Energy From Thorium Discussion Forum

This may seem like a dumb question, but bear with with me here...

For a reactor that let it's freeze plug melt and all fluids drained to their respective dump tanks, how does one go about reforming/reseting the freeze plug when setting up the reactor for a restart?

Do you slowly pour barely molten salts into the reactor vessel in the hope they reach the plug area, and have the freeze plug refrigeration running extra hard to cause accelerated solidification there? How do you deal with leakage until the freeze plug sets firmly?

There is another issue about freeze plugs in my mind.....

The decay heat in the fuel decreases as soon as it forms a frozen plug away from fission. If you open the freeze plug by stopping the cooling it will require the more time the longer the freeze plug did exist. What happens if the reactor is run for a couple of month. How long does it take to clear the freeze plug?

Does anyone have a recent design study about a freeze plug?

I would suggest a careful review of the ORNL MSRP documents in PDF form that I have made available online for many years now in order to answer your question.

Freeze plugs are not for rapid isolation. They are a nice simple "canned" compatible valve that can be used to assure long term isolation.

Obviously the decay heat of a tiny volume of frozen salt is nothing to the design of a freeze valve. The freeze valve, after all, has to be able to freeze a section of salt with margin to spare and salt has enormous heat of fusion.

The decay heat is for a MSFR or MCFR a couple of kW/Kg of fuel. For me as an engineer it is a major challenge. I did not really see yet a freeze valve suitable for a reactor with a high power density.

Another argument for a thermal MSR?
In our DMSR if you are willing to wait for six hours
after a drain before refilling, the decay heat is about 100 W per kg salt.

Can't you just use the freeze plug as a feedwater preheater?
While it is cooler than the actual salt its temperature is not exactly low.

The amount of salt in a freeze plug is not that big. Less than 1 kg. You can have multiple freeze plugs to avoid failure to thaw and to have increased

One interesting possibility I've looked at is to use a tiny PCHE as the freeze plug. One side contains salt, the other pressurized helium that is actively cooled. Big plenums accomodate volume changes.

You want a certain amount of self heating because otherwise it could take a long time to thaw the plug. So decay heat is good.

Cyril...that is a brilliant idea.

The valve can be made by EBM or diffusion bonding with a very efficient cooling/heating.

The main issue could be the plating out of noble metal fp on its surface. That would require a regular replacement of the valve to reduce the decay heat. Perhaps having 3 valves. Within each maintenance period the valve is exchanged... let the decay heat go down and reinstall it by a robot.

It would be great to have a file of great ideas.

Long time lurker, first time poster...

How much of a concern is it when the freeze plug starts melting? At some point before it melts completely, I expect the pressure on the freeze plug would be enough to push it out of the way. That starts the drain tank being filled up, the salt cools and possibly solidifies. Then after the situation is addressed the salt is melted and pumped back into the fission chamber.

But consider when the (still partly solid) plug starts getting moved to the drain tank. Does this damage the piping to a meaningful extent?

I haven't been able to find an answer in the ORNL documentation.

Why would it damage the piping? It's a very small amount of salt, about a kg or less as Cyril said. The fuel salt is close to atmospheric pressure, there wouldn't be much force to drive the freeze plug out very rapidly.

If you leave no space for the salt to expand out to, its going to rupture anything, even a thick Inconel vessel would be damaged. If your alloy is ductile enough it could just deform and be done with it but your coolant line/HX for the freeze valve is going to be ruined. So, enough space must be reserved for expansion.

Practically a good place for a freeze valve is in a loop seal. The loop's always got some salt in it so you don't have to worry about not having salt to re-freeze and seal the freeze plug again. This approach requires a separate way to get the salt back into the reactor vessel, there are various approaches to do that.

I just read this paper:
http://samofar.eu/wp-content/uploads/20 ... thesis.pdf



Is this statement correct? They may have heated the plug electrically to drain the reactor. But that does not mean that the freeze plug could not be melted by overheating the reactor as well.

I am speculating as you are and I believe you are correct. I've read portions of the paper you linked to and the concern is not that the freeze plug would not melt but that it might not melt before irreparable damage was done to the reactor. The paper even offers possible solutions to the problem, such as using a different material for the plug other than the carrier salt or using a heat activated valve much like that in an automotive radiator.

The use of a freeze plug does not mean one cannot also put a mechanical valve along side it. An actively operated valve would need power to work, but then so would a heater. If the MSRE had a heater to speed the draining process then I suspect this was a "hack" to avoid having to wait for passive heat to melt the plug and also avoid having to go through the much more involved process of adding another valve.

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Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.

People on this blog should read the MSRE docs before spreading misinformation..

The MSRE freeze plug consisted of a middle actively cooled small, crimped section of pipe
with a large section of actively heated pipe on either side.
The small amount of salt in the cooled section was solid.
The purpose of the heaters in the sections on either side was to keep the salt on either side
melted and hot. When you lost power, you lost both the cooling and the heating.
But the heat in the hot salt was enough to thaw the solid salt in the middle.
So the system is passive in the sense that once you lost power,
the reactor drained with no other action on the part of the system.
To drain on over-temperature, you need a thermoswitch that passively opens on over-temp,
and cuts power to the valve.
This should be put in the high temp region of the loop.
RTFM.