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PostPosted: Sep 30, 2013 12:02 am 
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Dear all, do you know how to determine the molten salt reactor temperature ?
As I know, inlet and outlet temperature for MSRE is 632 and 654 Celsius
FUJI-U3 is 840K and 980K (547 and 687 Celsius)
MSBR is 565.85 and 704.85 Celsius
Except fuel salt thermal properties and reactor power and flow rate, are there other limit for core temperature?


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PostPosted: Sep 30, 2013 12:21 am 
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The maximum allowed fuel salt temperature will be set by the mechanical strength of your reactor/pipes/pump impeller/HX. The minimum allowed fuel salt temperature is set by salt freezing or perhaps more precisely when the salt viscosity increases too much (more so with beryllium).

ORNL did worry about a large thermal difference between cold and hot since it creates more challenges with one form of corrosion where a component of the metal vessel is more soluble at high temperature than cold. This could continuously transport that element from the hot zone to the cold zone. I believe after the experience with MSRE they concluded that this wasn't really a problem. Chromium did indeed thermally transport but only from the surface of the vessel. The chrome did not diffuse within the metal so the chromium depletion was only at the surface and no harm done.

The average temperature in operation is set by the reactivity of the fuel salt. Add more thorium and the reactivity will go down and as a result so will the average temperature.

The actual temperature difference will be set by how much heat you extract from the reactor. The fuel salt heat capacity and flow rate will dictate a required temperature difference to achieve the heat transfer.

MSRE thermal production was limited by their heat sink. MSRE dumped its heat into the air using a salt->air radiator that could dump around 8MWatts. Hence we see a small temperature difference. It was sufficient for most of their purposes - though it did not test the maximum temperature difference that was planned for MSBR.

The high temperature differences are more challenging in the design. Emergency situations can cause severe thermal stresses. MSBR addressed these with thermal shields at the inlet and outlets of the core reactor. Another approach would be to use a pool of buffer salt.


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PostPosted: Sep 30, 2013 12:21 am 
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And aircraft reactor are 648 and 815 centigrade for core inlet and outlet temperature
There is little difference for different PWRs, its inlet and outlet temperature are about 290 and 330 centigrade.

There is so big temperature margin for molten salt reactor.
If we want a certian reactor power, we can change the flow rate of fuel salt and temperature rise between inlet and outlet of core, if only we keep the product of temperature rise and flow constant, so how to choose the best temperature and flow value for MSR operating?


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PostPosted: Sep 30, 2013 12:43 am 
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It is a complicated tradeoff. Much depends on your pump and heat exchanger. The higher flow rate means a lower temperature swing to delivery the same power. Generally, a lower temperature swing is easier on the power plant. The higher flow rate though means more power is needed in the pump. In addition, a higher fluid velocity will tend to create vibrations in the heat exchanger (it was this problem that shutdown San Onofre). MSR paper designs tend to presume around 100C temperature difference so this is a good starting point. Then as the design is refined you can adjust based on which is the harder problems to solve.

Note that if the reactor isn't built out of Hastalloy but rather out of a more exotic metal (moly for example) then a higher temperature can be tolerated and a large swing may be acceptable. One day we might even be able to use graphite based vessel, plumbing, hx, and pump impeller and allow very high temperatures and swings.


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PostPosted: Sep 30, 2013 1:43 am 
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Lars wrote:

Note that if the reactor isn't built out of Hastalloy but rather out of a more exotic metal (moly for example) then a higher temperature can be tolerated and a large swing may be acceptable. One day we might even be able to use graphite based vessel, plumbing, hx, and pump impeller and allow very high temperatures and swings.


Sounds exciting! Some people are talking about carbon-carbon material, better high temperature tolerance and irradiation stability than graphite, that means we could operate the reator from freeze point to boiling point of fuel salt, maybe the only restrict is from the steam generator(if it is a MSR plant)


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PostPosted: Sep 30, 2013 2:18 am 
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ORNL was always worried about the fuel salt inventory, because they wanted a good breeder and have low fuel salt cost. So they went for an extremely compact HX (for a tube in shell HX at least). It was so compact for such a tube in shell exchanger, that it needed large temperature differential across the tubes and within the fluid also, and even then the pump power and pressure drop was considerable. In fact one hesitates to call it a low pressure reactor, because of all that driving pump pressure needed. Today most of us are not so worried about a 20% increase in fuel salt so a bigger HX would be possible. That can be used to reduce the temperature drop while keeping pump power about the same. Or it can be used to reduce the approach temperature (ie increase the secondary coolant outlet temperature) while keeping the other things about the same. With HX design, optimizing for one aspect causes a tradeoff in other aspects. And there are a heck of a lot of aspects. Salt freezing, pump power, vibration, transient performance/natural circulation, dissolution driven corrosion, trifluoride solubility, and possibly also erosion from noble metal particles (if extremely large flow speeds are employed).

A more enticing prospect - for me at least, not everyone agrees - is to use advanced compact heat exchangers, so that most of the tradeoffs essentially disappear (no vibration damage, very compact HX even with small temp drops, low pump power, 99% reduced leakage probability).


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PostPosted: Sep 30, 2013 9:36 am 
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Cyril R wrote:
Today most of us are not so worried about a 20% increase in fuel salt so a bigger HX would be possible.

ORNL was concerned about the fraction of delayed neutrons lost and how that might impact stability (and make quickly stopping fission on stop of fuel flow harder). ORNL was targeting 1/3 of the fuel salt out of core. The French thought they could maintain stability even with 1/2 the fuel salt out of core. Do you think these are misplaced concerns?

Yes, the compact HX is certainly attractive. Not knowing enough about compact HX's I was thinking that the HX would also be the piping to get fuel salt from the top of the reactor to the bottom.


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PostPosted: Sep 30, 2013 10:38 am 
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Lars wrote:
Cyril R wrote:
Today most of us are not so worried about a 20% increase in fuel salt so a bigger HX would be possible.

ORNL was concerned about the fraction of delayed neutrons lost and how that might impact stability (and make quickly stopping fission on stop of fuel flow harder). ORNL was targeting 1/3 of the fuel salt out of core. The French thought they could maintain stability even with 1/2 the fuel salt out of core. Do you think these are misplaced concerns?


Yes I do believe that they are misplaced, as both doppler and salt expansion are much faster than any conceivable flow stoppage, and also the total reactivity worth of the delayed neutrons just isn't large so a 100 pcm more or less isn't going to be a major issue, but that's another discussion. For this discussion, let's assume they are real concerns.

ORNL's external loop had about an 18 m3 volume, but a minority of that is actually in the tubes of their HX.

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

30.4 m3 salt in the reactor vessel, 48.7 m3 total, so 18.3 m3 in the external area. Amount of fuel salt actually in the tubes of the primary HX is only 7.6 m3. Say you doubled that, to allow a halving of the temp rise to 70C in stead of the usual 140C. 7.6 m3 more fuel salt is an increase of 15% fuel salt, this isn't going to break the bank, and you still have more than half the fuel in the reactor. This means you have some design space.

Quote:
Yes, the compact HX is certainly attractive. Not knowing enough about compact HX's I was thinking that the HX would also be the piping to get fuel salt from the top of the reactor to the bottom.


How important is this? According to ORNL-4541, the amount of fuel salt in the piping is only 4.1 m3, or 8.4% of all the fuel salt. Pipes are big diameter so you can have high flow speed in them without great pressure drop/pump power. Erosion is no problem in wider diameter pipes because they are relatively thick and you can add a lot of erosion allowance (thickness). It's also easy to deal with vibration damage in pipes, as you can support them externally, the difficulty with vibration damage is in complex HX tube bundles and baffles and whatnot that all have to be supported together somehow in a very challenging fluid environment.

More important I think will be a design that promotes natural circulation, so fuel salt should flow down and coolant salt should flow up.

Compact HX solves many of the problems. It's possible to cut pressure drop (pump power) >10x while cutting the size 4-8x while also cutting the temp drop across to the secondary salt by more than a factor of 4. You can have all of this at the same time, rather than making a big tradeoff somewhere and having just one of the improvements. They are a surprisingly versatile product, much more so than tube in shell. You can make them very long if needed, with airfoil design the length is no longer the problem it was with the previous herringbone shaped PCHEs. They have much more flexibility in the inlet and outlet plenum locations, and can even support 3 different fluids in one exchanger (though probably not needed for our application). PCHE have no vibration damage, and no heat affected zone and weld heterogeneity corrosion. They can be designed to be leak-free, a very important advantage for any molten salt reactor.


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PostPosted: Sep 30, 2013 1:50 pm 
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I believe the reason UCB gave up on compact HX's
is the thermal stresses associated with a loss
of fluid on one side. Have not done the analysis myself.


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PostPosted: Sep 30, 2013 2:17 pm 
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djw1 wrote:
I believe the reason UCB gave up on compact HX's
is the thermal stresses associated with a loss
of fluid on one side. Have not done the analysis myself.


Yes, Dr. Peterson himself told me this. But it appears just a "hunch" in the sense that it wasn't a design variable. Inasmuch, Per wasn't able to substantiate the Berkelely decision. I have had extensive communications with a couple Ph.D people from Heatric, the primary PCHE manufacturer. Turns out that you can use geometry of the plates, thickness of the plates, and such, to accomodate the transient stresses. This has recently been confirmed by Iain Mcclatchie, who looked at a horseshoe curved HX. In addition in my own experience, highly reliable trip mechanisms, like SIL-3 classification trip loops, are available which will allow us to deal with offnormal events in a satisfactory way by simply tripping the pumps on redundant temp measurement logic. Of course there is always the one-in-a-million years event that will fail the trip and result in HX damage, but it's an investment protection issue really. Without a pumps trip, you get massive issues with fuel or coolant salt freezing, even with conventional HX design. Reliable trip logic is needed in any MSR design or you will risk freezing.


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PostPosted: Sep 30, 2013 8:17 pm 
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longwei1221 wrote:
Lars wrote:
Note that if the reactor isn't built out of Hastalloy but rather out of a more exotic metal (moly for example) then a higher temperature can be tolerated and a large swing may be acceptable. One day we might even be able to use graphite based vessel, plumbing, hx, and pump impeller and allow very high temperatures and swings.
Sounds exciting! Some people are talking about carbon-carbon material, better high temperature tolerance and irradiation stability than graphite, that means we could operate the reator from freeze point to boiling point of fuel salt, maybe the only restrict is from the steam generator(if it is a MSR plant)
I've often wondered if a boiling salt reactor might be possible. Hmmm.

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PostPosted: Sep 30, 2013 11:57 pm 
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Boiling aluminium chloride, boiling sodium, potassium and mercury have all been looked at as high temperature liquid/vapour cycles :shock:


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PostPosted: Oct 01, 2013 11:04 am 
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Boiling certainly has some advantages, such as near isothermal operation, possibly even on natural circulation.

But it also has loads of downsides, mostly relating to the issues of two phase flow. Just look at how much trouble the BWRs have had with flow instabilities and deposition of corrosion products, etc. And that's with the best known liquid and boiling process possible - water boiling to steam. Several decades of R&D would be needed to make this work for molten salt reactors, and even then you have lots of issues like fissile material precipitating somewhere, transient behaviour (possible pressurization, reactivity insertion from this) etc.

In fact, one of the advantages of the molten salt reactor is that everything is always in single phase, even during accidents.

Then there are issues with the coolants that Lindsay mentions. Mercury is rare and toxic and too volatile at elevated temperatures, sodium is chemically reactive which is nasty when combined with the activity from Na-24, potassium is even worse in every respect than sodium, aluminium chloride is not the most stable of salts and it's chloride rather than the fluoride we prefer...

To make this work at all, first you'd need a stable nonreactive salt. Possibly it would work with something like 7LiF, under vacuum it boils at reasonable temperatures (vacuum also solves some accident problems) and you'd want TRISO fuel not molten salt fuel to avoid many problems of fission products clogging and blanketing condensers and fissile precipitations etc. I'm personally quite interested in such a design, though most have been less than supportive of the idea so I don't mention it much. Probably no one would even be interested in paying for the basic R&D.


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PostPosted: Nov 05, 2013 1:07 am 
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Even though core temperature is lower than salt boiling point, there are several hundred degree range for operation temperature. Maybe we could try different temperature while operating with control rod adjustment.


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PostPosted: Nov 05, 2013 2:16 pm 
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KitemanSA wrote:
longwei1221 wrote:
Lars wrote:
Note that if the reactor isn't built out of Hastalloy but rather out of a more exotic metal (moly for example) then a higher temperature can be tolerated and a large swing may be acceptable. One day we might even be able to use graphite based vessel, plumbing, hx, and pump impeller and allow very high temperatures and swings.
Sounds exciting! Some people are talking about carbon-carbon material, better high temperature tolerance and irradiation stability than graphite, that means we could operate the reator from freeze point to boiling point of fuel salt, maybe the only restrict is from the steam generator(if it is a MSR plant)
I've often wondered if a boiling salt reactor might be possible. Hmmm.


Boiling salt doesn't sound attractive to me. Liquid -> gas phase change normally means enormous expansion in volume or pressure. In accident conditions you will get a temperature rise and volume is fixed so we are looking at a pressure increase. Increasing pressure at very high temperature doesn't sound like a route to really build something.


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