Energy From Thorium Discussion Forum

It is currently Jan 18, 2018 12:57 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 46 posts ]  Go to page 1, 2, 3, 4  Next
Author Message
PostPosted: Sep 12, 2012 1:24 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
We've previously discussed some decay heat removal options. Maybe it's a good idea to recapitulate - make a list to see if we have covered all the options, and maybe discuss some more. Only passive systems will be discussed because anything else won't sell (certainly not after Fukushima). Sorry for the length (it is an important subject).

Option 1. Dump tank with bajonet tubes cooling

Description: ORNL's choice. The familiar actively cooled freeze valve & underground dump tank design. Core overheating or power failing results in freeze valve thawing, draining the salt to a tank. The tank has bajonet tubes in it with coolant that removes heat to the outside. MSRE used this system (don't recall which coolant?).

Advantages:
- freeze valve is simple and fail-safe.
- does double duty as long term criticality prevention.
- having a tank to put the fuel in is required anyway for maintenance of the reactor internals, pumps, etc.
- in theory very good against long term station blackout (Fukushima).

Disadvantages:
- putting the salt in a tank doesn't remove the heat. That's where it gets tricky. Either a water based boiloff cooling system is used, or a sodium/NaK/fluoroborate salt coolant. Water and sodium/NaK are not compatible with the fuel salt. fluoroborate or other salt can clog, freeze, etc. Water can pressurize the tank in a big leak, and runs out so needs refilling (not passive). In all cases, pipes with coolant are used and that results in pipe breaking scenarios losing the cooling.
- difficult to inspect for integrity (below ground, lots of piping crammed together, high radiation).
- during on-power operation transients/accidents, may drain too late if cooling system continues working despite drain signal (Iain's concern).
- does not provide passive containment cooling.
- significant heat load remains in the vessel & graphite after fuel drain, in the form of noble metal and soaked up fluoride FPs. Need to cool with some system or there will be damage to the vessel/graphite.

Option 2. Dump tank with air cooling

Description: same as number 1, but with a natural circulation air cooling device.

Advantages:
- freeze valve is simple and fail-safe.
- does double duty as long term criticality prevention.
- having a tank to put the fuel in is required anyway for maintenance of the reactor internals, pumps, etc.
- in theory very good against long term station blackout (Fukushima).
- air cooling solves the loss of coolant accidents, freezing etc. listed above.
- may be easier to inspect than option 1.

Disadvantages:
- very difficult to engineer! Need a huge surface area whereas dump tanks are compact. A large number of dump tanks, perhaps annular tanks, are needed. Heat store, such as solid salt filled pebbles or cylinders, may also be needed to soak up the initial heat.
- if the tank ruptures (and perhaps also the guard vessel if present), there's a powerful means to put fission products into the air. The chimney becomes your enemy. If there's a guard vessel it is a low probability event, but still potentially high consequence.
- during on-power operation transients/accidents, may drain too late if cooling system continues working despite drain signal (Iain's concern).
- does not provide passive containment cooling.
- significant heat load remains in the vessel & graphite if the fuel is drained, in the form of noble metal and soaked up fluoride FPs. Need to cool with some system or there will be damage to the vessel/graphite.

Option 3. RVACS (Reactor Vessel Auxilliary Cooling System).
Description: natural circulation of outside air, through chimneys and ducting, remove heat from a guard vessel coupled thermally to the reactor vessel. Developed for gas cooled and sodium cooled reactors.

Advantages:
- simple
- no piping with coolant needed that can break or accidentally drain, clog etc.
- uses unlimited outside air supply
- ducting in the RVACS doesn't transfer heat, so can be thick (strong) steel plate concrete.
- provides passive containment cooling for the lower containment part (guard vessel is containment boundary).
- cools the below ground cavity concrete so no insulation and seperate cooling system on the cavity concrete is needed.

Disadvantages:
- doesn't work if there's dual guard vessel + reactor vessel failure (though very unlikely, this potentially leads to very large radiation release through the chimney)
- only works for very large vessels or very small reactor powers
- does not provide passive containment cooling of the upper containment
- need seperate system for dealing with criticality in the long term (eg passive control rod). Though more of a regulatory concern than a real safety concern; fission power will equalize to the maximum heat removal duty of the passive cooling system.
- needs lots of ducting to below the level of the reactor vessel (seems like a bad thing to me somehow).
- not modular

Option 4. DRACS

Description: passive heat exchangers remove heat directly from the vessel fuel salt, using a seperate coolant and natural circulation through pipes into a chimney. The chimney in turn sucks in air through natural circulation as well, cooling the coolant, sending it back to the heat exchanger.

Advantages:
- compact system (cheap and easy to fit in a design).
- modular (easy to scale/uprate reactor power, just add more modules).

Disadvantages:
- requires pipes with coolant circulating. Lots of failure modes there, ranging from fires/explosions (if sodium/NaK) to freezing (if molten salt) to accidental operator draining despite heat load duty present, plugging through corrosion (if poor metal eutectic), or just breaking draining the pipes. Such failure modes can bring down the heat transfer to zero. The pipes need to move out a lot of heat which favors thinner pipes, but they need to be very strong to resist breaking (conflicting requirements).
- does not provide passive containment cooling.
- need seperate system for dealing with criticality in the long term (eg passive control rod). Though more of a regulatory concern than a real safety concern; fission power will equalize to the maximum heat removal duty of the passive cooling system.

Option 5. combined passive containment cooling (AP1000 style but without the water reservoir on top) and radiative decay heat removal

Description: my suggestion. AP1000 air cooling containment with a pool of buffer salt, the vessel is in the buffer salt. Heat is transferred through the vessel into the salt. Buffer salt heats up, radiates onto the containment, which is cooled on the outside with air (AP1000).

Advantages:

- buffer salt provides large inherent heat sink and radiation shielding (can also put radioactive containers at the bottom for that reason).
- buffer salt reduces deadweight stresses on the vessel
- simple
- no piping with coolant needed that can break or accidentally drain, clog etc.
- uses unlimited outside air supply
- doesn't need subgrade ducting all around the reactor vessel cavity like the RVACS.
- does double duty as passive containment cooling
- aboveground ducting in the passive containment cooling system doesn't transfer heat, so can be thick (strong) steel plate concrete.
- works for smaller higher power vessels unlike RVACS (because of better cooling provided by the liquid buffer salt - air cooling being through the much larger containment).
- unlike the other concepts, doesn't appear to have a failure mode that impedes the passive cooling (says me). No pipes breaking, draining, freezing, no water to run out, sodium to burn. Doesn't appear to be any failure mode at all that puts large amounts of FP in the air (says me).

Disadvantages:

- need seperate system for dealing with criticality in the long term (eg passive control rod). Though more of a regulatory concern than a real safety concern; fission power will equalize to the maximum heat removal duty of the passive cooling system.
- not modular

Option 6. same as 5, but with in buffer salt dump tank to deal with the long term criticality protection

Description: Also my suggestion. Same as above, but with toroidal shaped dump tank in the buffer salt. Dump tank is the sump drain suction for the pump. Sump pump is in a lowered part of the dump tank. Pump actively empties the sump to feed the reactor. Pump failure overfills the sump, filling the rest of the dump tank, which is only a few cm thick to transfers heat to the buffer salt.

Advantages:

- buffer salt provides large inherent heat sink and radiation shielding (can also put radioactive containers at the bottom for that reason).
- buffer salt reduces deadweight stresses on the vessel
- simple
- no piping with coolant needed that can break or accidentally drain, clog etc.
- uses unlimited outside air supply
- doesn't need subgrade ducting all around the reactor vessel cavity like the RVACS.
- does double duty as passive containment cooling
- aboveground ducting in the passive containment cooling system doesn't transfer heat, so can be thick (strong) steel plate concrete.
- works for smaller higher power vessels unlike RVACS (because of better cooling provided by the liquid buffer salt).
- unlike the other concepts, doesn't appear to have a failure mode that impedes the passive cooling (says me). No pipes breaking, draining, freezing, no water to run out, sodium to burn.
- deals with reactivity in the subcritical dump tank.
- does not need safety grade fast acting freeze valve.
- having a tank to put the fuel in is required anyway for maintenance of the reactor internals, pumps, etc.
- very good against long term station blackout (Fukushima).

Disadvantages:

- need a very long shafted pump or a fully fluoride-submersible pump-motor unit.
- pump maintenance? How to replace??
- not modular


Top
 Profile  
 
PostPosted: Sep 15, 2012 6:25 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
Perhaps there are also hybrids that can be considered. For example, a combined PCCs, RVACS, and buffer salt concept. With the RVACS cooling the buffer salt tank from below. This seems more appropriate for an MSR, considering the high fission product activity in the vessel. The buffer salt tank would put a shielding buffer salt in between the RVACS ducting and the reactor vessel, while also increasing the surface area for cooling.


Top
 Profile  
 
PostPosted: Sep 18, 2012 1:23 am 
Offline
User avatar

Joined: Jan 10, 2007 5:09 pm
Posts: 501
Location: Los Altos, California
Just to be clear, my issue with the freeze valve is that there is a race between the freeze valve thawing and the reactor overheating. Since these two are not driven by the same variables, it's possible for the race to go either way, depending on how fast the reactor heats up (among other things).

So, even if a reactivity excursion causes the freeze valve cooling system to stop, how is it that the freeze valve melts before the reactor overheats? Was this actually tested on the MSRE?

Also, I'll note that plenty of respectable folks like Per Petersen and Kirk Sorensen don't appear to be worried about this issue, so maybe there's just some way to arrange the sizes of the freeze valve and the reactor so it's okay. But that should end up being a top-level system constraint, similar to the fractional volume of the primary HX constraint, and we should have seen that show up somewhere. I haven't seen it.

I'm not sure you listed it, but options 4, 5, and 6 might involve a continuous heat loss to the environment through the emergency cooling system. One advantage would then be that the emergency system is in continuous operation and is continuously verified working. Another is that the emergency system sees no change of state during an emergency, so there's no temperature shock to the system to break it.

-Iain


Top
 Profile  
 
PostPosted: Sep 18, 2012 10:56 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3069
The immediate heat load on shutdown is 7%. Most passive cooling systems target removing between 0.5% and 1% of full power. The excess shutdown heat is absorbed by thermal mass. So the thermal mass absorbs most of the heat for the first 24 hours. IIRC time to open the freeze valves is around ten minutes. So, freeze valves are useless to deal with a runaway fission process (but thermal expansion of the salt solves this one) but will function quite adequately for moving the fuel to a different configuration for decay heat removal.

Even so, there is an attractiveness to the concept of continuous removal of 0.5% of the heat generated by passive cooling. So even if the reliability of freeze valves is not a concern one may still choose forgo them for emergency use.

I like the idea of a large pool of buffer salt combined with continuous heat loss around 0.5%. This would seem to address the biggest safety issue with reactors.


Top
 Profile  
 
PostPosted: Sep 18, 2012 11:01 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 914
Location: Ottawa
Thanks so much for the summary Cyril, great work as usual.

Iain's comment needs addressing and I can't recall what was said in previous discussion threads. That being does one run passive containment cooling options (5,6) that always bleed off a fraction of the heat (option 4 of DRACS typically uses things like fluidic diodes to help only kick in when needed). The advantage of a system that is always leaking heat (say 0.5%?) is you of course know its always working but then do you need to deal with your building space surrounding the reactor's primary containment always being quite hot (i.e 100 C)?

One way I've thought was placing some solid material between the primary reactor cell and the surrounding building with a pre-determined melting point. Thus it would remain a hot solid and good insulator for operational conditions but then melt as soon as the temperature inside the primary cell started to raise in temperature. The liquid then runs off to expose the hot primary cell wall to radiate heat into the surrounding building. This also adds a little latent heat of fusion too.

Another comment is I as well like the idea of the outer containment building being the final decay heat step (a large surface area radiating at a modest heat under 200 C, maybe under 100 C). I'd add that one likely wouldn't even need a buffer salt surrounding the core vessel to do this since for most designs of modest power densities once can radiate enough heat from 500 to 700 C vessel walls to get ride of decay heat. For example, an early version of the big AHTR was shown to be able to radiate 0.5% of full power from the reactor walls at only 500 C (forget which paper mentioned this but easy to calculate yourself). Not saying this is better than having a buffer salt but an option to consider.

I should add in a link to the work some students at University of Cincinnati did last year where I acted as the "client". Their adviser directed them away from DRACS and to bayonet tubes.

http://thorium.mine.nu/content3/TEA/Senior_Design/Decay_Heat_DMSR_Report.docx

David LeBlanc


Top
 Profile  
 
PostPosted: Sep 18, 2012 2:34 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 247
David,

Re phase change from insulator to conductor.

We spent a lot of time and effort on this concept
and finally moved away from it.

The problem is that in any real world geometry
there is a sizable range of temperatures.
So the transition from fully solid to fully liquid
is quite complicated and takes place over a pretty big temperature range.
To make matters worse, there is a big change in density with the phase cahnge
with the liquid tending to overlay the solid.

We came to the conclusion that there was no way
we could reliably analyze this process.

Jack


Top
 Profile  
 
PostPosted: Sep 19, 2012 7:35 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
David, thanks for your kind words and for the student paper, very interesting. Too bad they went for the steam boiloff idea. All of the recent AHTR work explicitly stated that avoiding volatile incompatible substances such as water, is an important design criterion for the AHTR (in order for them to use the cheap compact filtered confinement containment). The issues with water are further that it can run out, or fail to inject (sheared connections, valves, tubing, etc.). It is simply not a category A passive cooling system, by IAEA definitions. If there's LOCA in the water pipeline, all cooling is gone. Futhermore, zinc is corrosive to almost all materials compatible with fluoride salt. The zinc will also boil at a temperature just above the normal operating temperature. Rupture of bayonet tubes could cause a steam explosion in the zinc. It seems like a very unworkable scheme that requires a lot of new R&D.

Iain, thanks for pointing out the continuous operation advantage. This is certainly a big advantage, as it avoids the failure mode of "what if the decay heat removal system doesn't activate". I had forgotten that your point about the freeze valves was also one of time required for drainage. Though this seems like an issue only for high power density cores, especially ones without graphite. Even there though it could be just operational safety, ie investment protection rather than public safety protection. Even if the vessel fails due to accellerated creep at elevated temperatures, the salt can still drain through the sink shaped hot cell floor. There it will drain to the dump tank. But that's where my concerns about failure modes in coolability start again. In my opinion we need to look out for failure modes that take away all decay heat removal, as that seems to be the only significant risk of fission products going out (along with putting volatile coolants into the reactor, that seems particularly stupid).

Jack, fully agree, multiple phases make things complicated. That also prompted me away from the CsF phase change insulator-around-dump-tank. Though I think that CsF or LiF filled spheres or logs in the dump tank are still viable options, especially for an air cooled dump tank. One of the big MSR selling points rarely mentioned, is that everything occurs in single phase. No things like dryout of fuel rods, cavitation etc. This greatly simplifies various aspects of the design, including accident analysis.


Top
 Profile  
 
PostPosted: Sep 19, 2012 8:07 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
David wrote:
Another comment is I as well like the idea of the outer containment building being the final decay heat step (a large surface area radiating at a modest heat under 200 C, maybe under 100 C). I'd add that one likely wouldn't even need a buffer salt surrounding the core vessel to do this since for most designs of modest power densities once can radiate enough heat from 500 to 700 C vessel walls to get ride of decay heat. For example, an early version of the big AHTR was shown to be able to radiate 0.5% of full power from the reactor walls at only 500 C (forget which paper mentioned this but easy to calculate yourself). Not saying this is better than having a buffer salt but an option to consider.
David LeBlanc


I looked into that before, but gave up after I noticed the heatup of the vessel before you get down to 0.5% of fullpower. Even for an ORNL DMSR. It gets hot enough to cause short term creep failure of Hastelloy N. This will work only if the heat loss is much bigger than 0.5%. Which I don't think is possible based on my software (it is firefighting based software but it is accurate on radiative heat transfer). If the vessel is in a pool of coolant, then it is much easier to lose ~3% continuously, and that heat can also be mostly used in a nonsafety buffer salt HX to make more electricity. If heat transfer from the vessel is insufficient, use a PRACS.

There are other issues too, for designs that don't use buffer salt. There's no gamma shielding. Putting steel blocks around the vessel complicates the decay heat removal. The thermal resistance of the blocks means that it needs to be convective which greatly increases the temperature of the hot cell. Without shielding everything becomes inaccessible, even to robotics. This is only acceptable for RVACS, and in my opinion you don't want to use RVACS without a buffer salt for a molten salt reactor.

With buffer salt there's real good shielding. It captures the gamma radiation and makes electricity from it. How cool is that. Plus the buffer salt can be used to eliminate deadweight stresses on the vessel, very important for a low power density design. Buffer salt can also be used to put radioactive stuff in. The offgas tanks for example. So they benefit from the same shielding. The buffer salt's thermal mass also puts of any freezing transient or accident for many days. In beyond design basis events (vessel rupture) the temperature stays lower and the buffer salt dilutes the fission products that helps to sequester and cool them in that buffer salt.


Top
 Profile  
 
PostPosted: Oct 08, 2012 7:07 am 
Offline

Joined: Dec 26, 2007 11:45 am
Posts: 191
I was wondering: in a pool-based design, does the buffer salt also act as the secondary heat exchange loop? What are the pros and cons of using the buffer salt for this vs. using a separate dedicated set of pipes?


Top
 Profile  
 
PostPosted: Oct 08, 2012 9:26 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
Owen T wrote:
I was wondering: in a pool-based design, does the buffer salt also act as the secondary heat exchange loop? What are the pros and cons of using the buffer salt for this vs. using a separate dedicated set of pipes?


Probably not, for three reasons. Gamma activation from delayed neutrons, possible fission products in the event of HX leaks, and tritium in the secondary loop. I'd prefer to not have that in the buffer salt. That way the buffer salt pool acts as rad shield and makes the hot cell relatively accessible (exept for the higher temperature of course).

Nevertheless, the buffer salt would have its own power-cycle heat exchanger installed, to keep losses to a minimum and generate useful power from the leaked heat of the primary loop.

It does mean an extra heat exchanger. But that generates useful work, so should be a good investment.


Top
 Profile  
 
PostPosted: Oct 11, 2012 11:13 am 
Offline

Joined: Dec 26, 2007 11:45 am
Posts: 191
If a buffer salt pool provides radiation shielding it may actually be possible for a person to enter the hot cell with the right kind of protective gear. Not saying it's a good idea, but I can imagine the hellish scene, with the orange blackbody glow and the blue Cherenkov radiation from the pool...


Top
 Profile  
 
PostPosted: Oct 15, 2012 2:51 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
Owen T wrote:
If a buffer salt pool provides radiation shielding it may actually be possible for a person to enter the hot cell with the right kind of protective gear. Not saying it's a good idea, but I can imagine the hellish scene, with the orange blackbody glow and the blue Cherenkov radiation from the pool...


Yes, that's the idea. In the unlikely event that the remote maintenance equipment isn't sufficient for a maintenance job, people could enter in reflective suits (like the ones volcanic lava researchers use).

Also, even with remote maintenance, having no gamma and neutron radiation helps a lot, just providing cooling to the robotics is all that's needed.

The equipment is all very similar to a standard aluminium manufacturing plant. Large fluoride salt baths, with some remotely operated cranes and manipulators. In fact, the temperature is lower for us and there's no volatile fluorides (HF) evolving from the bath. Not to mention having to deal with the liquid aluminium!


Top
 Profile  
 
PostPosted: Oct 17, 2012 3:33 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 914
Location: Ottawa
Cyril,

Just noticed a pretty significant disadvantage of using a buffer salt in contact with the reactor vessel that you didn't seem to list. In the standard MSBR design of the primary vessel in a sealed hot cell and using drain tank that any leak of the primary circuit (vessel wall, piping to IHX etc) would simply collect on the sloped floor and be redirected by gravity to the same decay heat dump tanks (ORNL had a thin metal membrane where the salt collected above the dump tank that they could puncture when needed if memory serves).

When you have a buffer salt around the primary circuit though, any leak would mean fission products and fissile elements will be mixed in with the buffer salt. No real safety concern to the public but it does mean any leak might be enormously expensive to clean up the buffer salt (might have to resort to disposing of all of it as high level waste). I suppose we have a similar situation with the primary to secondary salts but they typically arrange the pressures for coolant salt to leak inwards, not fuel outwards.

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.

In the SmAHTR design it is even better, they have a 9 m tall, 4 m wide vessel which would radiate 2500 kw at 500 C if they let it (2% of full 125 MWth power). I know ORNL didn't really want to consider this option for the SmAHTR as they wanted to stick to the usual engineering mantra of keeping the vessel wall as cool as possible during normal conditions. They have a new alloy they'll be releasing details on soon though that maybe will be more tolerant of higher operating temperatures. I can't recall what emissivity I was assuming (got these numbers from my notebook, sorry if any mistakes) but easy to get that up pretty high.

I'd also remind readers of the excellent discussion of all this in another thread by Cyril

http://www.energyfromthorium.com/forum/viewtopic.php?f=3&t=3311

David LeBlanc


Top
 Profile  
 
PostPosted: Oct 17, 2012 3:39 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 914
Location: Ottawa
Just noticed Cyril mentions 11 kw/m2 radiated from 550 C buffer salt to a 250 C surroundings (on another thread). I was just mentioning about 20 kw/m2 at 500 C and 50kw at 700 C so perhaps I'm being a bit optimistic (or maybe liquid surfaces have lower emissivity than solid metals?). Again, no promises on my math....

David LeBlanc


Top
 Profile  
 
PostPosted: Oct 18, 2012 5:30 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
11 kWt is a conservative lower bound. Because liquid fluoride emissivity is uncertain to me, I assumed it to be 0.5. So if the emissivity is 1 it is twice that, 22 kWt.

Re leaks. Yes, good point, but my defence is this. While true that leaks would be more a problem to clean up, they are also much less likely. The hydrostatic pressure from the buffer salt can be designed to just about match the hydrostatic pressure of the fuel salt. So you're eliminating the driver for large leaks in the first place, which is even better I would argue than cleaning up major spills.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 46 posts ]  Go to page 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:  
cron
Powered by phpBB® Forum Software © phpBB Group