Freon Moderated Reactor?

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

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…

Shippingport Reactor

Shippingport Reactor

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:

Materials used

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.

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).

But still…

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.

In Conclusion

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 Reactor at ORNL

Aqueous Homogeneous Reactor at ORNL

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.

Inside the reactor... a pot, some pipes, a pump and shielding.

Inside the reactor… a pot, some pipes, a pump and shielding.

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…

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About E.M.Smith

A technical managerial sort interested in things from Stonehenge to computer science. My present "hot buttons' are the mythology of Climate Change and ancient metrology; but things change...
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28 Responses to Freon Moderated Reactor?

  1. E.M.Smith says:


    Properties of Beryllium and Beryllium Oxide Moderator

    Beryllium is superior to graphite as a neutron moderator in nuclear reactors. Beryllium has been used in some reactors and beryllium oxide is proposed as a neutron moderator for high-temperature gas-cooled nuclear fission reactors. Beryllium is high in cost compared to graphite and heavy water, which often replace it.

    Some other bits are that the light weight of Be makes it more effective at scatter, and it tends to spit out extra neutrons sometimes… But it does look very similar to C and D2O in effectiveness, that would imply Freon has a shot too…

    Is an interesting one too…

    LIUM OXIDE REACTOR. MARITIME GAS-COOLED The Experimental Beryllium Oxide Reactor, EBOR, will be constructed at the National Reactor Testing Station as the AEC portion of the joint Maritime Administration–AEC Maritime Gas Cooled Reactor Program. The ultimate goal of the Program is the development of nuclear power plants employing a helium cooled and beryllium oxide moderated reactor directly coupled to a closed cycle gas turbine. The objective is to obtain compact nuclear engines suitable for use either in a merchant ship propulsion system or an intermediate size central station power plant in the 20 to 100 Mw(e) size range. The EBOR is a l0 Mw(t) test of the basic fuel element and moderator designs. It is capable of being up-graded in power at a later date to a test of the nuclear reactor turbine concept. The objective of the experiment is outlined. The principal reactor components to be tested and the test facility are described. (auth)
    Moore, W.C.
    Publication Date:

    So in 1961 someone was looking to make a Be oxide / gas cooled reactor of very small size for ships.

    Click to access 19667.pdf

    says it got cancelled due to progress on carbon moderators… (page 8) but only says “before Uranium fuel” could be loaded and says nothing about enrichment level needed…

    Further down:

    3.6 Organic Moderated Reactor Experiment, OMRE

    The Organic Cooled and Moderated Reactor has been considered as a thermal neutron
    spectrum shipboard power plant. The Terphenyl waxy organic coolant was considered
    promising because it liquefied at high temperatures but did not corrode metals like water.
    Also, it operated at low pressure, significantly reducing the risk of coolant leak and loss of
    coolant through depressurization. A scaled-up reactor, the Experimental Organic Cooled
    Reactor, was built in anticipation of further development of the concept.

    The rectangular-plates fuel clad in aluminum can be natural uranium since the organic
    coolant can have good moderating properties.
    The cladding temperature can reach 800 oF
    with an average cycle time of 2,160 hours or 2,160 / 24 = 90 days. The overall heat transfer
    coefficient of the coolant is low with the formation of polymers under irradiation that
    require an adequate purification system. The perceived advantages are negligible corrosion
    and the achievement of low pressure at a high temperature.

    A Diphenyl potential coolant broke down under irradiation. The hydrogen in the compound
    turned into a gas, forming bubbles. The bubbles reduced the moderator density and made it
    difficult to maintain the chain reaction. The initially clear liquid turned into a gummy and
    black breakup product. No uranium fuel ever was loaded into the reactor and it never
    operated or went critical before the program was cancelled.

    So if organic moderators let you make natural U “go”, then fluorocarbons ought to work even better! (And likely will have similar problems with beakdown products… perhaps reduced by the stronger tendency for F to hang onto the C bond…)

    I think I have my answer… Freon or Teflon moderated reactors would likely work on natural Uranium, with occasional issues of breakdown products and / or cleaning up the stuff before it got too gooey. So maybe make a reactor with the Freon or Teflon in some kind of isolated pipes / tanks and have another liquid as the heat transfer fluid with pumps and all. Like the CANDU, where Heavy Water is in one space and light water is used to carry heat out in other spaces.

    OK, now I really want one ;-)

  2. M Simon says:

    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”.

    As a long ago Naval Nuke – I found this paragraph confusing. Because it depends. Is U235 the main fuel? Or are you specifically addressing a breeder?

    The preferred terminology for “slow neutrons” is thermal neutrons. They are in thermal equilibrium with the medium they traverse. This is due to the numerous collisions required to get them to slow down.

    The distance a neutron has to travel to be thermalized depends on the mass ratio. If it is 1 to 1 (hydrogen) the energy loss per collision is greatest. As the mass ratio goes up (or down depending on numerator – denominator) the energy loss per collision goes down.Considerably. And then there is the scattering cross section. And absorption cross section. The ratio of those affects the neutron economy.

    Well this discussion is bringing back a lot of stuff I thought I’d forgotten.

  3. M Simon says:

    BeO dust is very poisonous. It is why BeO is no longer sold for power transistor insulators.

  4. M Simon says:

    Water corrosion is not too bad if you monitor and control the water chemistry. pH in the 9 to 10 range. Slightly alkaline.

  5. M Simon says:

    The problem with carbon as a moderator is the boron impurities. Just 1 ppm can ruin the neutron economy.

  6. Another Ian says:


    One of my lecturers in radiation courses noted that the designers/builders of the British graphite/CO2 power plants knew what they were doing

  7. poitsplace says:

    BTW, NaK is actually even more reactive than either sodium or potassium.

    And while yes, you can breed Thorium, the extremely low output levels are not considered good enough for bomb making…it’s not like uranium/plutonium cycles where you get out twice as much as you put in. Plus we’d basically have to use uranium/plutonium to jump-start a thorium program (not that I have any problem with that…I’m all for it since there’s enough thorium to power the entire earth at US per capita energy consumption rates (including transportation/industry) for over ten thousand years…LOL, sustainability where energy is concerned is effectively not an issue.

    One reactor type I didn’t see you mention, Bill Gates had thrown some backing behind a company that was planning to make traveling wave reactors. Basically you separate out the fissile uranium, at put it all at one end of a gigantic, sealed, life-time supply of fuel…and then just have the control rod assemblies slide farther and farther down the pile as it consumes the fuel.

  8. omanuel says:

    Thanks for an interesting review of reactor technology.

    The main impediment to developing and using nuclear energy is an error purposely introduced in the foundation of nuclear physics after WWII to hide the source of energy in heavy elements, reactors, atomic bombs, the Sun and galaxies of starships – NEUTRON REPULSION:

    Aston’s nuclear packing correctly reveals both
    1. Long-range Coulomb repulsion between protons, and
    2. Stronge, but short-range repulsion between neutrons

    Weizsacker’s nuclear binding energy equation replaced Aston’s nuclear packing fraction in nuclear physics textbooks after WWII, although that equation FALSELY
    1. Exaggerates the force of Coulomb repulsion between protons, and
    2. Underestimated the force of REPULSION BETWEEN NEUTRONS.

  9. omanuel says:

    CORRECTION: and galaxies of stars

    Many nuclear engineers prefer to ignore the obvious error of greater values of nuclear binding energy per nucleon (BE/A) in

    1. Radioactive H-3, C-14 , I-129 etc., than in
    2. Stable decay-products He-3, N-14, Xe-129

    This post-WWII, biased view of nuclear energy distorts understanding of energy stored in each of the 3,000 types of atoms that compromise all matter.

    Solar physicists face an even greater challenge than nuclear physicists in accepting reality that like nuclear reactors and atomic bombs,

    1. Stars make and the discharge hydrogen to interstellar space

    2. The H-rich surface of the Sun is hotter than its Fe-rich interior

  10. E.M.Smith says:


    Yeah, “simplifying things” for a common audience often glosses over bits that are very important, but only to folks “experienced in the art”. I dodged the issue of differential U-235 vs U-238 absorption of fast vs slow neutrons. Fast neutrons tend to turn U-238 into Pu and NOT give more neutrons. Nice in the long run as you are getting more fuel, but in the short run it quenches your reaction… Slow neutrons absorb better into U-235, fission, and give you more neutrons. Thus a reactor with mostly slow neutrons can use regular unenriched U.

    Probably ought to have not oversimplified to oblivion ;-)

    Yeah, part of my “design goal” is to avoid Be at all costs. The stuff is nasty in a person.

    Water is good as a coolant, but the H in it soaks up too many neutrons to use it with natural U. As “my goal” is to bypass completely U enrichment and make a very neutron rich small reactor to breed Th to U-233 (and chemical separate) I’m trying to minimize neutron loss. Thus the idea of using Freon as moderator / coolant. Yeah, the whole reactor has to be run at a lower temp ( I think, but ought to check the breakdown / reaction temps for Freon…) but since my goal is not spinning a turbine to make electricity, but rather to make a lot of thermal neutrons for Th to soak up…

    The boron impurities came from the way graphite was made, on boron rods. I’m assuming that isn’t the way carbon fibre is made as it is typically done from a plastic (made in turn from natural gas) that has been high temperature de-hydrogenated. Ought to be near zero B in it. Yes, ti’s that “outside the box” or sometimes “where did I leave the box?” thinking in me ;-) So back in 1940 they didn’t have carbon fiber. It was graphite or carbon black or diamond. Now we do have carbon fibre. It’s strong (at least until the neutron flux rearranges too many of the carbons ;-) and ought to bypass that ‘graphite on boron rod contamination’ issue. So by making a rod holding structural core out of carbon carbon (or similar) it ought to be very strong, heat tolerant, and a great moderator. Then use flood cooling with Freon (IFF it doesn’t tend to make gummy bears after a while ;-) or with CO2, and you have a reactor core that is nearly 100% fuel and moderator. Nice.

    Add natural U for fuel (in what form? I’d likely go for U Oxide, but that’s open). Now here the issue is making a structurally strong fuel that “gets the heat out” quickly. This is where the heat is greatest first, and if you don’t get it out, it just builds up till things are melted. The thermal conduction of carbon carbon casing is likely not going to cut it for high heat loads, so one might need to resort to the classical clad fuel. I’d use the Magnox cladding as it is shown to work well in a carbon moderated gas cooled design. Also pretty easy for the DIY guy to reproduce. No need for buying Zr and setting off alarm bells ;-)

    “The name magnox comes from the magnesium-aluminium alloy used to clad the fuel rods inside the reactor.”

    Which brings me to:

    @Another Ian:

    I have to agree. While the Wiki calls it a “now obsolete” design, I look at it and think Classical British Design. A bit staid and proper, but everything is where it belongs, does what it ought, and most certainly does not misbehave. Oh, and when pressed, it is happy to go to war and make weapons grade Pu, though it would rather stay home and heat the kettle for tea…

    Only real issue is that the MgAl cladding melts when too hot and reacts with water. Since my target is Th breeding, not heat, I don’t care so much. I’m fairly sure Freon is inert to magnox but “needs checking” especially since 300 C is not the normal usage temp for Freon…

    It used metallic U for fuel. I lean a bit toward U oxide for the added O moderation, but it doesn’t conduct heat as well and is harder to reprocess. I could see going either way.

    In many ways my “design point” originated with the Magnox, and I added a step of “find carbon without the B in it via plastic from natural gas” and then “can we swap Freon in for CO2 as coolant?” and “Can a fluorine based plastic replace the graphite as an even better moderator so the whole thing can be made damn small?”

    My very favorite general purpose reactor design is the CANDU, but just a moment behind it is the Magnox. It is so sparing of special materials and processes, and so effective and reliable. No, not as flexible as a CANDU. No, not as efficient as a PWR. Yes, the fuel reprocessing is a pain. But for a chunk of graphite in a concrete box with beer gas as working fluid, it does sooo much…


    Um, the whole point of my little thought experiment is to bypass the whole U / Pu enrichment breeding game and get strait to Th breeder with natural U. IMHO, the USA did something like that early on. Look at the early history of nukes and there’s Th and U-233 all over the place. Then it suddenly goes quiet and disappears…. So I”m pretty sure they were breeding up buckets of that U-233 fairly easily (likely with chemical separation) and that’s all just been hidden. This is an attempt to see the same space they were seeing. Natural U reactors and then use that neutron flux to do the Th to U-233 in a distinct fuel channel. Chem separate and Bob’s Yer Uncle.

    I dispute the notion that Th breeding is not efficient. In LWR at thermal neutron ranges, Th very nicely converts to U-233 in a breeder blanket.

    Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

    One neutron in, one Th-232 to U-233 out (eventually). Since you have a Protactinium in the middle, it’s a nice place to chem separate. Have a thin layer of Th flow by a very high neutron flux, then out the other end you wait about a day. Now you have a pot of Th and Pa and a 3 week timer. Separate, recycle the Th back to the fuel feed and send the Pa off to ‘holding’. In a couple of months, it’s U-233 and very nice “boom stuff”.

    Slow? Yes, but not nearly as hard as making a Pu reprocessing plant… and way way easier than doing isotopic separation.

    Both India and the USA have made bombs with U-233 and it is “about as good as Plutonium” for that use… i.e. it’s good boom stuff.

    Now, personally, I care about Th breeding more because it would let me get U-233 to use to enrich natural U so I could make fuel for a more standard set of reactor designs. It is a VERY nice reactor fuel and added to natural U lets you run all sorts of other reactors.

    So my “vision” would be of one Freon/Teflon/Carbon-Fibre low temp breeder who’s main purpose is just to generate enough U-233 to then enrich natural U to the ‘couple of percent’ range where it can be used in other reactors without enrichment facilities. Now you can “jump start” an entire nuclear industry and power system without ever dealing with the enrichment barrier.

    Kind of like the CANDU ;-) but more attainable to the “home gamer” ;-) and without the need for heavy water that is expensive to make…

    So it dodges heavy water, enrichment, fuel reprocessing (other than a chemical separation step) and isotopic separation. A “poor man’s path to nuclear power”.

    That’s the idea, anyway. Once neutrons hit the novel materials and structures all hell breaks loose and you find out what you missed / forgot / need to change etc. etc.


    I’ve been intrigued by the “packing function” approach, thanks to your pointer.

    It is in many ways a better fit.

    I’ve also, I think, noticed a tendency to slightly ‘bend’ things and erase bits of history that seems directed at obscuring the easy paths to nuclear materials, reactors, and eventually “boom stuff”. While I laud the goal and to some extent the effectiveness of it ( 70 years later and we’ve still avoided nuclear wars and especially nuclear regional wars): I have learned to not assume that what I’m reading in any post 1950s official literature is undistorted.

    Frankly, that “negative space” nagged at me and was what got me looking at Th to U-233 to nuclear bombs, and at cheap simple easy to make reactors with common ingredients and materials. That was all about 1974 or so during the Arab Oil Embargo (and when I was hanging out with the children of the Nuke Folks at LLNL – Lawrence Livermore lab rats sent their kids to UCD a lot… I hung out with them. At one time dated the daughter of the Director of LLNL.) Then “we all talked” about power issues and designs and things. (And Hot Fusion was only 40 years away ;-)

    Once I thought I saw this pathway to ‘boom stuff”, I sat on it for many decades. Now there’s been enough of all of it “in the news” with the LFTR advocates poking the Th bear and others pointing out the potential for Pa harvesting that I’m willing to put it forward as an observation. Especially after India set off a U-233 bomb. That pretty much confirmed they knew the path, and had taken it. Which means all sorts of other folks knew. Once everybody already knows, what’s the point in being silent?…

    Oh, and yes, it has also nagged at me for a very long time that the solar wind is full of H. I’d ask myself “Where does all the H come from?” and “IF the sun has been doing this for 5 Billion Years why doesn’t it run out of H?” And answer came there none… until your point on N decay…

    To me, it now looks more like a star sucks in all sorts of matter. Lots of heavy stuff eventually crashes into it. Then it gets gravity crushed to degenerate form. Eventually neutrons that start to decay into H, and we get to see the solar system as it looks today. Vast sheet of stellar wind blowing H out into the empty spaces…

    At least that’s my thumbnail view of things based on your N descriptions…

  11. omanuel says:

    @E.M. Smith

    1. If global cooling occurs, instead of global warming, and . . .
    2. If NASA is unable to hide the evidence,

    We will probably see:
    a.) The Sun get hotter
    b.) The Earth and other planets cool
    c.) Ratios of lightweight/heavy elements and isotopes, like O/Fe and Ne-20/Ne22, decrease at the solar surface.

    I believe these changes occur on a regular basis with each solar cycle but NASA has so far succeeded in hiding the evidence from most observers.

  12. Larry Ledwick says:

    Like all cases of security through obscurity, the information will eventually leak out, In the middle ages they could not keep the formula and method for black powder production secret, and they never could keep nuclear methods secret to prevent proliferation. The internet just speeds up that info leakage as so many minds trading ideas will eventually stumble on all possible options (often many times).

    I also concluded many years ago that even a research reactor that supposedly could not be used for breeding fissile material could be used for that surripticiously with very simple shade tree mechanic methods and lots of patience.

    My thought was a channel (or channels) that would allow a wire of appropriate seed material to pass through a high neutron flux zone. (dangle a loop of wire into a pool reactor so the bottom of the loop hangs near the core) and periodically move that wire and then take the portion which had been exposed to high neutron flux and process it for the new isotope you are looking for. Small simple and nearly impossible to monitor. When the inspectors come, just yank the wire loop out and smile.

    Just like in Japan and Germany during WWII, such backyard production methods actually were out producing large factories at the end of the war and they were nearly impossible to find, and target.

    I would be absolutely astonished if Iran and N. Korea have not tried similar dispersed production methods in small random mines and other sheltered locations, and have a significantly larger stock pile of boom stuff than is widely believed. Highly motivated clever people almost always out fox the “experts” who make limiting assumptions of massive production facilities which are not necessarily required.

  13. Larry Ledwick says:

    Ref your comment about carbon fiber:
    From Wiki

    In 1960 Richard Millington of H.I. Thompson Fiberglas Co. developed a process (US Patent No. 3,294,489) for producing a high carbon content (99%) fiber using rayon as a precursor.

  14. p.g.sharrow says:

    A small bit of information from a refrigeration man. I have operated a very hot system for an extended time with no known problems.. F-22 or Freon 22
    lubricated with mineral oil, head and condenser temperatures near 450F. This was a water cooled fast freeze walkin box.
    While the mix was stable at that temperature, I’m not sure about how stable it would be under radiation.
    I am sure everything has been tested under radiation but locating the information is the hard thing. The Navy Nuc boys are the best place to dig. They have tried everything that they might think of and have no agenda except, What works and is SAFE…pg

  15. E.M.Smith says:


    Interesting… I thought of a ribbon (flat sides to max flux) with thickness and speed optimal for the production of what you want with minimal secondary products (2 neutrons making bad stuff). And with loop retraction automatic on door open… But yes, all it takes is a fast fuel channel and patience.


    Nice to know. That’s hotter than I think needed, and chemicl stability tends to track neutron damage stability for light elements (where it is mostly recoil from impact displacing bonds).

    Golly, this thing might actually work! 8-)

  16. Larry Ledwick says:

    I like the ribbon idea, let’s go with ribbon 2.0, where at the point of maximum neutron flux. the ribbon passes around a sheave made of a good neutron reflector of high atomic weight (so neutrons are not thermalized too much). By having a reflector behind it, the neutrons which passed through the foil ribbon without interaction make a second pass through the material after reflection, approximately doubling their chance of interacting with the foil.

    I love designing things I will never build ;) as a mental exercise in problem solving.

  17. gallopingcamel says:

    “The chemistry will be unexplored and who knows what neutron bombardment will do to it (though it is known that Teflon degrades).”

    I used to think of Teflon as a superior material. However when I used Teflon insulated wiring in the Duke Free Electron Laser it disintegrated owing to the gamma ray flux. PVC insulated wires worked much better but the colors tended to fade. Materials that are insensitive to gamma rays include fiber glass and cross linked polyethylene.

    Telecommunications grade fiber optic cables are not suitable for use in high radiation environments unless you use fluorine doped fiber:

  18. Tom says:

    Per the Robert Alvarez report, the US had as of 2012 a bit more than 1500kg in inventory of U233 for a jump start of a thorium fuel cycle lineage.

    Indications are that a power cycle based on super-critical Co2 working fluid at 760C and above the 7.38MPa critical pressure are feasible.

    Click to access 47-6-1.pdf

  19. Bennett In Vermont says:

    E.M. Smith,

    I love how much I learn reading your posts, and the comments your learned readers submit.

    Thanks Gents!

    Now it’s off to tell the 10 year old why his interest in chemistry and math is a really good thing.

  20. R. de Haan says:

    Here is the stite, Dutch cheerleader of Thorium Reactors at the TU Delt.

    Last year I attended a kind of seminar about Thorium and listed to a presentation from Kloosterman. The TU Delft thanks to the late Wubbo Ockels is completely soaked in Climate Change mambo jambo and I left their premises in total shock and awe about the level of gnorance and the opposition from greenies who were convinced that Thorium would threaten their clean sustainable wonderland of solar PV and wind mills.
    Kloosterman claimed it would take at least 25 years before a working Thorium Reactor coukd be operational and according to the Greenies that was much to late so better firget about it.

    Just like you I think the DIY development based on Boiler and Plumber skills, good research and common sense and the same mindset lke the guys from the sixties could short cut the entire process.
    All the materials can be acquired and as I see it ample will be involved.
    Just build that thing and talk later.
    The TUDelft clan isn’ t going to do it.

  21. R. de Haan says:

    Another Thorium Enthousiast who believes he can capture a 150 billion dollar market with his Thorium Laser Turbine able to power trains. planes and automobiles as well as the local soup kitchen and provide us with a bright sustainable future.

  22. E.M.Smith says: Haan:

    That last link looks dodgy to me. Nothing there makes sense…


    I’ve got another poor man’s reactor idea…

    Cross that last homogeneous with a pebble bed… So use small ceramic Uoxide balls in a steel pot, flow freon through it as moderator and coolant. A melt plug in the bottom dumps the pebbles into a steel catch pan, spreading them out, and the moderator drains to another space, eliminating criticality passively if too hot. In normal use, pebbles are added to the hopper of freon until you have the reaction rate desired. I’d also have borated rods available to quench without a pumpout or melt, if needed.

    So a pot, fluid pump, pellet handler, emergency systems. For Thorium breeding, add a Th blanket area as desired…

    As O is a good moderator, and UO ceramic pellets don’t melt until crazy hot (couple of k C and near 5k F) the fuel never melts (all freon moderator long gone before that, and even before the melt plug spreads the pellets in the catch tray…) it ought to be inherently self safing.

    So being entirely fuel and good moderator atoms ( O, C, and F ), it ought to be way small and run on natural U w/o enrichment. Running it fairly cool, Magnox (MgAl alloy) ought to be usable for the melt plug and the Th channels (though hot freon on magnox alloy needs proving – it doesn’t like water…) I’d be reluctant to just mix Th pellets in the mix as they would not be optimally distributed and prone to forming irregular sized concentrations… so either a blanket shell or tubular fuel channels around the edges.

    I’m pretty sure this would work, and even if the freon needed frequent changing, would be much cheaper than heavy water… The Poor Empires SNM Breeder :-)

  23. cdquarles says:

    Hi EMS.

    My last undergraduate bits of chemistry were x-ray crystallography and symmetry. To me, the concept of a packing fraction does not necessarily make a binding energy concept wrong, or vice-versa. They are two ways to view something.

    Heck no, I don’t want to be anywhere near a NaK sample without it being kept very dry and away from halogens and oxygen.

  24. Larry Ledwick says:

    Interesting item on nuclear reactors and accidents like Chernobyl.

  25. Graeme No.3 says:

    Following up I found
    this gives detail on the history and problems which might interest some readers.
    There were early attempts using slurries not solutions, so when I found
    which mentions a solid homogeneous reactor: e.g. mixture of uranium (UO2) in polyethylene, I wondered about a slurry of micronised fuel in perfluorinated oils. The latter are non toxic and considered inert. Whether they would break down releasing fluorine I don’t know and any way it would probably react with the uranium (or thorium) oxide. At least it wouldn’t result in radiolysis making a potentially explosive hydrogen/oxygen mix.
    Corrosion would be less of a problem with a non aqueous mix. Abrasion problems might be minimized by a non-stick coating (or even Tedlar film) on the reactor walls, which would mean some heat removal from the circulating mix.

    Still, “Rehabilitation of the aqueous homogeneous reactor is a favorite pastime of nuclear engineering undergrads. They have all found that there is no combination of materials, pressure, and chemistry at any given power density that will yield an economically viable power reactor.”

  26. Fabbersmith says:

    Looks like it might let me post. the little project I was researching when I found this was a nuclear thermal rocket on the cheap. It was the premise of a sci-fi story of a billionaire building a radical escape vehicle for him and his cohort, thus had to avoid detection, keep it simple and reliable, etc.

    I wanted to avoid any kind of processing if at all possible, so just extracting uranium(or maybe thorium) from seawater, turning it into fuel, and throwing it in–liquid fuel was very tempting.

    I was looking for info on silicon as a moderator, since I wanted to use a fused quartz matrix (3d printed via SLS) for the reactor core. It looks like silicon dioxide will moderate, though it’s life as a core is debatable. Hey, as long as you can reach the asteroid belt and make a new one. This also provides temperature resistance–the thought was a higher temp system that’s more efficient, and can handle just sitting there possibly without any cooling at all.

    I’m also now looking at CO2 as a propellant, instead of high purity water. Probably lower ISP, but will be easier than hydrogen or heavy water. It actually sounds almost buildable.

  27. E.M.Smith says:

    Silicon is about the same barns as zirconium, which is used for reactor fuel cladding, so ought to work, though likely needs enriched U to work.

    It is used in fuel cladding so is ok in the reactor radiation environment. Silicone rubbers ought to work too. I could easily see a silicone with mediun levels of C and H along with Si–O working as a moderator, but would need a neutron economy calculation to determine enrichment level needed.

  28. E.M.Smith says:

    Confirmation of SiO2 moderator:

    Another vital condition for self-sustaining nuclear reaction is the high content of a moderator to slow the neutrons, Meshik said. Water, carbon, most organic compounds, silicon dioxide, calcium oxide and magnesium oxide all are natural neutron moderators. Also, the concentrations of neutron absorbents – iron, potassium, beryllium, and especially gadolinium, samarium, europium, cadmium and boron – should be low.

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