In most of these reactors, pipes and heat exchangers are made out of high strength stainless steels, for corrosion resistance and the need to contain hot (300-950 C or 572-1742 F, depending on the reactor design) coolant at modestly high pressures (usually 1000 psia or more). Power plant thermal efficiencies (and thus final electrical generation efficiency) improves with higher temperatures. Current Gen II and III reactors are on the low end of that core outlet coolant temperature range above.

As good as high strength stainless steels are for corrosion resistance, they still require inspection and replacement in current Gen II/III light water reactors. I should point out that corrosion rates, like most chemical reaction rates, increase significantly with increasing temperature of the reactants. Thus, for a power plant design, you want to operate at temperatures as high as practical. For designs more susceptible to corrosion reactions, you either limit the operating temperature to limit the corrosion reaction rates, or you look for alternate corrosion resistant materials/coatings, or operating conditions that promote formation of stable corrosion byproducts (oxides, or fluorides for MSRs) that create a protective layer of scale that inhibits further corrosion.

I'm not the materials or corrosion expert, so I don't know what other alternatives may exist to handle high temperature, corrosive fluorides at modestly high pressures. I would guess there are few alternatives to high-strength stainless steels currently used and would be more expensive.

Again, I wish those MIT students the best of luck. But when you develop new power conversion design concepts, the last things that usually get attention and resolution are issues of maintenance, operability, reliability, and total life cycle cost. The thermodynamic conversion cycle is easy. The reactor core physics work gets most of the early development. But the early estimates of life cycle cost always underestimate costs associated with maintenance and reliability of components and any oddities associated with operability or complications during construction.

Hello robertmbeard,
Fascinating. Thank you for detailing the issues in a most comprehensive way. As a layman I really appreciate your explanations. Question: Could materials other than metals be used to provide the piping for the fluorine in the MSR reactors, thus mitigating the corrosion problem?
This problem seems similar to a problem common to investment casting plants that use Kolene to dissolve ceramic from inside their castings. It too is a molten salt bath. The process is so corrosive that it will dissolve aluminum castings, and can therefore only be used on certain steel alloys. I don't know what material the units tanks and piping are made of, or lined with, but I know they have been used reliably for 75 years... If this problem could be overcome, would this be the best type of reactor compared to the others you have discussed?
Respectfully,
O.A.

I completely agree with your aversion to the use of the term "waste" for used nuclear fuel from commercial light water reactors. The main limit of fuel rods in reactors is the zircaloy cladding on the outside of the fuel rod, providing structural support and containment of gaseous fission products, like radioactive xenon gas.

Zircaloy has more radiation damage resistance than alternative materials, but it still has its limit of useful life before a locally weak spot cracks and starts leaking radioactive fission gas byproducts into the cooling water. When such a leak is detected, the fuel assembly containing the bad fuel rod is removed, even though the other fuel rods in the fuel assembly still likely have some useful life left in their zircaloy cladding.

The used nuclear fuel of the fuel rods in the "spent" fuel assembly still contain a significant amount of actinides and heavy metals to support additional fission energy release. To more fully utilize that fuel, it could be extracted and reprocessed into a form that some of the advanced reactor concepts can use.

I'm sure they know about the concerns I pointed out above. I certainly did not want to sound overly critical of their interest in MSRs. I wish them the best. I just recently completed a master's degree in nuclear engineering and have spent a little time studying Gen IV nuclear reactor concepts. Of the 6 Gen IV concepts receiving attention, MSR's are getting the least R&D and are considered more of a "long-term" alternative due to some of the concerns above.

what we need is an island on which to try a SFR,
recommended by robertmbeard, on this post, crossed
with a thorium reactor using set-aside (Not Waste)
fuel rod pellets, to prove it in. then, strong-arm the
NRC with evidence that it works. rotating blackouts
produced by BHO's curtailment of the coal-power
industry should get the public on our side.

if it were mine to design, I would have multiple
fuel flow paths, multiple coolant flow paths, and
multiple cooling tower flow paths -- to allow long
runs during alternate-pipe maintenance.
too expensive? how expensive are shutdowns?

just thinking ! -- j

Molten Salt Reactors (MSRs) are a Generation IV reactor concept that has been around since the 1950s (like most concepts). It always has looked like the most promising concept for utilizing used nuclear fuel from light water reactors. There are several significant technical challenges. The biggest is the fact that it uses fluorine salts that the actinides and other heavy metal fuels are dissolved in. Fluorine is one of the most corrosive elements in the periodic table of elements.

Pumping a molten, heavy metal fluoride salt mixture through the reactor's primary loop and going through a heat exchanger to dump heat to the secondary loop (which uses either a Rankine or Brayton cycle to drive the electric generator) introduces a couple of big problems that nobody has wanted to tackle with much R&D spending:

1) Severe chemical corrosion of all components in the reactor's primary loop (pipes and the intermediate heat exchanger) is a big life-limiting and maintenance problem. In a Gen II (current) or III (being built) reactor (light water reactors), chemical corrosion of pipes from cooling water is the biggest maintenance challenge (significant cost) over the typically 60 year life of a nuclear power plant. Using fluoride salts makes all of these material compatibility and maintenance issues an order of magnitude more problematic.

2) Pumping a radioactive, molten fluoride salt mixture through the primary loop pipes and intermediate heat exchanger degrades their material properties faster (in addition to the corrosivity problem) due to radiation damage of the metal alloys in the structural material. This significantly shortens the useful life of the structural material, requiring more frequent replacement of pipes and intermediate heat exchanger (difficult inside the reactor's radiation containment...).

There are other less challenging problems with MSRs, but the 2 above are the ones that have been show-stoppers to any significant R&D investment over the years. There are 5 other Gen IV reactor concepts receiving most R&D funding, due to being less challenging, etc... Two of those also help close the nuclear fuel cycle (minimize waste) but are breeder reactors (a proliferation concern since the plutonium can be extracted for use in weapons) -- LFRs and SFRs:

1) Lead-cooled Fast Reactors (LFRs), being advanced by Russia in their BREST family of reactors, use more conventional solid nuclear fuel elements in the reactor core that mix in used nuclear fuel. The coolant is liquid lead instead of water. Molten lead has a high heat capacity, making it thermally a great coolant. However, lead has a high melting point. If the temperature of the lead drops too much, it starts solidifying inside all the pipes of the primary cooling loop (which usually have heaters to try to avoid this...).

2) Sodium-cooled Fast Reactors (SFRs) advanced mostly by the U.S. and a few other countries are similar to LFRs but use liquid sodium as primary coolant. Sodium has a significantly lower melting point than lead. The drawback is the risk of sodium fires, if hot sodium comes in contact with air (like if a pipe springs a leak at a joint). So, the reactor core has an inert atmosphere and inspections are more frequent for pipes, etc... The U.S. DOE successfully built and ran test SFRs (EBR-I and EBR-II) from the 1950s through 1994. While successful, they were expensive projects.

I think the likely long-term future combination for U.S. nuclear power plants will use a mix of 2 types -- VHTRs (commercial) and SFRs (DOE). For closing the nuclear fuel cycle, a DOE-operated SFR (with required adjacent used fuel reprocessing plant) would be dedicated to more fully utilizing spent nuclear fuel from commercial power plants. For commercial power plants, VHTRs (Very High Temperature, gas-cooled Reactors) would operate with higher efficiency than Gen II light water reactors. VHTRs, due to the higher temperature of the helium gas exiting the reactor core, can also be used as process heat for hydrogen production and other industrial facilities, in addition to electricity generation. R&D on VHTRs is much further along than most of the other 5 Gen IV reactor designs.

Of course, the biggest roadblock to innovation is the U.S. Nuclear Regulatory Commission (NRC, which is separate from the DOE). Any new reactor design takes at least 8 years (usually far more) to obtain NRC licensing approval for commercial construction and operation. The nuclear power industry is the most heavily regulated industry in America, which results in a depressingly slow pace of innovation...

India is not alone. China is working on it, mostly to capture as much IP as they can. I head a story the Toyota and Mitsubishi are working on it in Siberia. Recently it appears that Norway is getting interested. They are an interesting new entry since they have a trillion extra dollars to spend and know they are sitting on one of the better supplies of thorium. So, its a race at this point and our DOE and NRC need to figure that out.

Most of the hard engineering and material science was done in the 60s and 70s. Sadly the folks who did that are mostly dead now.

India is active in the liquid-salt thorium reactor development venue. India has no uranium deposits, but it has a lot of thorium and since only a small amount of 'waste' from a conventional reactor is needed to seed a thorium reactor, this would greatly benefit their plan to bring power to all parts of their country.

I would like to take this opportunity to quibble with the term 'waste'. This is a good example of derisive labeling. "What are we going to do about all of this horrible nuclear waste?" people wail. It is not waste, it is byproducts; it is not something that needs to be gotten rid of but something that needs to be preserved as a resource. To whit: use in seeding the reaction in the next generation of reactors. To call it 'waste' is to fall smack dab into the agenda of people who are against nuclear power.

I would also like to mention that, back in the 60's, before the environmental movement shut down nuclear development, there was discussion as to whether, with nuclear plants as the source of electricity, it would cost more to bill for electricity than the effort would be worth. India may be the nursery of the thorium reactor but we would all like to have the benefit.

Jan

these young people should know what you know and if not that is unfortunate.

That the article did not mention the caustic reactions problem indicates to me this is money grab that will produce nothing. If an article is in Business Week I generally ignore it.

I have read about reconstituting spent fuel before, but this sounds even better. Very intriguing. They predict the cost of nuclear produced energy would be cut in half. Cheap energy is a prerequisite for a growing economy.