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PostPosted: Nov 30, 2013 4:09 am 
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You'd also want to check the temperature profile for that. Heat transfer area through the barrier and then through the blanket vessel is limited and you have a big decay heat load to transfer with such a large reactor. Heat transfer through the blanket wall (reactor vessel) into the buffer salt is a particular problem as the buffer salt has low velocity (poor heat transfer coefficient). It works for low power density design. For high power density design extra short term cooling is needed (say through the HXs with natural circulation and some passive cooling system there) to prevent vessels failing.

If your blanket vessel is only 3.5 m ID but you use the full 2400 MWth core power then this is very hard indeed. We're talking about moving hundreds of MWth through that reactor vessel.


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PostPosted: Nov 30, 2013 11:43 am 
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It is a lot of heat to be sure but not hundreds of MWth. Initially it is 7% of 2400 MWth or 168MW but this will go first to the thermal mass of the fuel salt. By the time fuel salt has heated up 100C the heat generated has dropped to 45MW. By the time the blanket is also 100C hotter the heat generation has dropped below 30 MW. While this is a very crude thermal analysis I think it shows we are not dealing with hundreds of megawatts to the buffer salt but rather tens.


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PostPosted: Nov 30, 2013 12:05 pm 
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Ok I was exaggerating but still, 30 MWth through a vessel with no pumped flow on the buffer salt side is likely asking too much for such a limited surface area.

I also wouldn't be too certain of the barrier transferring enough heat to the blanket to limit the core salt temp to a 100 degree heatup on loss of forced circulation. Its surface area is much smaller than the reactor vessel. Without flow in the blanket and core the heat transfer coefficients become poor and the reactor vessel is thick unlike a heat exchanger wall so its thermal resisance isn't negligible.

Additional heat sinks and surface area will be required. I ran into this problem in thermal modelling of high power cores. The combined film (at low flow speeds of natural circulation) and vessel wall resistances add up to a lot of heat impedance. It is very tenacious.


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PostPosted: Nov 30, 2013 2:02 pm 
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Heat flow is a new problem area for me - and yes the 100C was completely arbitrary. It is a perfectly acceptable temperature rise that showed the heat capacity of the fuel salt takes care of the highest thermal flows.

What if we build a chimney?

Imagine the wall between the core and the blanket as a simple right cylinder, say 1 cm thick.
Now place another wall say 5mm thick with a gap of 2cm outside of the real wall.
Make the secondary wall be open to the blanket salt on the top and bottom.
For even more effectiveness include flow guides along the top and bottom of the blanket salt that steer the majority of the hot blanket salt across the top and then down the outer wall of the blanket which has another heat flow dividing wall just inside it. This will double the convective flow and virtually eliminate the thermal resistance of the thickness of the blanket salt. Finally the cold(ish) blanket salt is guided from the outer wall bottom to the inner wall bottom to complete the circle. (Perhaps this is just the first step toward building in natural convention HX's between the fuel and blanket salts and between the blanket and buffer salts.).

Assuming 100C rise from bottom to top, we need to move the fluid at 0.35m/sec.
Is this a reasonable value?


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PostPosted: Nov 30, 2013 3:52 pm 
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We can look at the MSBR primary HX. The designs were typically around 2 m2/MWth. Not sure exactly the flow speed in thos HXs, but it's probably in the ballpark of 10x the value you used above (3-4 m/s maybe). So to match that you'd be thinking of 20 m2/MWth of heat that you want to remove. Never mind that the tubes are 1 mm thick and the core/blanket is thicker. We're already talking about a large surface area which does not appear present.

Say you want to move 50 MWth from core to blanket in a pinch. At the 20 m2/MWth we'll need 1000 m2. Seems like we don't have that.


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PostPosted: Dec 01, 2013 5:45 am 
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The Berkeley work on PRACS may be of value here.

http://pb-ahtr.nuc.berkeley.edu/papers/ ... riveau.pdf

They use 12800 meter of tube of 2.5 cm diameter. That works out to some 1000 m2. Of thin tubes.

This is for the same 2400 MWth reactor power.

The Berkeley PB-AHTR has a lot lower power density core and a lot of salt compared to Lars' suggested design. Thus far more thermal capacity in the core.

Now the Berkeley work also showed that a lower surface are (numbertubesxheight) is acceptable in terms of peak temperatures. On the other hand the tubes would be only 1 mm or so in thickness versus a barrier of multiples of that, plus a 1.5 or 2 fluid reactor has very little thermal capacity in the core compared to the PBAHTR.

So I think you'll want in the ballpark of 1000 m2 of surface area on natural convection of the core into the blanket.


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PostPosted: Dec 01, 2013 11:58 am 
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It looks like this idea won't work and the problem of passive decay heat removal is significantly harder than I imagined. Could we use the primary HX with a large pool of salt as the thermal mass?

Still need to get the passive cooling to something like 1% of full power which looks like a challenge.


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PostPosted: Dec 01, 2013 1:14 pm 
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AW was pretty smart guy and his solution was a dump tank (which you need anyway) and a freeze plug. Why try to guild the lily?

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PostPosted: Dec 01, 2013 4:38 pm 
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KitemanSA wrote:
AW was pretty smart guy and his solution was a dump tank (which you need anyway) and a freeze plug. Why try to guild the lily?


I don't like the heat rejection systems proposed for dump tanks. Lots of thin walled pipes in the dump tank, lots of little welds, carrying dump tank coolant, that can leak or break. Also dump tank line break below the catch pan level would be a serious issue (if part of the salt gets somewhere where there's no cooling like in the drain cell but out of the drain tank).


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PostPosted: Dec 02, 2013 3:39 am 
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Cyril R wrote:
KitemanSA wrote:
AW was pretty smart guy and his solution was a dump tank (which you need anyway) and a freeze plug. Why try to guild the lily?


I don't like the heat rejection systems proposed for dump tanks. Lots of thin walled pipes in the dump tank, lots of little welds, carrying dump tank coolant, that can leak or break. Also dump tank line break below the catch pan level would be a serious issue (if part of the salt gets somewhere where there's no cooling like in the drain cell but out of the drain tank).
So improve the dump tank design. My design has no such small welds.

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PostPosted: Dec 02, 2013 3:40 am 
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KitemanSA wrote:
Cyril R wrote:
KitemanSA wrote:
AW was pretty smart guy and his solution was a dump tank (which you need anyway) and a freeze plug. Why try to guild the lily?


I don't like the heat rejection systems proposed for dump tanks. Lots of thin walled pipes in the dump tank, lots of little welds, carrying dump tank coolant, that can leak or break. Also dump tank line break below the catch pan level would be a serious issue (if part of the salt gets somewhere where there's no cooling like in the drain cell but out of the drain tank).
So improve the dump tank design. My design has no such small welds.


I'm hanging on the edge of my seat.


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PostPosted: Dec 02, 2013 3:51 am 
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Cyril R wrote:
KitemanSA wrote:
So improve the dump tank design. My design has no such small welds.
I'm hanging on the edge of my seat.
Already described it a while back. Phase change salt, bimetallic strip thermo-diodes across a dewar. Single large heat-pipe around tanks, two if needed for redundancy. Yada yada.

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PostPosted: Dec 02, 2013 3:59 am 
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KitemanSA wrote:
Cyril R wrote:
KitemanSA wrote:
So improve the dump tank design. My design has no such small welds.
I'm hanging on the edge of my seat.
Already described it a while back. Phase change salt, bimetallic strip thermo-diodes across a dewar. Single large heat-pipe around tanks, two if needed for redundancy. Yada yada.


Have you checked the temperature profile of that? Especially at the tank wall. Heat pipes are good at transporting heat, but you still have a tank wall surface area and thickness limit, even if the tank volume is big (from being mostly PCM). PCM is good for an initial sink, but after that it gets tricky.

Even if the PCM occupies 90% of the tank volume, the tank surface area isn't anywhere near 1000 m2. At 20 m3 core volume, the tank is then 200 m3.

You'd need a larger number of tanks than 2.

This does mean more piping to break and spill salt where you don't have PCM, hence larger peak temperatures (on the plus side the salt would then be in direct contact with the heat pipes, but not anywhere near to the full height). It may also challenge the integrity of the drain cell.

How do you propose to deal with these issues?


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PostPosted: Dec 02, 2013 4:30 am 
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Cyril R wrote:
KitemanSA wrote:
Already described it a while back. Phase change salt, bimetallic strip thermo-diodes across a dewar. Single large heat-pipe around tanks, two if needed for redundancy. Yada yada.


You'd need a larger number of tanks than 2.
Sorry, two pipes per tank.

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PostPosted: Dec 02, 2013 4:52 am 
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Still the questions remain. First as to whether all drain tank/piping LOCA has been addressed. Salt outside the drain tanks is likely to challenge the containment.

The heat pipe could also break or even develop a small leak and lose its coolant (one of my laptops was killed in this way - lil' heat pipe LOCA causing the whole thing to overheat and shutdown).

Second. It's hard to get to the required 1000 m2 of tank surface area for 2400 MWth, 20 m3 fuel salt core. A single tank of 200 m3 with 90% PCM would only get you around 150 m2 or so of coolable tank wall surface. I can already tell you without running any numbers that's not anywhere good enough.

With 10 tanks you can get 300-400 m2 which is still not good enough. Melting 180 m3 of PCM isn't THAT big a heat sink for a 2400 MWth reactor afterheat.

With 20-30 tanks we're talking about a LOT of piping and welds that can break and stuff to generally go wrong.


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