I just want to point out this nonsense piece that's been making the rounds. It is a long, technical, detailed critique of the molten-salt reactor (MSR) concept, from someone who claims to be a PhD engineer (with expertise in "renewables"). It is absurd hogwash, full of utterly insane misconceptions. But it looks superficially credible, so I expect this "expert" will be widely quoted in the near future.
The 8th in a series (!).
I'm not a PhD engineer, or any sort of engineer at all. Part of this blog's extremely small readership is such engineers. To those of you, here's a quick summary of some of the things in this critique which are complete nonsense, so you can judge for yourselves (I'll go into a bit more detail after the summary):
Summary of some of the biggest howlers
- Claims MSRs have "Isotope Separation Plants" which separate 233U and 232U (the trace contaminant)
- Warns of hazardous fission products, such as thorium isotope "T-232" [sic], which supposedly is a disadvantage of thorium-fuelled reactors because of its 14 billion year half-life
- Warns that electrolyzing nuclear fuel salts is energy-intensive
- Warns that heat inputs in fluoride reprocessing are energy-intensive
- Asserts that thorium MSRs are constrained to a lower temperature limit of 1,110 °C, the melting point of pure ThF4. Concludes MSRs must be built entirely from ceramics
- "Obviously, once we exhaust the world’s U-235 stockpiles, LFTR’s and any other Thorium fuelled reactors will cease to function."
- Argues against using molten fuel salt as a working fluid in a gas turbine
Well, that pretty much says it all, don't you agree?
I'll go into a bit of detail on individual points, pointing out where in the 23-page screed they are from (so no one else has to waste time).
Isotope separation plants
The Isotope Separation plant and waste output
[...] Also the supporters of the LFTR seem to assume that this ISP can operate with 100% efficiency (i.e remove all the radioactive poisons). This would be very technically challenging, especially in the LFTR case given the importance (if you followed the points made by IEER earlier) about separating out of U-232 (and its Thallium-208 payload) from U-233.
He has the misconception that 233U/232U isotopic enrichment is necessary for MSR operation. Spends many paragraphs speculating on this imaginary thing, finally concluding it will consume up to 25% of an MSR's electric output:
All in all my suspicion is that our heat exchanger would struggle to produce thermal efficiencies any greater than 70-75%. Assume a good high efficiency Gas driven Brayton cycle the other end (55-60%) so that yields us an overall efficiency of 38% – 45%, oh! but we almost forgot about that isotope separation plant and its net energy inputs, say we deduct 5-10% of reactor power output to account for running that, so overall between 29% – 40%, with a 35% overall efficiency being my best WEG
Dangerous fission products
Certainly the fission products from a Thorium reactor are a worry, Technetium-99 has a half life of 220,000 years, uranium-232 produces thallium-208 (a nasty wee gamma emitter), Selenium-79 (another gamma emitter with a 327,000 year half-life), even Thorium-232 is a problem with its half life of 14 Billion years (and while the T-232 isn’t a major worry, all the time during this 14 Billion years it will be decaying and producing stuff that is!).
Names the long-lived fission products as the most "worrying". Thinks thorium-232 is a fission product (abbreviates it "T-232"). Is very worried about its daughter products -- they're continuously generated over 14 billion years! Thinks this is somehow a disadvantage of the thorium reactor (the one that consumes 232Th dug up out of the ground).
Where will we find so much energy?
Sounds the alarm about the energy intensiveness of using electrolysis in reprocessing (§8.4):
Notably, the LFTR supporters have suggested (see here) using electrolysis to help improve the filtering efficiency of their plant. An excellent idea, it would solve a number of problems, but unfortunately electrolysis systems practically eat electricity! Where’s all that electricity going to come from?
And (same section), the energy intensity of heating fuel salt to high temperatures in reprocessing:
As the working fluid will be coming off the exhaust from the heat exchange cycle it will be relatively cool (in the MSRE it was at around 570 °C) yet some of these processing stages will require the fluid to be heated back up to 1,600 °C. Where’s that energy going to come from?
Perhaps he is confused about the difference between temperature and heat? There is enough energy coming out of a nuclear reactor at full power to heat a ton of fuel salt by a thousand degrees in one second. (...within an order of magnitude. Don't have temperature-dependent heat capacities on hand, so this number is rough.) That a multi-gigawatt-thermal MSR "only" operates at 600° C or at 800 °C, is hardly a statement against its enormous power!
One section (§8.8) of his critique is based on the bizarre error that thorium MSRs are limited to operating at above 1,110 °C, the melting point of pure ThF4:
The dendrite problem above demonstrates that the LFTR/LFUR has a relatively narrow thermal window. Its filtering plant will not work if the temperature of the fluid drops much below a certain threshold and the danger of fuel solidification raises the risk of the reactor being damaged. With UF4 the solidification temperature is 1,036 °C and its vapourisation temperature is 1,417 °C. [...] With TF4 our “window” is 1110 – 1,680 °C, but again we can potentially move this by lowering the pressure (or raising it if we want to go the other way…not that we do!). A low vapour pressure also creates a few potential problems in terms of keeping the reactor sealed (air is more likely to leak in if the pressure inside is less than atmospheric…possibly starting a fire!) and maintaining a good flow rate from our pumps.
I don't know what confusion of his provoked this nonsense. Maybe he hasn't done his basic research, that all MSR proposals involve solvating actinide fluorides in other fluoride salts -- mixtures of LiF, NaF, BeF2, ZrF4, and/or others -- with the mixture having far lower melting points than actinide fluorides. Or maybe he's under the illusion that individual components of a chemical solution precipitate out at their pure melting points. At any rate, his chain of reasoning starts from this major error and leads to others:
With the LFTR however, I doubt you could operate one made out of any Nickel alloy, contrary to everything said on the internet. Bare in mind I’m thinking in terms of a good lengthy service life with a sensible factor of safety, not a flimsy test reactor in a lab (with a 100 mile exclusion zone!).
Thus the pressure vessel of any LFTR would likely have to formed out of Ceramics (very expensive and difficult to form, especially given how critical getting an air tight seal is given the graphite core) and key internal components out of Refractory metals, as would be the case for certain high temperature parts of any ISP (in both the LFTR and LFUR cases) given talk of operating temperatures in the range of 1600 °C.
So my instinct from a materials science point of view would be to drop the LFTR idea altogether and focus instead on a LFUR. While this isn’t able to use the Thorium cycle, the point was raised earlier about how the Thorium cycle isn’t all its cracked up to be. Its going to be a lot easier to build a LFUR than a LFTR, cheaper (relatively speaking) and likely safer too. Of course it does come at the disadvantage of a slightly awkward acronym! but overall that would be my focus of attention.
In reality, an MSR runs at fuel temperatures of only 700 °C (at least the first generation), which is why "internet" people like Kirk Sorensen, PhD assert that Hastelloy alloys are perfectly acceptable structural material.
What is breeding?
His presentation of the thorium fuel cycle, is, well, I don't know what to make of it!
...and the fact we’ll still need supplies of Uranium to get Thorium reactors going again whenever we have to turn it off (which will happen at least once a year or so during its annual maintenance shutdown).
Obviously, once we exhaust the world’s U-235 stockpiles, LFTR’s and any other Thorium fuelled reactors will cease to function. The LFTR fans usually groan at this point and state that “all we need is a little plutonium”. Now while I’m quite sure that in the fantasy world which the LFTR fans inhabit Plutonium is available in any good hardware store but back in the real world, it’s a little harder to come by! As with the HTGR’s using Thorium (if its possible) would certainly help stretch things out….a bit! But not by nearly as much as the supporters of Thorium reactors would have you believe.
It's not a gas turbine if it's not a gas
Unlike Dr. PhD, I'm not an engineer, and my background knowledge does not help me understand how a Brayton cycle can run on liquid. I just don't get how it would spin, what with the difficulty of getting a liquid to expand. But because the author says he has a "PhD in a thermodynamics-related field", and is trained as a mechanical engineer -- I assume he understands something that I do not.
Another misconception is that a LFTR or LFUR can operate on an open cycle with a gas turbine. While true, it could be run this way, there are a host of practical reasons not to do it. Not least of them the fact that our turbine would have to be designed to withstand having a mixture of molten salt and fluorided fuel passed through it at very high temperatures. This would be tricky to say the least, likely requiring the use of those super expensive refractory metals, and while using such materials to make the odd turbine blade is one thing, an entire turbine casing is an entirely different matter. It would likely cost much more than the reactor itself!
Of course, the concept he is supposedly debunking, it seems he has somewhat miscomprehended its nature. The Energy from Thorium discussion isn't talking about running molten salt through a turbine as a working fluid. They're actually talking about running atmospheric air through a gas turbine ("open cycle") -- analogous to a jet engine or a gas-fired internal combustion turbine. (The air being heated by the reactor, through a gas/liquid heat exchanger). But hey, if we actually read and understood the thinks we were ridiculing, that wouldn't be much fun, would it?