cold as long as possible and thus protects plant investment is my primary argument.


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PostPosted: Jul 25, 2017 9:51 am 
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Concrete is fine for short term exposure to 100C. Long term needs to be under 65C per most codes.

Quote:
If we have access to a 40m tall column of water and the reactor vessel (likely with integral steam generators) is at the bottom, could we perhaps not use the column of water itself as an alternative feedwater supply to one or both of the steam generators?


Sure, you could connect some of the compartments to the SGs. Maybe have 8 compartments and connect 2 of the 8 to the SGs.


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PostPosted: Jul 25, 2017 10:26 am 
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Quote:
You would need a couple of powered valves (one in the intake and one on the steam vent line), a one way valve on the intake and the set pressure valve per steam generator. You want to have direct control over it so it does not activate in a steam generator tube break that could vent radiation out of the containment through the pipework, or allow water to drain from the containment through a broken feedwater line. But such things will be detectable immediately by a high containment or steam cycle radiation count signal, so a computer logic circuit could be used to make the initial decision on whether to use it.


Nuclear power plants use too many valves already. Future plants should reduce the number not increase it!


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PostPosted: Jul 25, 2017 12:15 pm 
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Cyril R wrote:
Quote:
You would need a couple of powered valves (one in the intake and one on the steam vent line), a one way valve on the intake and the set pressure valve per steam generator. You want to have direct control over it so it does not activate in a steam generator tube break that could vent radiation out of the containment through the pipework, or allow water to drain from the containment through a broken feedwater line. But such things will be detectable immediately by a high containment or steam cycle radiation count signal, so a computer logic circuit could be used to make the initial decision on whether to use it.


Nuclear power plants use too many valves already. Future plants should reduce the number not increase it!


Yes, but relatively small valves in a feedwater system that would remove the need for some sort of fully seperate core cooling system is probably a net reduction in complexity!

We need a system to blowdown the steam generators anyway, if for no other reason than to simply remove the energy from the containment in an accident.


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