Energy From Thorium Discussion Forum

Molybdenum alloys must be welded in a vacuum chamber or under a very pure protective gas. That means a primary circuit consists of many pieces assembled on the construction site by flanges screws and of course gaskets.

Yesterday I visited Gysi manufacturer of valves and gaskets mainly working for chemistry, pharmacy, power generation and a bit nuclear (Switzerland).

For high temperature applications they offer gaskets made of expanded graphite - expanded metal (SS) up to 450°C, and mica (phlogopit KMg3[(F,OH)2|AlSi3O10][)- expanded metal suitable up to 1000°C. The later one is used for exhaust channels.

Does anyone know if phlogopit is corrosion resistant vs. chloride/fluoride salts?

Holger

If its oxide/hydroxide then it will be generally poor since all oxides have at least some solubility in fluoride salts. Attack via ion exchange is probably also an issue.

Gaskets submerged in fluoride salt are, in my opinion, both a major risk and completely unnecessary. All welded fluoride contacting parts is feasible. You can have metallic gaskets (like a cavity seal) in the gas space above where needed.

Hi Cyrill,

we are here in the chloride salt reactors....

the phlogopit is indeed a silicate. I read reports about the corrosion of ceramics - SSIC corrodes - Al2O3 does not corrode in chloride salt 850°C.

Are there any reports about the corrosion of such silicates in chlorides?

If you will not have flanges in your reactor design...Will you weld pumps and heat exchangers to the reactor vessel? Will you ship the whole primary circuit to the construction site?

Best regards

Holger

According to this,

http://www.sintef.no/home/Publications/ ... TEF+S10287

alumina has some solubility in chloride salt. Lower than in fluoride but not zero, so unlikely to be acceptable in long term flow salt service.

Aluminium metal tends to chlorinate, and free oxygen will pass into salt as oxychloride impurity so if the alumina is not precisely stoichometric it would be worse. Ceramics aren't as low permeable as alloys.

A small reactor could have integral design, with the HXs and pump impellers in the vessel. A large reactor would weld pump/HX modules to the reactor vessel onsite.

A molten salt reactor vessel is a nasty thing, you don't want to open it on a regular basis. Probably all welded design for the vessel is feasible. For some components a long lifetime may be more difficult, a metallic cavity seal in the cover gas phase is a good option there (like the pump impeller).

Metal seal rings work well with Graylock type connectors.....

Hy Cyrill,

graphite gaskets face the challenge of carburization?

That means a gold-platinum, or gold-nickel alloy is the ideal material for gaskets.

These alloys are somehow soft resistant vs. corrosion.

Holger

Hi Jaro,

what is the main advantage of the Graylock vs. a standard screwed flange?

Best regards

Holger

Graylocks might work for smaller piping. Difficult for large vessels and such. Also isn't the metallic part subject to pressure welding? Are there pressure welding resistant alloys that also resist fluoride? Higher carbon steels tend to be good in terms of pressure welding but are poor in fluoride salts.

Graphite carburizing - why would this be a problem in a molten salt reactor environment? Some carbon diffusion would occur but only in the surface layers. Carbon diffuses very slowly. But graphite is not a good sealant from its poor strength and brittleness. You want something strong and ductile, exactly opposite to graphite.

Re graylocks, like the picture shows they don't force the seal apart like a conventional flange. So seal leakage is far less likely.

Hi Cyril,

2 days ago I visited Gysi a company making valves and gaskets for chemistry, pharmacy, power plants and as well nuclear.

Graphite and expanded graphite is widely used for gaskets usually with a expanded metal insert.

A restriction is that it should not be used in contact with air at such temperatures.

Best regards

Holger

For piping and flanges something like this looks ideal, some kind of Vee band closing mechanism with either face to face sealing (gasket free) or a metallic insert of some kind. For large diameter flat flanges say for a reactor vessel head, a metallic O ring or multiple O rings may work well. Most pure metals are quite soft and complaint, it should not take long to find a compatible metallic O ring material.

For the big flanges these could always be backed by a helium pressurised zone and monitor space if one wanted to go to active backup sealing and monitoring. I don't think that this is required, but may be worth considering.

High pressure, high temperature metallic cavity seals are commercially available from a few specialized suppliers. There's no need to reinvent the wheel, as long as we provide the cavity seal in the cover gas space (not in fluoride salt).

The highest performing material is typically Inconel 718. For ultra high temperature it is less suitable though.