Energy From Thorium Discussion Forum

It is currently Aug 15, 2018 12:08 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 46 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
PostPosted: Oct 18, 2012 5:32 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Quote:
I certainly like the idea of a buffer salt around the primary system and agree with the many advantages. I still think though that at the expense of having a pretty hot primary vessel wall, that for many MSR designs (those with reasonably low power density) one can just allow the reactor vessel itself to radiate enough heat into the containment building and use the same AP1000 tricks to get it the ultimate removal to outside air.

For example of a low power density core, an early version of the big AHTR salt cooled design (9.2 m diameter, 18 m high and 2400 MWthermal) could radiate 0.5% or 12 MW with its sides and bottom at only 500 C as it has such a big surface area. That reactor has a huge amount of surface area but letting a vessel wall rise to 700 C gives you about 2.5 times as much radiant heat per square meter (50 kw/m2). Certainly not an option for a lot of the higher power density concepts but might fit nicely with the small modular versions which typically tend to be low power density.


Yes this is possible, but the problem is that the heatup to the point of dropping down to 0.5% decay heat power is really huge! Way higher than 700 (way higher than 1000 C in fact). I don't think it acceptable so dropped this concept in the modelling.


Top
 Profile  
 
PostPosted: Oct 18, 2012 8:33 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
Cyril R wrote:
Quote:
I certainly like the idea of a buffer salt around the primary system and agree with the many advantages. I still think though that at the expense of having a pretty hot primary vessel wall, that for many MSR designs (those with reasonably low power density) one can just allow the reactor vessel itself to radiate enough heat into the containment building and use the same AP1000 tricks to get it the ultimate removal to outside air.

For example of a low power density core, an early version of the big AHTR salt cooled design (9.2 m diameter, 18 m high and 2400 MWthermal) could radiate 0.5% or 12 MW with its sides and bottom at only 500 C as it has such a big surface area. That reactor has a huge amount of surface area but letting a vessel wall rise to 700 C gives you about 2.5 times as much radiant heat per square meter (50 kw/m2). Certainly not an option for a lot of the higher power density concepts but might fit nicely with the small modular versions which typically tend to be low power density.


Yes this is possible, but the problem is that the heatup to the point of dropping down to 0.5% decay heat power is really huge! Way higher than 700 (way higher than 1000 C in fact). I don't think it acceptable so dropped this concept in the modelling.


Not sure I follow, by heatup to way higher than 700 C do you mean the salt and internal graphite if the vessel wall is at 500 C or up to 700 C radiating off the 0.5% decay heat? Again, I'm only suggesting this for quite low power density designs that have lots of surface area per MWatt. It was awhile back when I did some simple number crunching, in my notes I have Hastelloy N at a thermal conductivity of 20 W/mK so even the standard thick 5 cm vessel walls (overkill in my book) would only need a 50 C delta T to give the 20 kilowatts/m2. Since the internal salt could thermosyphon to move heat to the vessel walls I think you could do pretty good. Even with an external buffer salt you still have these issues (just easier to get heat from outer vessel walls into buffer than just by radiant).

Oh wait, now that I re-read your comment do you mean the overall heatup of the core and vessel for the first hour or two when decay heat is significantly beyond 0.5%? I'd have to crunch numbers again I think for a low power density design we'd be likely OK and again is there really that huge a difference in how much better a buffer salt will draw off heat by conduction versus what will simply be radiated? I know that allowing the vessel wall to heat up significantly can give all sorts of material concerns but I guess that has to be weighted against how often we'd actually ever employ such a system since the primary cooling system, even by thermosyphoning through the loops, should in just about every imagined case be your main decay heat removal.

I'll add my usual caveat that I'm not saying I think this method without buffer salt is best, just trying to sort out the details...

David LeBlanc

David


Top
 Profile  
 
PostPosted: Oct 18, 2012 9:54 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
David,
I expect the key difference is that the buffer salt provides a huge thermal inertia that can absorb the energy given off during the first several hours of decay heat when heat production exceeds the final passive heat removal capacity.


Top
 Profile  
 
PostPosted: Oct 18, 2012 10:35 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Lars wrote:
David,
I expect the key difference is that the buffer salt provides a huge thermal inertia that can absorb the energy given off during the first several hours of decay heat when heat production exceeds the final passive heat removal capacity.


Yes, this is a big difference. The buffer salt helps to absorb the heat till you reach 0.5% power. It actually takes in the ballpark of 2-3 hours to get there! Plenty of time for a Hastelloy vessel to heat up to short term creep failure temp.

Buffer salt is also considerably better in heat transfer from the vessel wall, than just thermal radiation. Drop a piece of hot metal in a pool of coolant, versus letting it sit outside in dry air. Big difference! Okay, the buffer salt isn't all that "cool" (>500C) but still it is a pool of coolant.

Without the buffer salt, you'd have to go to much lower power densities than even a DMSR, to absorb the heat down to 0.5% decay heat. In fact, you'd need even lower power density than a PBMR, since PBMRs can run their central parts at 1600 degrees Celsius, whereas we really want to stay well below 1000 C. It could work for a very small output DMSR, as in a prototype. So yes that could be a good idea for a prototype. But the hall that the reactor sits in would be extremely radioactive. Buffer salt means the hall is just hot.


Top
 Profile  
 
PostPosted: Oct 18, 2012 12:58 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
Quote:
In fact, you'd need even lower power density than a PBMR, since PBMRs can run their central parts at 1600 degrees Celsius, whereas we really want to stay well below 1000 C.


That's actually a completely different situation since a PBMR has to get the heat from its solid fuel by conduction through many feet of solid graphite and fuel to the outer vessel then out. That's why the fuel might reach 1600 C. In any MSR the heat is in the liquid salt which will thermosyphon to the vessel walls. In an FHR (salt cooled) it is in between as the coolant can move heat between fuel and wall.

Quote:
David,
I expect the key difference is that the buffer salt provides a huge thermal inertia that can absorb the energy given off during the first several hours of decay heat when heat production exceeds the final passive heat removal capacity.


OK but in the AP 1000 analogy, if you are radiating heat out from vessel walls to the surrounding building walls you effectively have the entire thermal inertia of the containment building to absorb this radiant heat.

David


Top
 Profile  
 
PostPosted: Oct 18, 2012 3:05 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
The thermal inertia of an AP1000 style containment is very small. It needs to be thin enough to move out the heat. It's possible to add solid material in the containment, but heat transfer, from thermal radiation of the vessel, into solid material, is poor. I wouldn't expect this idea without buffer salt to work, except for a tiny prototype with low power density and large surface area to volume of vessel.


Top
 Profile  
 
PostPosted: Nov 12, 2012 5:06 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
ORNL 4541 has a very useful table on the afterheat of various fission products and actinides.

http://www.energyfromthorium.com/pdf/ORNL-4541.pdf

Table 3.11.

It's interesting to see what the hot-heads are.

Now we can see that the offgas system is a hothead, even after 28 hours. But the graphite and metal are also hotheads, because of the big fission product load that remains after draining.

That's another reason to not drain the core rapidly. ORNL wanted to use a nitrogen circulating system to move out the graphite and metal heat load after rapid fuel drain. But that's obviously not a passive safety system (they planned on using the primary electric pumps for nitrogen circulation).

Another thing we can see from the table is the significant heat load in the processing systems. Protactinium being the biggest at 5 MW, but the fission products also amount to 1.6 MW in addition to that. So without Pa seperation we'd still have to deal with that.

ORNL planned on using circulating water for cooling the various processing and offgas systems. With boiling water in an emergency. Obviously the water is not compatible with most of the reprocessed streams, and is also a means to pressurise things. It could also run out and with no water makeup it could be a bad accident scenario.

We will need a robust solution for these various systems with lots of decay heat. Let's discuss this.

I would suggest to submerge all the reprocessing systems with large heat loads in the buffer salt. This avoids needing lots of seperate cooling systems that can all fail from loss of cooling.


Top
 Profile  
 
PostPosted: Nov 12, 2012 10:38 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
The offgas system is a large volume with near zero mass and the dominate heat load for processing. Is the hydrostatic pressure that results if you submerge that a problem? Perhaps we send the gas through a very long coil of piping that is reasonable close to the surface of the buffer salt?

Another approach is to treat the offgas system separately. Its heat load drops off pretty rapidly so providing a thermal mass for it separately may be reasonable.

By the way, do we want one or two containments between the fission products and the buffer salt?

For servicing things do we:
a) work undersalt
b) hoist the processing modules up
c) pump the buffer salt out into a holding tank
d) a better option I haven't thought of


Top
 Profile  
 
PostPosted: Nov 12, 2012 11:20 am 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
Lars wrote:
d) a better option I haven't thought of
No buffer salt? :mrgreen:

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Nov 12, 2012 12:20 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Lars wrote:
The offgas system is a large volume with near zero mass and the dominate heat load for processing. Is the hydrostatic pressure that results if you submerge that a problem? Perhaps we send the gas through a very long coil of piping that is reasonable close to the surface of the buffer salt?


Yes, the long coil of piping is a good idea to get surface area, and enough piping length for flow delay. I´m thinking of packing the piping with grannules of a fluorine donor that we talked about. Possibly particles of CrF2 or FeF2. I looked at CuF2 earlier, a better donor, but it corrodes the pipe if made of Hastelloy N. In any case, the particles are heavy and will deal with the bouyancy problem.

Quote:
Another approach is to treat the offgas system separately. Its heat load drops off pretty rapidly so providing a thermal mass for it separately may be reasonable.


We have thermal mass, plenty of it, in the buffer salt. So the coil of pipe goes at the bottom of the buffer salt pool. This is good, we don´t have to move that 20 MW through the vessel in an emergency. Same for the noble metal cartridges.

Quote:
By the way, do we want one or two containments between the fission products and the buffer salt?


A double walled pipe isn´t too much trouble, so two containments we can use.

Quote:
For servicing things do we:
a) work undersalt
b) hoist the processing modules up
c) pump the buffer salt out into a holding tank
d) a better option I haven't thought of


I´m thinking about option b). The entire array of processing equipment and reactor can all be ganged together in one big superstructure. The entire thing can then be submerged slightly, the stuff that needs servicing will have things like removable lids that are just barely surfaced so they can be maintained. This way we get a robust design yet benefit from the reduction in deadweight stresses at all times. The design philosophy would be to make most of the tanks and such last the lifetime of the plant. What cannot last must be accessable above the salt level. So things like a flush salt drain tank and fuel holdup tank would be designed for zero maintenance. The coil of offgas piping could be zero maintenance as well, just make it big enough to accomodate all the cesium as cesium fluoride. Since the volume must be large for gas holdup, this seems feasible. If it is double walled then that helps for making it last a lifetime without leakage.


Top
 Profile  
 
PostPosted: Nov 12, 2012 12:40 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
KitemanSA wrote:
Lars wrote:
d) a better option I haven't thought of
No buffer salt? :mrgreen:


It´s easy to give criticism to an idea, but much more difficult to come up with a detailed, superiour alternative.

There are other ideas that get some of the advantages of a buffer salt, but not all of the advantages, and you may have to make other sacrifices in different parts of the system. Buffer salt may seem like a maintenance problem at first glance, but on second thought you can exploit the shielding advantage of a buffer salt and the reduction in stresses due to bouyancy. You have to look at the whole picture. A dump tank looks nice, but NaK heat pipes, active nitrogen recirculation cooling, and baths of boiling water may not look so nice. The devil is in the details. There´s dozens of things I don´t like about ORNL´s work on decay heat cooling. You have to come up with a robust plan for decay heat management.


Top
 Profile  
 
PostPosted: Nov 12, 2012 1:59 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
Cyril R wrote:
Lars wrote:
The offgas system is a large volume with near zero mass and the dominate heat load for processing. Is the hydrostatic pressure that results if you submerge that a problem? Perhaps we send the gas through a very long coil of piping that is reasonable close to the surface of the buffer salt?


Yes, the long coil of piping is a good idea to get surface area, and enough piping length for flow delay. I´m thinking of packing the piping with grannules of a fluorine donor that we talked about. Possibly particles of CrF2 or FeF2. I looked at CuF2 earlier, a better donor, but it corrodes the pipe if made of Hastelloy N. In any case, the particles are heavy and will deal with the bouyancy problem.

The gas is awfully light. If I recall correctly ORNL planned on around 20 m^3 at 1 atm for the first off gas holding tank. Assuming your pellets are double the density of the buffer salt and you need a similar capacity those tubes are going to be 40 m^3 - that's a lot of tubing. Would it be reasonable to have this at higher pressure to reduce the volume? Also this volume of tubing at density of the buffer salt will have significant mass. If we lift the tubing out of the buffer salt then we need to think about its strength.
We get a lot of decayed fission products in the offgas. If we are going to allow that to build up over the lifetime of the reactor then this gets to be a significant volume. I was thinking of the coils being a coil with the radius horizontal and gas flow being downward. This should push the fission products down toward the bottom of the coil where we could allow it to drip down into a larger tank.

Quote:
Quote:
Another approach is to treat the offgas system separately. Its heat load drops off pretty rapidly so providing a thermal mass for it separately may be reasonable.


We have thermal mass, plenty of it, in the buffer salt. So the coil of pipe goes at the bottom of the buffer salt pool. This is good, we don´t have to move that 20 MW through the vessel in an emergency. Same for the noble metal cartridges.

Quote:
By the way, do we want one or two containments between the fission products and the buffer salt?


A double walled pipe isn´t too much trouble, so two containments we can use.
What happens to the heat transfer properties with a double walled pipe? Should we fill the gap with a liquid that is monitored for fission products as a detection mechanism for leakage?
Quote:
Quote:
For servicing things do we:
a) work undersalt
b) hoist the processing modules up
c) pump the buffer salt out into a holding tank
d) a better option I haven't thought of


I´m thinking about option b). The entire array of processing equipment and reactor can all be ganged together in one big superstructure. The entire thing can then be submerged slightly, the stuff that needs servicing will have things like removable lids that are just barely surfaced so they can be maintained. This way we get a robust design yet benefit from the reduction in deadweight stresses at all times. The design philosophy would be to make most of the tanks and such last the lifetime of the plant. What cannot last must be accessable above the salt level. So things like a flush salt drain tank and fuel holdup tank would be designed for zero maintenance. The coil of offgas piping could be zero maintenance as well, just make it big enough to accomodate all the cesium as cesium fluoride. Since the volume must be large for gas holdup, this seems feasible. If it is double walled then that helps for making it last a lifetime without leakage.

We still have to provide for deadweight stresses when the superstructure is out of the buffer salt. Would it make sense to only allow pulling the superstructure out as the fuel and blanket salts are pumped out? (Buoyancy then will help lift the superstructure out of the buffer salt).


Top
 Profile  
 
PostPosted: Nov 12, 2012 2:25 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
The optimal pressure in the first holding tank can be higher with buffer salt, because of the hydrostatic head provided by the buffer salt. A 10 meter buffer salt cover having a weight of 2 kg/liter put 2 bars of pressure on the tube. So it makes sense to operate the tube at 2 bars gauge, 3 bar absolute. This also eliminates the driver for a leak.

Double walled means insulating, but that´s easy enough to deal with without resorting to liquids in between. Just add cooling fins/ribs in between the tubes. Then maybe pressurize the annulus with 3 bars helium, if necessary.

Iron fluoride is heavy, around 4 kg/l. If the buffer salt weighs half that, you only need 50% packing fraction of grannules. The added weight of the pipe itself will make it heavier than the buffer salt. I guess you need more like 1/3 packing fraction.

Why does the pipe need resurfacing? It´s a simple thing that can be designed to last the life of the plant. Enough pellets can be loaded for the life of the plant. I wouldn´t want to resurface that without good reason, with all that Cs-137 in there. It could be recovered during decommissioning and then vitrified.

In the superstructure concept, the parts that need access for maintenance are mounted a bit higher. So they submerge first upon lifting, so the rest of the superstructure stays down, where it doesn´t suffer deadweight.

The downward spiralling pipe seems like an idea to explore. The cesium drips down, then drains into the tank where it is converted to the fluoride by pellets. Could be a useful way to reduce the amount of piping (if that is a problem at all).


Top
 Profile  
 
PostPosted: Nov 17, 2012 5:44 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
An ORNL document, ORNL 3145, describing the issues of decay heating of the primary HX when it is drained. The HX will actually overheat in the unlikely event that both primary and secondary sides are drained.

http://moltensalt.org/references/static ... M-3145.pdf

So, another reason not to drain quickly!

Quote:
Hastelloy N pressure vessels, piping, e t c . , are not expected to
sustain serious damage if held at low stress for short times (< 20 hr)
at temperatures of 2150°F (1177"C).*
fluence w i l l lose ductility. Ultimate strength at t h i s temperature w i l l
be very low.' If a component is to survive at this temperature, we must
ensure that the high temperature regions be v i r t u a l l y f r e e of stressproducing
imposed loads. It is appropriate to point out that it is routine
fabrication practice to specify a stress-relieving anneal at 2150°F
for welded Hastelloy N pressure vessels.


This is very promising, if we can eliminate gross stresses by suspending the vessel in a buffer salt, it could clearly tolerate much higher short term temperatures than 800 degrees Celsius that we've previously assumed. If, in an unlikely serious accident such as long term total station blackout without scram, we can let the temperature increase to a 1000 degrees Celsius or so, this will make it much easier to withstand these extreme scenarios.


Top
 Profile  
 
PostPosted: Nov 30, 2013 12:28 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
Been meaning to run this calculation for a while. Given a 1GWe, 2.4GWth reactor, and a passive cooling x% of full power, and an allow temperature rise of 100C how much buffer salt is needed? I used FLiNaK (FLiBe results are similar - slightly less salt is needed).

Cooling Salt (m^3)
1.00% 198
0.90% 301
0.80% 482
0.70% 823
0.60% 1524
0.50% 3161

If we imagine a graphite free core 1.5m radius, 3 m tall, with 20m^3 fuel salt in core, with a 1m thick full surround blanket, and an HX with 20m^3 fuel slat and 40m^3 secondary salt in a full surround pool 2m thick beyond the blanket. This is provides 400m^3 buffer salt.

I was surprised by how fast the volume of salt climbs with poorer passive cooling. (I started the exercise thinking I could just stick in a huge pool of buffer salt - but absolutely no way).


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 46 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group