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PostPosted: Mar 24, 2014 2:08 am 
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A common safety feature of LFTR designs is the drain tank. If the reactor happens to get too hot the fuel is dumped into this drain tank where the geometry does not allow criticality. The concern is then that the fuel is hot, molten salt hot, and it must be cooled off. This leads to concerns like cooling pumps and fans failing. One solution that came to my mind is adding more salt to the tank to cool it off, like putting ice cubes into a water glass.

I'm thinking that the cooling salt could be kept in the tank like gravel. The chunks of salt would be big enough to allow space in between for the molten salt to flow but small enough to give plenty of surface area.

Another idea is to have the cooling salt stored in a tank above the drain tank but ground up like sand so it flows easily. That way if the primary cooling systems work then the cooling salt will not add to the volume in the drain tank. It can be gravity fed like the drained molten fuel so that it does not have to rely on power to move.

The composition of the cooling salt will have to be considered. The obvious choice would be to have it composed of the same material as the carrier salt, that way it will not contaminate the fuel. I suppose it could be the same salt as the fuel salt, adding more ThF to the drain tank might be beneficial since it absorbs neutrons. I assume this might be problematic politically since policy makers would have to be convinced that adding fuel to the fire is actually a good idea.

The cooling salt can be a different salt than the molten fuel. Properties can be chosen to increase cooling capacity and/or ease separation so the fuel can be reused. Adding ThF as a neutron absorber might be a policy no-no but perhaps some other neutron absorber could be used to reduce any possibility of the fuel going critical in the drain tank. Not sure what might fit this criteria, cadmium fluoride perhaps?

I suppose the cooling material doesn't even have to be a salt but it does have to be a material compatible with the LFTR fuel. Putting in lithium shavings would be compatible but it would also ignite once it touches the molten salt, so that is out. I suppose it does not have to melt into the mix like an ice cube in a soda, but just soak up the heat like whiskey stones. You lose the heat soaking capability of a phase change though.

Adding something that goes through a phase change to a gas would likely really dissipate some heat but I think that anything boiling off the drain tanks would be thrown out as a bad idea.

What might be an added benefit is having cooling salt contains a denaturing material. If proliferation is a concern then put some U-238 salt into the mix. That would keep people from robbing the fissile U-233 from the drain tanks. Perhaps add some Pu-242 as well to prevent theft of any fissile plutonium isotopes.

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PostPosted: Mar 24, 2014 12:16 pm 
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Fundamentally though you are talking about adding heat capacity. This will help with the immediate problem but the total stored energy in the decay heat is enormous - too big to solve with the heat capacity of materials you put there (like your salt, or a pool of water). In addition to heat capacity to handle the short term high heat load one must have a cooling system that dumps heat to the environment. The final cooling system should be passive and will typically dump to the atmosphere or a large body of water.


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PostPosted: Mar 24, 2014 7:40 pm 
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What is the integral of the power released by the salt in the post shutdown environment?

Proper choice of eutectic salts could generate v. large effective heat capacity values.


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PostPosted: Mar 25, 2014 4:28 am 
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Perhaps I need to clarify the problem I'm trying to solve if we are going to establish if the solution I gave is a good one or not.

As Lars points out the primary means of removing the heat from the drain tank should be passive if we want to maximize the safety of the reactor. I agree, there should be some sort of passive heat sink such as convective movement of air. It seems to me that there is still a concern that the passive cooling might be insufficient and/or the fuel might have too many neutrons bouncing around.

The problem is that something went wrong, not sure what but the fuel is getting way too hot. So hot that there is the possibility of the reactor vessel and pipes getting soft and the fuel salt could start to boil. If not shut down immediately and the fuel cooled off the reactor could pop like a water balloon and you have radioactive boiling salt getting splashed all over your power plant.

An operator or an automatic system triggers the draining of the reactor, scramming the system. So, now you have hot salt sitting in a passively cooled tank that is built out of the same material as the reactor. It's a sturdy metal, built to get hot but not this hot, at least not for very long. So, what do you do? You can improve the cooling by using forced air instead of just allowing convection. But what if you lose power too?

What do you do if your cup of tea is too hot but you don't want to wait for it to cool down on its own? You drop an ice cube in it.

Even though the drain tank is designed to passively cool the fuel, and its designed to put the fuel in a configuration that inhibits criticality, that might not be enough. I think of how a modern heavy water reactor is cooled if it gets too hot, they dump in light water. It does two things, the water boiling off removes a lot of heat (a phase change) and it inhibits criticality. If things get real bad they dump in boronated water, the boron soaks up the neutrons while the water cools it down.

In a MSR dumping in boronated water sounds like a bad idea, it's not particularly compatible with the fuel. I was thinking what would be an analog to that, something that can soak up the heat in a LFTR salt and also soak up some neutrons and obviously not add to the problem of already having boiling hot radioactive salt.

Perhaps the solution is that it's not really a problem. Passive cooling might be sufficient, especially since the hotter the salt is the faster it will cool down. Perhaps the active cooling has enough redundant fans, pumps, and battery packs that the chances of them not working is very slim. I just think of the safety systems on current solid fuel reactors, in Fukushima the highly redundant fans, pumps, and battery packs was not enough. In modern LWR and HWR designs there is a tank of boronated water situated where gravity can carry it into the reactor core if the situation calls for it.

It sounds like dumping boronated water into a solid fuel water moderated reactor is essentially a last ditch effort. The reactor might not survive the acidic nature of the water. The good thing about adding cooling salt to the drain tank of a LFTR is that it does not affect the reactor. What it will most definitely do is change the chemistry of the fuel salt so it cannot be used as fuel any more, it'd have to be reprocessed first. It might also have to be jackhammered out of the drain tank, but the reactor would be intact and a bigger disaster averted.

As I type this I was reminded of a talk I saw on Youtube with, IIRC, David LeBlanc talking about MSRs. A safety system he proposed was dumping depleted uranium salt into the reactor as a last resort to shut it down. This would bring the reactor subcritical and destroy any ability to take fissile U-235 or U-233 from the reactor chemically. I assume if this was done, and the salt allowed to solidify in the reactor, that reactor will never operate again.

My original questions were first, why hasn't anyone proposed this before? I just answered my question, my brain just didn't put all the pieces together until just moments ago. Secondly, is this a good idea for LFTR? It's been at least considered for some MSR designs and perhaps rejected for good reason. I guess my real question now is why haven't I seen more discussion of this besides one remark on it in the many talks I've seen on the topic?

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PostPosted: Mar 25, 2014 12:22 pm 
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You will want to read this thread: http://www.energyfromthorium.com/forum/viewtopic.php?f=3&t=3311

I believe there is a large discussion about decay heat removal.

Your "ice cube" proposal will not work, or work for very long because the decay heat keeps being generated. The tea in your example does not keep producing heat like the variety of isotopes in the fuel salt produce heat.


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PostPosted: Mar 25, 2014 3:09 pm 
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Well said. The whole decay-heat-that-can't-be-turned-off-ever thing is hard to understand because its unlike anything in our daily lives. If your milk boils over, shut off the cooking gas. Can't do that with decay heat.

Having more heat capacity like ice in the tea glass is useful for a number of reasons:

1. It reduces the size of the passive cooling pathway. Not that this will save much cost but it makes it easier to design the passive cooling pathway to be more robust - cooling and robustness are largely at odds with each other.

2. It delays the time to freezing of the fuel salt.

3. It can help reduce peak temperatures in container material, depending on design.

The problem with putting ice in the tea glass is that something can happen so that the ice ends up somewhere else. Or the tea glass could break and then spill hot tea all over the floor, not much point in putting ice in the glass then. In my opinion these problems can be eliminated by design so having at least some extra heat capacity around is an optimal solution. Its not quite as important as the passive cooling systems though.


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PostPosted: Mar 25, 2014 4:53 pm 
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Alexander Litvinenko's tea kept producing heat.


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PostPosted: Mar 25, 2014 5:52 pm 
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Jim L. wrote:
Your "ice cube" proposal will not work, or work for very long because the decay heat keeps being generated. The tea in your example does not keep producing heat like the variety of isotopes in the fuel salt produce heat.


I do not intend to replace passive cooling but augment it with cooling salt. I realize decay heat will be an issue, and a lot of heat will be generated for a long time. Here's some assumptions I was working with, they may not be correct assumptions but this is what I was thinking:

- Fuel was supercritical at the time the reactor was drained
- Fuel is hot enough that there is a threat to the drain tank physical integrity and/or the fuel salt may begin to boil
- Any fission products that boil off at normal operating temperatures would have been removed by the inherent nature of an operating LFTR, fission products that remain would be trans-uranics, noble metals, and a few others
- Passive cooling systems are operational but active cooling is not due to power loss or damage

Therefore the goals are to:

- Prevent the fuel from going critical in the drain tank
- Cool it down before drain tank integrity is lost
- Raise the boiling point, and perhaps melting point, to ease containment
- Reduce cooling load and time to cool so that the reactor can be brought back on line more quickly

Some secondary goals:

- Reduce proliferation risk
- Minimize contamination of fuel so that it can be reprocessed and reused with minimal time and effort

Seems to me that a good way to reach those goals is to dump some room temperature salt into the drain tank. Thinking about it some more a mix of LiF and depleted UF4 would work nicely. I would leave out the BeF so that the fuel salt would no longer be at its eutectic point, the boiling point and melting point would rise. Adding depleted UF4 would soak up some neutrons and make the uranium content of the fuel useless for weapons.

Then again maybe just add depleted UF4, would that be cheaper than LiF?

What about adding some PuF3? Is there already enough isotopes of plutonium in the salt to prevent weapon proliferation?

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PostPosted: Mar 26, 2014 9:07 am 
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It is impractical for a large reactor to have sufficient passive cooling to handle the initial heat load. The initial heat load is around 6% of full power. For a 1GWe plant that amounts to 144MW! Typically passive cooling will handle 0.5 to 1% of full power and we use heat capacity to absorb the initial surge. By the end of the first day you are down to a power level the passive cooling system can handle.


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PostPosted: Mar 26, 2014 2:04 pm 
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Folks, remember that many designs have much of the longer term isotopes removed by the reprocessing system so the "continued heating" won't continue for long. A big enough pot of salt may be of reasonable size given the continuous reprocessing envisioned.

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PostPosted: Mar 26, 2014 5:26 pm 
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KitemanSA wrote:
Folks, remember that many designs have much of the longer term isotopes removed by the reprocessing system so the "continued heating" won't continue for long. A big enough pot of salt may be of reasonable size given the continuous reprocessing envisioned.


You'll have to process REALLY fast for this to be a big help. For reasonable processing rates, it is a marginal gain.

Processing all the fuel rapidly has all sorts of risks on its own.


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PostPosted: Mar 26, 2014 5:48 pm 
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KitemanSA wrote:
Folks, remember that many designs have much of the longer term isotopes removed by the reprocessing system so the "continued heating" won't continue for long. A big enough pot of salt may be of reasonable size given the continuous reprocessing envisioned.

Reprocessing fast doesn't eliminate the decay process - it just moves it from one spot in the reactor to another. You still have to dump the heat. The heat rate is around:
1% 3 hours
0.5% 1 day
0.1% 4 months
0.06% 3 years
0.025% 30 years
and is roughly linear on a log-log scale. The 30 year power is mostly from 137Cs and 90Sr.

I tried to do a huge pot of salt and concluded it simply wasn't going to get there. Adding heat capacity is needed to get the passive cooling system reasonable BUT heat capacity by itself isn't going to do the job.


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PostPosted: Mar 26, 2014 8:11 pm 
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The aim is an automatic,instant reduction of heat and neutrons to avert an accident. Even the fuse plug sounds iffy and subject to failure. Later, you have to take action to return to 'as you were' situation.
Another idea would be a wider spill area immediately over the safety level. This area could also have a neutron absorbent like boron. The fuel can flow back once it cools down.
You could also have Scram rods of Boron Carbide. That would be removable ice-cubes.


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PostPosted: Mar 26, 2014 10:42 pm 
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Yes we have stop the fission process relatively rapidly. But this is a task that is routinely accomplished and so far we have never failed to accomplish it. I expect the same will hold true for MSRs. The hard part is dealing with the heat in days two through the fourth month or so. By day two you likely have used up your heat capacity so you have to have the passive cooling working. Remember how long it took for Fukushima to be in real trouble? TMI also took several days.

There is no chance for an instant reduction of heat except to stop the fission process. After that you are stuck with the decay heat as outlined in an earlier post.


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PostPosted: Mar 29, 2014 3:14 pm 
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Stopping the fission process in the short term is not important for a MSR with properly designed power coefficients. It just heats up marginally more in the short term and is easy to design for for most designs.

In the long run with a thorium reactor you must put in negative reactivity somehow or the salt will boil (and before that vessel will fail) from Pa233 decay adding U233 into the salt.

Despite the inherent advantages of liquid salt fuel, there are also a few key disadvantages. Two of them are that the reactor is poorly accessible for accident recovery, and that injecting water is not an option (this would in fact make things much worse). We can't do something to fix things in a pinch even if given a lot of time. The decay heat removal has to work, and for thorium cycles the long term negative reactivity insertion has to work.

Failing the reactor vessel is one way to add negative reactivity in a really serious accident, with multiple failures in normal cooling and reactivity control. To some extent we have to design for this anyway since primary loop integrity can't be 100% assured in all extreme events. So in terms of safety we are good on reactivity, but it bears down even more on the passive cooling. If the vessel has failed the containment has to be cooled to prevent the containment from failing too.

So bottom line it is all about having a fail safe cooling pathway. Once you have this a large release of radionuclides can be virtually eliminated by design.


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