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 Post subject: Molten Salt Pumps
PostPosted: May 24, 2013 7:25 pm 
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Here is a link to some solar based molten salt pumps. They have long shafts to reduce heat to the motor and heat loss, but it would also help reduce the radiation dose to the pump motor and allow better motor maintenance access.

http://www.nrel.gov/csp/troughnet/pdfs/ ... _pumps.pdf


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 4:06 am 
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Long shafted pumps indeed allow easier shielding. But that's not really the biggest problem area.

The biggest problem area are seals and bearings. Seals in particular, in my experience, are unreliable with conventional pumps. They are best used when pumping non toxic fluids such as water heaters and such.

Vibration is often an issue with long shafted pumps. If you put a bearing in the salt then vibration is greatly reduced, but that switches the problem to a bearing. Bearings in nitrate salt are relatively easy, as the nitrate salt provides and maintains the protective oxide coating in stainless and higher alloy steels, superalloys etc. With fluorides, these don't provide a coating, they in fact dissolve it (flux agent). The bearings will then diffusion weld when idle, and require hard metals to prevent abrasion.

Silicon carbide and tungsten carbide bearings can be used.

The seal problem is more tenacious. If a serious seal failure would occur, potentially a large amount of activity would contaminate the buildings and worst case leads to a scandal and decommissioning of the reactor. Not a small risk. Ideally a canned rotor is used that eliminates seals, in stead you have welds. But it would require huge R&D to develop these for high temperature fluoride salt application.

If a low pressure drop core and heat exchanger can be designed, it may be possible to put the pumps in the cold leg, and it may be feasible to engineer canned rotor pumps at that temperature. It won't be very efficient but we don't need as much pump power as a PWR or gas cooled reactor.

Needless to say the pumps remain a major trouble area.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 12:22 pm 
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A couple of questions:
1) for the bearing - would a magnetic bearing work? Could we put an electromagnet on the rotor and another on the outside using similar technology as a maglev train?

2) for the seal - I would guess we want to seal the shaft so that off gases don't propagate into the motor area. We could overpressure the motor area so that any minor leak would pump helium from the motor into the offgas system. But I think we could seal the whole motor as a secondary (and likely a third) layer of isolation of the fission products from the interior of the building. The first seal is tough and we should anticipate R&D to improve it and expect that this might be a maintenance item. But the second and third seals are at low pressure, low temperature, and benign materials so I don't see why they should be challenging. So the risk is contamination of a motor not a building.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 2:08 pm 
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Lars wrote:
A couple of questions:
1) for the bearing - would a magnetic bearing work? Could we put an electromagnet on the rotor and another on the outside using similar technology as a maglev train?
Probably not. The salt temperatures exceed the Curie temperature of most strong magnet materials.

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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 4:25 pm 
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KitemanSA wrote:
Lars wrote:
A couple of questions:
1) for the bearing - would a magnetic bearing work? Could we put an electromagnet on the rotor and another on the outside using similar technology as a maglev train?
Probably not. The salt temperatures exceed the Curie temperature of most strong magnet materials.


The salt outlet temperature from the core does, but the inlet doesn't. That's why, if we can put the pumps in the cold leg (requires low pressure drops in core & HX), a canned rotor pump might work.

However, if we can use a canned rotor pump in the cold leg, we likely want a short shafted pump, and there may not be issues with bearings at all. It won't be effcient in any case at temps of 550 Celsius, but who cares much about that really, we have tiny pumps compared to PWRs.

Canned rotor pump technology was actually invented for the nuclear industry, for safety reasons. That's with a relatively low radioactivity pressurized water coolant. With molten fuel salt there would be a far greater need for canned rotors.

The seal problem is mostly due to the need for a rotating seal at high temperature and possible high radiation without degradation. If the seal is not moving, high reliability metal seals, heat and radiation resistant, are available commercially.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 8:01 pm 
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Cyril R wrote:
KitemanSA wrote:
Lars wrote:
A couple of questions:
1) for the bearing - would a magnetic bearing work? Could we put an electromagnet on the rotor and another on the outside using similar technology as a maglev train?
Probably not. The salt temperatures exceed the Curie temperature of most strong magnet materials.


The salt outlet temperature from the core does, but the inlet doesn't. That's why, if we can put the pumps in the cold leg (requires low pressure drops in core & HX), a canned rotor pump might work.
What magnetic material would you use and what is your assumed "inlet" temp?

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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 8:04 pm 
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Wouldn't a hydraulic motor powered pump using the fuel salt as the hydraulic fluid be a simpler design?

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 Post subject: Re: Molten Salt Pumps
PostPosted: May 25, 2013 8:21 pm 
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If we have electro-magnets on both the rotor and stator does the Curie temperature matter?


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 1:51 am 
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Lars wrote:
If we have electro-magnets on both the rotor and stator does the Curie temperature matter?


It does, because even iron loses its magnetism above a certain temperature. It's much higher than the inlet of 550-575 degree Celsius.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 1:53 am 
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KitemanSA wrote:
What magnetic material would you use and what is your assumed "inlet" temp?


Probably iron, possibly with a coating if necessary. The temp would be around 550 C in the pump, then through the core, 700 C out of the core.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 5:02 am 
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If your pool type reactor vessel is suspended from the top in a vat of buffer salt which takes all the weight, and the reactor internals/ graphite/ inlet and outlet ducts were all suspended independently inside that, would it be possible to spin the whole vessel, and use vanes built in round the bottom of it as the pump? The rollers under the lip of the vessel would have to be cantilevered back from it so they could be actively cooled, but would not be load bearing. There would be drag from both the buffer salt and the fuel salt,and the pump would be high volume, low velocity, but coast-down in a power failure would be impressive. I suppose if it's a two fluid reactor the spinning vat would be between fuel and fissile salts.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 6:06 am 
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jon wrote:
If your pool type reactor vessel is suspended from the top in a vat of buffer salt which takes all the weight, and the reactor internals/ graphite/ inlet and outlet ducts were all suspended independently inside that, would it be possible to spin the whole vessel, and use vanes built in round the bottom of it as the pump? The rollers under the lip of the vessel would have to be cantilevered back from it so they could be actively cooled, but would not be load bearing. There would be drag from both the buffer salt and the fuel salt,and the pump would be high volume, low velocity, but coast-down in a power failure would be impressive. I suppose if it's a two fluid reactor the spinning vat would be between fuel and fissile salts.


Now that's outside the box thinking!

But how do you affect a seal between the ducting for secondary coolant salt, and the rotating vessel? If we need a rotating seal on the vessel, we haven't improved on the situation.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 6:35 am 
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Cyril R wrote:
KitemanSA wrote:
What magnetic material would you use and what is your assumed "inlet" temp?


Probably iron, possibly with a coating if necessary. The temp would be around 550 C in the pump, then through the core, 700 C out of the core.
Do you think plain iron magnets will provide enough strength to perform as a magnetic bearing?

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 Post subject: Re: Molten Salt Pumps
PostPosted: May 26, 2013 6:40 am 
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KitemanSA wrote:
Cyril R wrote:
KitemanSA wrote:
What magnetic material would you use and what is your assumed "inlet" temp?


Probably iron, possibly with a coating if necessary. The temp would be around 550 C in the pump, then through the core, 700 C out of the core.
Do you think plain iron magnets will provide enough strength to perform as a magnetic bearing?


No good for bearings. But good for a rotor. Salt lubricated bearings could be commercially available silicon carbide bearings in tungsten carbide sliders.


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 Post subject: Re: Molten Salt Pumps
PostPosted: May 27, 2013 11:31 pm 
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jon wrote:
If your pool type reactor vessel is suspended from the top in a vat of buffer salt which takes all the weight, and the reactor internals/ graphite/ inlet and outlet ducts were all suspended independently inside that, would it be possible to spin the whole vessel, and use vanes built in round the bottom of it as the pump? The rollers under the lip of the vessel would have to be cantilevered back from it so they could be actively cooled, but would not be load bearing. There would be drag from both the buffer salt and the fuel salt,and the pump would be high volume, low velocity, but coast-down in a power failure would be impressive. I suppose if it's a two fluid reactor the spinning vat would be between fuel and fissile salts.


Cyril R wrote:
But how do you affect a seal between the ducting for secondary coolant salt, and the rotating vessel? If we need a rotating seal on the vessel, we haven't improved on the situation.


Suppose you have Jon's spinning vat. The graphite in the vat is attached to the vat. At the periphery of the vat, you have the primary heat exchangers. The PHXs do not spin. The only rotating "seal" in the system is the top surface of the vat to the cover gas. The cover gas is contaminated with fission products that have to be separated from it.

The cover gas seems like a fairly bad contamination path from the fuel salt to the buffer salt. You could route the suspension and pipework for the PHXs through the central axis over the rotating vat, and then have a partial "lid" on the vat, so that the gap between the lid and the nonrotating PHX suspension/pipework was small. Then you could suck the cover gas from above the vat, through a pipe in that pipework, through a separator, and blow the clean gas back to the space over the buffer salt. The rotating "seal" now prevents fission products from escaping to the buffer salt gas cover by forcing them to head upstream.

This is an active solution. When a plant power failure happens, the cover gas separation system stops, but presumably some amount of fission product gases will continue to evolve from the fuel salt. I don't see how to stop these from getting to the gas space over the buffer salt. And I think that plant power failures are a given. If a time limit can be put on the evolution of fission product gases from the salt, then a passive vacuum tank might be provided to maintain the flow of cover gas for some time after power stop.

But I don't think a time limit can be put on fission product evolution to the cover gas. If I understand correctly, alpha particle and perhaps beta particle decays at the surface of the radioactive fuel salt will cause microscopic explosions, which will form microscopic fuel droplets, which will form an aerosol and stay airborne. In combination with afterheat evolving from the fuel salt, these will tend to rise with the fuel salt cover gas and out into the surrounding buffer salt cover gas.


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