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 Post subject: Meltdown Risk?
PostPosted: Oct 23, 2012 4:07 pm 
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Speaking as a citizen scientist, one of the big threats of nuclear power still present in the mind of the public is the risk of reactor core meltdown, I was wondering what the risk of meltdown(if it exists) is with a Liquid-Fluoride Thorium Reactor.

As the fissile material is contained in solution within a molten salt circulating in the reaction loop, would this not make the possibility of a runaway fission reaction and a core meltdown impossible?

More specifically as the molten salt heats with an increase in the rate of fission does this not provide a natural limitation on reaction, the salt would expand and decrease the rate of fission as the distance between the suspended fuel increases?


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 23, 2012 4:38 pm 
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The risk of meltdown in a typical reactor is driven by the simple fact that the temperature at which the fuel is molten is greater than the temperature at which the structural materials that make up the reactor vessel will melt.

Melting point(uranium dioxide) > melting point(stainless steel)

In a LFTR, the melting point of the fuel mixture (LiF-BeF2-UF4) is far less than the melting point of the reactor materials (Hastelloy-N). Thus melted fuel doesn't melt through the reactor vessel.

Melting point(LiF-BeF2-UF4, ~380C) < melting point(Hastelloy-N)


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 23, 2012 4:51 pm 
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So basically by having the U233 contained in a molten salt in a LFTR, you remove it's melting point as part of the safety risk of running a nuclear reactor?

I'm just thinking in terms of presenting LFTRs as a large scale alternative to "conventional" power sources, having it clear that it lacks one of the major safety(whether real or imagined) concerns of PWRs is probably going to make acceptance easier.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 23, 2012 5:52 pm 
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A snarky answer is that the fuel is already molten so there is no risk of meltdown - it is already melted!

More reasonably is to address the resulting concerns. If a solid fuel reactor gets in trouble there are two concerns:
a) recriticality - much postulated but not a real risk with LWRs. This is a real risk with fast reactors where the fuel getting into a different configuration could place it in an environment where there is additional moderation. This would increase the reactivity of the fuel and we might get recriticality. Likewise, for a thermal fluid reactor the risk of recriticality is basically nil. But there is a concern for fast fluid reactors - especially if you allow hypotheticals where the fuel melts through the containments and hits moisture in the concrete etc.
b) decay heat melting through containment and causing radioactive release to the environment.
here a fluid fuel has an advantage in that we can move the fuel from the core area where all trades have to be made in favor of better neutronics to a dump tank area where trades are more centered around guaranteeing poor neutronics and excellent cooling.

For both concerns I think all new designs for thermal reactors will be safe in these regards. LFTRs have a simplicity advantage in that there is no pressure wanting to push stuff into the environment. Fast reactors though have a more difficult engineering problem to solve and - in my view - a very difficult PR problem dealing with worse case speculations about re-criticality.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 23, 2012 6:22 pm 
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Having a reactant stage that if it gets too hot it simply drains into a cooled containment vessel underneath the primary vessel seems like a strong selling point for LFTRs.

If they are resistant to the kind of incidents that have got so much publicity in the past with nuclear power and even in a worst case scenario all you end up with is the radioactive material safely contained and cooling down, then it becomes hard to even lump them in with PWRs when it comes to certification and public acceptance.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 23, 2012 7:30 pm 
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For me the biggest safety feature is the fluoride. Some simple chemistry. Look at a periodic table. The column the furthest to the right are the nobles. They don't need anybody - they are totally self-sufficient. The next column is just one shy of being noble and they know it! They desperately want to share an electron with someone so they can become noble. And once they are noble they are very reluctant to change. As you up this column the chemical get even more aggressive. The top of the heap is fluorine. Once he pairs up with someone (say cesium) he makes a fluoride and it is most difficult to change that chemistry.

This is good in that our fission products are rapidly tied up to become fluoride compounds and they don't go wandering off as gasses nor do they react with other chemicals much. So you spill some of the salt by whatever means (even say a bunker buster missile) and stuff goes only as far as you throw it. It isn't prone to spread into the biosphere.

Further, there is nothing inside the reactor that wants to throw stuff out. In an LWR you have water under high pressure. Add heat and you will have to release some of it or burst your containment. As we learned at Fukushima you also have zirconium which when combined with steam at high enough temperature will steal the oxygen from the water and let the hydrogen float away to become the explosion risk we saw at Fukushima. Inside a LFTR there is no such reaction. Inside sodium cooled reactors you have the risk of fire. Careful engineering makes the risk small - but not zero. Inside a LFTR there is nothing to burn.

These features are more important for safety to me than the flexibility to drain the fuel. If we totally depended on being able to drain the fuel then one could ask what would happen if an earthquake managed to bend the drain pipes in a way to restrict the flow.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 9:54 am 
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Lars wrote:
So you spill some of the salt by whatever means (even say a bunker buster missile) and stuff goes only as far as you throw it. It isn't prone to spread into the biosphere.

The way I have imagined this in my own mind is the molten salt is like candle wax. If the containment structure was somehow violently breached, the liquid salt would splash out, hit the much cooler surrounding air, and solidify very quickly. You wouldn't have gaseous plumes of radiation spreading across many square miles of land and sea; instead you would have solidified lumps of salt near the reactor structure. These lumps would be easy to locate and they would be relatively easy to collect compared to the remediation sites that exist today.

Is that a correct description?


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 11:58 am 
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Mostly. Thinking on it I did exaggerate in a way I should not have. If you really were to hit a reactor with a bunker buster that would be a bad deal. The problem is the noble gases and their precursors. There are many fission products that have a short half-life and in the process decay to a radioactive noble gas. These gases would be prone to escape and spread in the air. I haven't thought hard about how bad that would be but I don't expect that would be nice. Pretty unpleasant thoughts.

The primary radioactive exposures in from reactor accidents stem from three isotopes, 131I (8 days), 134Cs (2 yr half-life), and 137Cs (30 yrs). So assuming that the containment holds for a day then these are the guys to worry over. In an MSR with off-gas removal we don't generate any significant quantity of 134Cs because we remove virtually of the precursor 133Xe. Cesium gets chemically captured with fluorine so once it is cesium it won't travel. The precusor is 137Xe with a 4 minute half-life. So we need the containment to hold for 40 minutes for this to decay and the concern about 137Cs to be gone. Iodine is a strange animal that we aren't quite certain what it does. ORNL could not trace down where all the iodine went in MSRE. They had hoped to see it in the off-gas since that would allow better off-gas collection (iodine is a precusor to xenon). They seem to have given up on that thought later in the program so I'm interpreting this to mean most likely the iodine sticks to something in the reactor (one ORNL paper suggested it stuck to the noble metal conglomerations). This is one area where we need more experimentation.

Summary. Shut the reactor down, have the containment hold for a day, and then there isn't much that wants to go anywhere.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 1:35 pm 
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There's no 134Cs in the reactor. It's not formed from fission product decay, rather from neutron absorption on stable 133Cs. There's no 133Cs in the neutron flux because it got removed as 133Xe in the offgas. So no 134Cs to contend with.

Update: oh, you mentioned that.... **sheepish**


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 2:33 pm 
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Lars wrote:
For me the biggest safety feature is the fluoride. Some simple chemistry. Look at a periodic table. The column the furthest to the right are the nobles. They don't need anybody - they are totally self-sufficient. The next column is just one shy of being noble and they know it! They desperately want to share an electron with someone so they can become noble. And once they are noble they are very reluctant to change. As you up this column the chemical get even more aggressive. The top of the heap is fluorine. Once he pairs up with someone (say cesium) he makes a fluoride and it is most difficult to change that chemistry.

This is good in that our fission products are rapidly tied up to become fluoride compounds and they don't go wandering off as gasses nor do they react with other chemicals much. So you spill some of the salt by whatever means (even say a bunker buster missile) and stuff goes only as far as you throw it. It isn't prone to spread into the biosphere.

Further, there is nothing inside the reactor that wants to throw stuff out. In an LWR you have water under high pressure. Add heat and you will have to release some of it or burst your containment. As we learned at Fukushima you also have zirconium which when combined with steam at high enough temperature will steal the oxygen from the water and let the hydrogen float away to become the explosion risk we saw at Fukushima. Inside a LFTR there is no such reaction. Inside sodium cooled reactors you have the risk of fire. Careful engineering makes the risk small - but not zero. Inside a LFTR there is nothing to burn.

These features are more important for safety to me than the flexibility to drain the fuel. If we totally depended on being able to drain the fuel then one could ask what would happen if an earthquake managed to bend the drain pipes in a way to restrict the flow.


It sounds like a more accurate description of an LFTR would be a chemical/nuclear reactor.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 3:24 pm 
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DougC wrote:
It sounds like a more accurate description of an LFTR would be a chemical/nuclear reactor.


I'm not an expert, but I believe "nuclear" is a more appropriate designation than "chemical/nuclear," as the source of the energy is bonds within the nuclei of the atoms. There wouldn't be any significant amount of energy released from the chemical bonds between the atoms. There are chemical processes involved with the fuel, but it's my understanding that this is true to some degree with all types of nuclear reactors - for example the preparation of the fuel which goes into traditional light water reactors.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 4:03 pm 
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DaveSchroeder wrote:
DougC wrote:
It sounds like a more accurate description of an LFTR would be a chemical/nuclear reactor.


I'm not an expert, but I believe "nuclear" is a more appropriate designation than "chemical/nuclear," as the source of the energy is bonds within the nuclei of the atoms. There wouldn't be any significant amount of energy released from the chemical bonds between the atoms. There are chemical processes involved with the fuel, but it's my understanding that this is true to some degree with all types of nuclear reactors - for example the preparation of the fuel which goes into traditional light water reactors.


Right, the power is being produced as a result of the energy released from the U233 undergoing fission, the chemical part would be the use of the fluoride salt as a medium to contain and circulate the fuel and exchange the energy for electrical generation.

There's also the online removal of fission products that the fluoride chemisty makes possible.

Maybe it's confusing to use the term chemical in describing LFTRs, it does seem important to communicate the unique characteristics that using the molten salt gives to thorium MSRs.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 24, 2012 4:24 pm 
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The term I've heard is that LFTR is a chemists nuclear power plant.


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 Post subject: Re: Meltdown Risk?
PostPosted: Oct 26, 2012 7:00 am 
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Kirk Sorensen wrote:
The risk of meltdown in a typical reactor is driven by the simple fact that the temperature at which the fuel is molten is greater than the temperature at which the structural materials that make up the reactor vessel will melt.

Melting point(uranium dioxide) > melting point(stainless steel)

In a LFTR, the melting point of the fuel mixture (LiF-BeF2-UF4) is far less than the melting point of the reactor materials (Hastelloy-N). Thus melted fuel doesn't melt through the reactor vessel.

Melting point(LiF-BeF2-UF4, ~380C) < melting point(Hastelloy-N)


Well technically the accellerated creep rupture temperature of the structural material will be decisive, but a more important point I think has to do with cooling and coolability. The issue with nuclear reactors is decay heat and containment, and they are linked crucially in cooling. If you have sufficient cooling, the fuel, vessel and containment will remain intact (or at least one of them will in for example a severe criticality accident). There won't be a meaningful release of radiation to the environment.

It's not really possible to design cooling systems without failure modes, so what we need is fail-safe cooling. If you have fail safe cooling, everything will be fine, solid fuel or liquid fuel. In my opinion the cooling design is more important than solid or liquid fuel, though it is true that liquid full will limit the extent of a radiation release (eg very little cesium, containment failure very unlikely due to lack of internal pressure creation). But a liquid fuel reactor with no cooling will still heat up to the point of failing the vessel, which is very bad at a minimum for your investment and public relations. Compare this to a solid fuel reactor with a pipe bursting but cooling systems working, this will not be worse and probably look better than the liquid fuel reactor with no cooling.

Even at Chernobyl, technically a criticality accident, the biggest driving force for the contamination was in fact the heat from decaying fission products and neptunium (which set off a graphite-steam reaction that made it worse). If there was a good emergency cooling system there wouldn't be a large release, and if there was also a good containment there wouldn't even be a small release.


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 Post subject: Re: Meltdown Risk?
PostPosted: Nov 01, 2012 11:37 am 
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To extend this topic a bit, what is the most serious failure mode of a Thorium based MSR?

If the reactant is already in a molten state, it won't meltdown and burn through the bottom of containment, from what I understand a frozen salt plug in the bottom of the reactor vessel will melt if the salt/fuel mix overheats draining it into passively cooled containment vessels. There's no water present to provide disassociated hydrogen to explode and the reactor isn't pressurized so there won't be a release of radioactive steam. Even if the core material is released, as Lars describes, it will mostly be contained within the salt that quickly solidifies as it cools.

So what is the main risk, it almost seems as if you'd need to place explosives inside the reactor vessel to even get the molten salt/fuel mix out of containment?


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