So over on the LENR thread a discussion of Thorium reactors broke out. For anyone unfamiliar with the current “push” for a safer nuclear reactor fuel cycle, the True Th Believers are pushing a Thorium Molten Salt Reactor (MSR) and in particular, one using Florine salts in the https://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactor
The liquid fluoride thorium reactor (acronym LFTR; pronounced lifter) is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based, molten, liquid salt for fuel.
Molten-salt-fueled reactors (MSRs) supply the nuclear fuel in the form of a molten salt mixture. They should not be confused with molten salt-cooled high temperature reactors (fluoride high-temperature reactors, FHRs) that use a solid fuel. Molten salt reactors, as a class, include both burners and breeders in fast or thermal spectra, using fluoride or chloride salt-based fuels and a range of fissile or fertile consumables. LFTRs are defined by the use of fluoride fuel salts and the breeding of thorium into uranium-233 in the thermal spectrum.
Note the generic qualifications of a MSR as using F or Cl salts of U or Th (or, one presumes Pu) and various isotopes for fission or breeding. It’s a wide ranging scope of things you can do in a MSR, and it’s been done at various times in the past. The “crazy idea” of a nuclear powered airplane used a MSR, for example.
But, the present “hot button” of the LFTR is all about the Thorium, and as a Florine salt.
In a LFTR, thorium and uranium-233 are dissolved in carrier salts, forming a liquid fuel. In a typical operation, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine. This technology was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s. It has recently been the subject of a renewed interest worldwide. Japan, China, the UK and private US, Czech, Canadian and Australian companies have expressed intent to develop and commercialize the technology. LFTRs differ from other power reactors in almost every aspect: they use thorium rather than uranium, operate at low pressure, receive fuel by pumping without shutdown, entail no risk of nuclear meltdown, use a salt coolant and produce higher operating temperatures. These distinctive characteristics give rise to many potential advantages, as well as design challenges.
The “design challenges” are generally directly related to those advantages of a molten corrosive salt at extraordinary high temperatures. So, yeah, no meltdown as the thing is already melted…
But you can do it with 1960s technology…
For technical and historical reasons, the three are each associated with different reactor types. U-235 is the world’s primary nuclear fuel and is usually used in light water reactors. U-238/Pu-239 has found the most use in liquid sodium fast breeder reactors and CANDU Reactors. Th-232/U-233 is best suited to molten salt reactors (MSR).
Note well that “historical reasons”… There is nothing that ties a given fuel to a given reactor type. It is mostly just a matter of who made the first one of a particular kind. The last few decades have seen folks revisiting some of those “historical reasons” and putting Mixed Oxide (MOX) of U and Pu into fuel bundles for use in light water reactors, along with Th in fuel bundles in some other light water reactors. The first MSR at Oak Ridge was fired up on Uranium. The exact operational requirements and materials details change with the fuel, but it’s pretty much any fuel in any reactor type (with appropriate design choices).
Alvin M. Weinberg pioneered the use of the MSR at Oak Ridge National Laboratory. At ORNL, two prototype molten salt reactors were successfully designed, constructed and operated. These were the Aircraft Reactor Experiment in 1954 and Molten-Salt Reactor Experiment from 1965 to 1969. Both test reactors used liquid fluoride fuel salts. The MSRE notably demonstrated fueling with U-233 and U-235 during separate test runs. Weinberg was removed from his post and the MSR program closed down in the early 1970s, after which research stagnated in the United States.
Note that they used both U-233 and U-235 in separate runs to get the thing going. Either one works. Then the Thorium can absorb the extra neutrons and make more U-233. That’s the basic idea.
Note too that they don’t bother to say why Weinberg got dumped and the MSR with him. They were working fine, after all. The reason is political. Nixon came in and the “push” was to make a Liquid Metal Fast Breeder reactor. Can’t have an easy Th breeder competing with the gigantic and expensive LMFBR, so kick out the guy who likes it and kill his projects, replace with your guy… Why use a simple, safe, cheap, proven reactor design when your cronies can rake in buckets of money trying to make a gigantic, expensive, difficult, complicated, and unknown design go?…
The Wiki has more on that:
In 2006 all large-scale fast breeder reactor (FBR) power stations were liquid metal fast breeder reactors (LMFBR) cooled by liquid sodium. These have been of one of two designs: [Open Loop, and Pool descriptions elided]
All current fast neutron reactor designs use liquid metal as the primary coolant, to transfer heat from the core to steam used to power the electricity generating turbines. FBRs have been built cooled by liquid metals other than sodium—some early FBRs used mercury, other experimental reactors have used a sodium-potassium alloy called NaK. Both have the advantage that they are liquids at room temperature, which is convenient for experimental rigs but less important for pilot or full scale power stations. Lead and lead-bismuth alloy have also been used. The relative merits of lead vs sodium are discussed here.
It goes on from there. What I find most interesting in that quote is that NaK is a eutectic liquid at room temperature! I need to get me some ‘o that! ;-)
Now I’d rather have a molten salt than a highly reactive hot metal. I’ve had both hit my skin (and have the scars to show for it) and generally the liquid molten metal does a worse job on you… but that’s just me. I’d also rather have a slow breeder than a fast breeder (as a lot of materials issues can be caused by a high fast neutron flux).
That link goes on to mention the MSR breeder:
Another proposed fast reactor is a fast molten salt reactor, in which the molten salt’s moderating properties are insignificant. This is typically achieved by replacing the light metal fluorides (e.g. LiF, BeF2) in the salt carrier with heavier metal chlorides (e.g., KCl, RbCl, ZrCl4).
Several prototype FBRs have been built, ranging in electrical output from a few light bulbs’ equivalent (EBR-I, 1951) to over 1,000 MWe. As of 2006, the technology is not economically competitive to thermal reactor technology—but India, Japan, China, South Korea and Russia are all committing substantial research funds to further development of Fast Breeder reactors, anticipating that rising uranium prices will change this in the long term. Germany, in contrast, abandoned the technology due to safety concerns. The SNR-300 fast breeder reactor was finished after 19 years despite cost overruns summing up to a total of 3.6 billion Euros, only to then be abandoned.
As well as their thermal breeder program, India is also developing FBR technology, using both uranium and thorium feedstocks.
Now several interesting points there. You can get a “fast breeder” with Cl salts, or a moderated thermal breeder with F salts. So F acts as a moderator… Nice to know… Note, too, that a FBR was built as early as 1951. This isn’t hard to do. But being not-hard to do didn’t stop Germany from getting scared and running away from a pile of money.
Finally, that last line about India. They have both a thermal breeder set, and are developing FBR, and can run either U or Th as desired. These things are flexible…
Thermal breeder reactor
The Shippingport Reactor, used as a prototype Light Water Breeder for five years beginning in August, 1977
The advanced heavy water reactor (AHWR) is one of the few proposed large-scale uses of thorium. India is developing this technology, their interest motivated by substantial thorium reserves; almost a third of the world’s thorium reserves are in India, which also lacks significant uranium reserves.
The third and final core of the Shippingport Atomic Power Station 60 MWe reactor was a light water thorium breeder, which began operating in 1977. It used pellets made of thorium dioxide and uranium-233 oxide; initially, the U-233 content of the pellets was 5–6% in the seed region, 1.5–3% in the blanket region and none in the reflector region. It operated at 236 MWt, generating 60 MWe and ultimately produced over 2.1 billion kilowatt hours of electricity. After five years, the core was removed and found to contain nearly 1.4% more fissile material than when it was installed, demonstrating that breeding from thorium had occurred.
Now a couple of things. First off, this was done way back in 1977. Thorium and thermal breeding are NOT new. Nor hard. Second, it made 2.1 BILLION KW-hrs of electricity and ran for 1/2 a decade. This is pretty well production quality. Finally, it had a seed region of U-233. That comes from Th. That means they had already bread U-233 from Th somewhere else and that this reactor was 100% Th or U-233 from Thorium powered. Not Pu. Not U-235 or U-238. Thorium, start to end.
And in the end it made 1.4% more fuel than it used.
It is at this point that I think The Powers That Be had an Ah Hah moment and figured out a few things. One of them being that having other folks realize breeding U-233 was easy, and that it was GREAT “boom stuff” and could be chemically separated: was a Very Bad Thing for proliferation. I figured this out about 1983, and deliberately sat on the idea until just about 5 years ago. Once it was openly discussed that India was using Th to U-233 AND had made a U-233 bomb AND that MSR using Th could do online fuel reprocessing / chemical extraction of Pa (then wait 30 days and it becomes U-233); at that point me not mentioning it was kind of silly. Then there’s that issue of it being hard to control nuclear fuel if you have reactors making more than they use up. Not a lot of money in that…
But that’s what I think killed the early use of Thorium. The USA pushing “hard to do LWR designs” with NO breeding and breeding needing a FBR equally hard (or harder) to make. Reprocessing and enrichment mandatory to run a fuel cycle. And generally disappearing U-233 from discussions of “boom stuff”. They also actively discouraged Graphite and Heavy Water reactors (like the absolutely stellar CANDU) for the same reasons. So we have a lot of giant, complex, expensive, hard to fuel reactors with piles of nuclear garbage. But hey, anything was worth it to prevent India, Pakistan, China, etc. etc. from getting The Bomb… Oh, wait…
The liquid fluoride thorium reactor (LFTR) is also planned as a thorium thermal breeder. Liquid-fluoride reactors may have attractive features, such as inherent safety, no need to manufacture fuel rods and possibly simpler reprocessing of the liquid fuel. This concept was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s. From 2012 it became the subject of renewed interest worldwide. Japan, China, the UK, as well as private US, Czech and Australian companies have expressed intent to develop and commercialize the technology.
And, IMHO, why there’s been a “passive aggressive” pushback against the LFTR and Thorium thermal breeders in general. But now the cat is so far out of the bag that some sanity is starting to show up, and new reactor designs are being considered on their merits a bit more (who contributed to the campaign and what company stock the Senator holds a little less… and maybe even a bit less paranoia about proliferation since it’s already established fact.)
Now back at the first wiki again.
Reactors that use the uranium-plutonium fuel cycle require fast reactors to sustain breeding, because only with fast moving neutrons does the fission process provide more than 2 neutrons per fission. With thorium, it is possible to breed using a thermal reactor.
Neutrons have some speed. When going very fast with lots of energy, they are called fast neutrons. Going slowly, they are more like warm soup, and are called thermal neutrons. (You can also make cold neutrons, but that’s outside the scope here, but just realize you can pipe neutrons around in a plastic pipe if they are cool enough…) When a fast neutron whacks into a U, it can split it and spit out more neutrons. Or it can escape the system as it blows on by. As the neutrons get more “thermal”, they tend to be absorbed more. At some point you are not making enough new neutrons from splitting atoms to replace the absorbed ones and the reaction halts. This is “neutron economy”.
For Thorium, it’s able to keep the neutron flux high enough even with thermal neutrons. So a Th breeder is easier to make than a U to Pu breeder. In many ways “that’s clue”… Want a back door to Special Nuclear Material? (aka “boom stuff”) Well, you need to breed it. So what’s the easiest starting point for breeding SNM?… but I digress… Though I’m pretty sure that “back door” method was why the USA actively killed off the early Thorium programs and advanced the hard to make go U to Pu cycle with light water reactors.
At any rate, take your typical moderated thermal reactor (be it light water or the CANDU style heavy water or even the Carbon Moderated ones) and chuck some Th into it, pretty soon you have a nice supply of U-233 that can be chemically separated. Turn it over fast enough, you avoid a lot of the nasties like U-232 and such, but at the price of many cycles of chemical separation. Turn it over very slowly, you have a nice power reactor that makes as much fuel (or more) than it consumes. That’s your basic starting point.
This was proven to work in the Shippingport Atomic Power Station, whose final fuel load bred slightly more fissile from thorium than it consumed, despite being a fairly standard light water reactor. Thermal reactors require less of the expensive fissile fuel to start, but are more sensitive to fission products left in the core.
There are two ways to configure a breeder reactor to do the required breeding. One can place the fertile and fissile fuel together, so breeding and splitting occurs in the same place. Alternatively, fissile and fertile can be separated. The latter is known as core-and-blanket, because a fissile core produces the heat and neutrons while a separate blanket does all the breeding.
Reactor primary system design variations
Oak Ridge investigated both ways to make a breeder for their molten salt breeder reactor. Because the fuel is liquid, they are called the “single fluid” and “two fluid” thorium thermal breeder molten salt reactors.
And on it goes…
But that trip down memory lane isn’t why I’m writing this. I’m fairly sure the folks of 2020 can recreate a technology from 50 to 60 years ago and not screw it up.
What caught my eye was that F as moderator. There are a few basic points of reactor design. You pin them down, then the rest of the design flows from those constraints. What fuel? It isn’t always a constraint (see all the rectors retrofit with MOX or Th fuel bundles), but if you want a flexible design, it’s best to have the range of fuel in mind up front. What speed neutrons? (It impacts neutron economy, breeding, fission products, materials aging, all sorts of stuff). IF thermal neutrons, what moderator? (Gas – CO2 or He, Liquid – Light Water, Heavy Water, mix, metal. Solid – Carbon and some others). Then how to get the heat out (gas, water, steam, liquid metal, whatever). After that, you are off in the weeds of Engineering Design Land.
Notice I did not list Fluorine in the moderators… It just isn’t typically mentioned.
The Wiki on neutron moderators says ‘the usual’:
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide.
Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.
Currently operating nuclear power reactors by moderator Moderator Reactors Design Country none (fast) 1 BN-600 Russia (1) graphite 29 AGR, Magnox, RBMK United Kingdom (18), Russia (11) heavy water 29 CANDU PHWR Canada (17), South Korea (4), Romania (2), China (2), India (18), Argentina, Pakistan light water 359 PWR, BWR 27 countries
Most use Light Water, despite it making it much harder to make a decent reactor and requiring enriched fuel.
Some use either graphite or heavy water. Both of which make for dandy reactors that are also good for making “boom stuff”.
Russia has a working fast reactor without a moderator. (Oh Joy…)
Note, too, that the Magnox also used CO2 gas as a heat extraction fluid, and CO2 is also a nice moderator, but not listed in the wiki chart…
Waaayyy… down further it gives a more complete list:
Hydrogen, as in ordinary “light water.” Because protium also has a significant cross section for neutron capture only limited moderation is possible without losing too many neutrons. The less-moderated neutrons are relatively more likely to be captured by uranium-238 and less likely to fission uranium-235, so light water reactors require enriched uranium to operate.
There are also proposals to use the compound formed by the chemical reaction of metallic uranium and hydrogen (uranium hydride—UH3) as a combination fuel and moderator in a new type of reactor.
Hydrogen is also used in the form of cryogenic liquid methane and sometimes liquid hydrogen as a cold neutron source in some research reactors: yielding a Maxwell–Boltzmann distribution for the neutrons whose maximum is shifted to much lower energies.
Hydrogen combined with carbon as in paraffin wax was used in some early German experiments.
Deuterium, in the form of heavy water, in heavy water reactors, e.g. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.
Carbon, in the form of reactor-grade graphite or pyrolytic carbon, used in e.g. RBMK and pebble-bed reactors, or in compounds, e.g. carbon dioxide . Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium. Graphite is also deliberately allowed to be heated to around 2000 K or higher in some research reactors to produce a hot neutron source: giving a Maxwell–Boltzmann distribution whose maximum is spread out to generate higher energy neutrons.
Beryllium, in the form of metal. Beryllium is expensive and toxic, so its use is limited.
Lithium-7, in the form of a lithium fluoride salt, typically in conjunction with beryllium fluoride salt (FLiBe). This is the most common type of moderator in a Molten Salt Reactor.
Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and it requires special design to achieve sufficient density; lithium-6 and boron-10 absorb neutrons.
But little mention of Fluorine…
So now I’m wondering just what it’s Barns might be… (Neutron absorption cross section is measured in “barns” as a joke about hitting the broad side of a barn… Nuclear physicists have an active sense of humor, based on the ones I’ve met ;-)
Periodic Table of Elements Sorted by Cross Section (Thermal Neutron Capture) Name Sym # 0.00019 σa/barns Oxygen O 8 0.0035 σa/barns Carbon C 6 0.007 σa/barns Helium He 2 0.0092 σa/barns Beryllium Be 4 0.0096 σa/barns Fluorine F 9 0.03 σa/barns Polonium Po 84 0.034 σa/barns Bismuth Bi 83 0.04 σa/barns Neon Ne 10 0.063 σa/barns Magnesium Mg 12 0.171 σa/barns Lead Pb 82 0.171 σa/barns Silicon Si 14 0.172 σa/barns Phosphorus P 15 0.184 σa/barns Zirconium Zr 40 0.232 σa/barns Aluminum Al 13 0.3326 σa/barns Hydrogen H 1
Golly, Oxygen is GREAT, at 0.00019 barns. No wonder CO2 and H2O or D2O are good. Zirconium, often used for fuel cladding since it does few captures, is way down the list at 0.184 and just above aluminum and hydrogen. Here, too, we see why light water is kinda crummy. H is 0.3326 barns… Going back to the top, Carbon is just below Oxygen. This strongly suggest a CO2 moderated reactor could run on unenriched U (and likely why it was used in the Magnox). Then just below it you get Helium, used in the HTGCR as coolant and Beryllium, stated as working, but expensive. Then, just below Be is F at almost the same barns. 0.0092 vs 0.0096 barns (carbon at 0.0035 notably better, but all of them ‘way better’ than H).
Oddly missing from their list is Deuterium…
Thermal cross section (barn) Fast cross section (barn) Scattering Capture Fission Scattering Capture Fission Moderator H-1 20 0.2 - 4 0.00004 - H-2 4 0.0003 - 3 0.000007 - C (nat) 5 0.002 - 2 0.00001 -
So this puts it about 0.0003 or in the area of oxygen (well, 3 x as bad, but 3 x nearly nothing is still nearly nothing…) No wonder D2O works so well…
Unclear to me is if Beryllium is ‘good enough’ to make a reactor that does not need enriched fuel to run. Carbon is good enough, but Beryllium is not as good as carbon, but is it ‘good enough’? IF it is, then Fluorine ought to be near “good enough” as well.
Now one of the games I play with myself from time to time is “How can the Average Joe do something technically advanced using common junk?” This is especially fun when dealing with nuclear things. Like that kid that made a reactor of sorts out of the stuff (Americium) in smoke detectors… Think of it as “a Sheldon moment” ;-)
First off, I got to pondering making a carbon moderated reactor, but with an update. Why use graphite? Doing it today, I’d use Carbon Fibre or Carbon Composite. It already ought to be fairly clean in terms of no Boron contamination (graphite was commonly made on Boron rods and that contamination of their graphite with boron poisoned the German reactors, which is why we speak English today and the Iron Cross is not over England and the USA…) So that ultra clean carbon used to make carbon fibre ought to work nicely. Not as cheap as graphite, but way prettier ;-)
Then that “paraffin” mention got me thinking. Yeah, you could use a petroleum product too. But those H atoms… I don’t want to do enrichment… How about replacing them with Fluorine?
Now you could not run this thing very hot or it would cook, but for a nice little DIY toy, or as a low temp reactor mostly used to breed Th into U-233 to avoid enrichment… How about a reactor using Teflon or Freon moderators?
Think about it for a minute.
Lots of Carbon – good moderator.
Lots of Fluorine – good moderator.
No hydrogen – mediocre moderator.
For a while I toyed with the idea of using a fluorocarbon alcohol, but they are more reactive chemically and adding just one oxygen (or two or even three) is unlikely to make that big a difference in neutron economy. (BUT, if just on the edge of running unenriched… using a carboxy perfluoro compound… might be enough. https://en.wikipedia.org/wiki/Perfluorinated_carboxylic_acid
Yeah yeah yeah, I know, why bother. Graphite is well understood and all. Having some plastic or a gas just gets into thermal limits and pressure issues. The chemistry will be unexplored and who knows what neutron bombardment will do to it (though it is known that Teflon degrades).
The idea of a nuclear breeder reactor made by piling up blocks of Teflon, with unenriched U oxide in fuel bundles, and with Th Oxide in a breeder blanket, then the whole thing in big pot full of Freon as coolant… Well, I think I could actually build one of those… Run cool enough, it ought to make a decent amount of U-233 in the breeder blanket too. (IF needed, put the whole thing in a swimming pool of regular water as a large cheap neutron reflector. That’s how “pool type research reactors” work…)
I mean really, what would be more cool than a plastic nuclear reactor? 8-)
I know, not only are there all sorts of unexplored materials issues, but the locals would likely not appreciate the FBI / CIA / TLA du jour raid nor my home glowing in the dark… and I’d not really want the neutron flux (thus, the swimming pool…) cooking me. Besides, I don’t have a swimming pool.
Just as a closing point, having gotten this far:
In the early days of nuclear reactors, before people learned that they had to be terribly complicated, cost millions of dollars, need massive fuel cycle facilities, and could only be done by governments and giant multinational corporations; folks made some very interesting very simple and cheap reactors.
I’m especially fond of this one as a “DIY” candidate:
The Pot, Pipe, and Pump reactor.
Aqueous homogeneous reactors (AHR) are a type of nuclear reactor in which soluble nuclear salts (usually uranium sulfate or uranium nitrate) are dissolved in water. The fuel is mixed with the coolant and the moderator, thus the name “homogeneous” (‘of the same physical state’) The water can be either heavy water or ordinary (light) water, both of which need to be very pure.
A heavy water aqueous homogeneous reactor can achieve criticality (turn on) with natural uranium dissolved as uranium sulfate. Thus, no enriched uranium is needed for this reactor. The heavy water versions have the lowest specific fuel requirements (least amount of nuclear fuel is required to start them). Even in light water versions less than 1 pound (454 grams) of plutonium-239 or uranium-233 is needed for operation. Neutron economy in the heavy water versions is the highest of all reactor designs.
AHRs were widely used as research reactors as they are self-controlling, have very high neutron fluxes, and were easy to manage. As of April 2006, only five AHRs were operating according to the IAEA Research Reactor database.
Corrosion problems associated with sulfate base solutions limited their application as breeders of uranium-233 fuels from thorium. Current designs use nitric acid base solutions (e.g. uranyl nitrate) eliminating most of these problems in stainless steels.
Homogeneous reactor experiment
The first aqueous homogeneous reactor built at Oak Ridge National Laboratory went critical October 1952. The design power level of one megawatt (MW) was attained in February 1953. The reactor’s high-pressure steam twirled a small turbine that generated 150 kilowatts (kW) of electricity, an accomplishment that earned its operators the honorary title “Oak Ridge Power Company.” However AEC was committed to development of solid-fuel reactors cooled with water and laboratory demonstrations of other reactor types, regardless of their success, did not alter its course.
Gotta love it. A reactor you can put in the bathroom. Something the local welding shop can put together.
I’m also wondering just what Uranium salts are soluble in Freon or some other CF- polymer… like an alcohol group on the end. If, just maybe, those exceptional neutron efficiencies coupled with a very simple design could make a DIY small scale home SNM program possible, and a DIY nuke… Not that I want one… well, maybe just one ;-)
And that is where this muse must end.
A way dinky reactor that is dirt simple in design, yet works very well. Pot, pipe, pump.
An alternative fuel cycle well suited to “boom stuff” and easy fuel breeding. Fuel made from beach sands (monzanite).
A rampant speculation about an alternative to Heavy Water moderator yet with similar ability to use natural uranium, at least in low pressure low temperature low power designs. Like a ‘research’ reactor.
Would it work? I have no idea. Most likely it would kill the operator and melt, IMHO. Or the perflurocarboxyethanol would would break down and free Fluorine is not your friend. I’d be happy to watch from a safe distance, though. Maybe over an internet video link… The key point of it? Most reactors were designed with the goal of making LOTS of heat. They were designed before our era of advanced structured materials. IF you shift your POV to making modest heat, but enough thermal neutrons to breed better fuel, there are other interesting and perhaps easier paths to tread. Unexplored paths. Or those with 1/2 century of dust on them.
Were I to actually try making a reactor, I think I’d try making Carbon Carbon fuel tubes, all in a carbon carbon fiber based core, immersed in liquid CO2 under pressure. Don’t know what temperatures it would be good to, but with the whole insides being Very Good Moderator, it ought to run on plain unenriched U-oxide and be quite small. It would likely need a thick steel containment to keep pressure high enough for the CO2 to stay liquid at usable temperatures, and I’d use that to drive an ammonia cycle turbine (i.e. temps low, but heat flow high). Any leak or break, the CO2 moderator goes away and the thing shuts down. OTOH, one could use a liquid fuel as in the homogeneous reactor and just pump that up into carbon fiber fuel channels.. Or maybe one could put enough carbon nano-tubes into suspension in light water to make that homogeneous design “go” without heavy water… decisions decisions…