Liquid fluoride thorium reactor

[floss]

I was wondering what are the implications of a Liquid fluoride thorium reactor   to future spaceflight  are they useable in a engine or only as a traditional nuclear plant ?

[QuantumG]

Every design I've seen intended for flight has terrible power/weight. i.e., to the point that they don't even make sense for powering an aircraft.

[floss]

Thanks would you have any pointers as where to see some of those designs ?

[QuantumG]

Here's a brief history, with some pictures.

http://energyfromthorium.com/2006/04/22/a-brief-history-of-the-liquid-fluoride-reactor/

I expect it was on NTRS that I found various other documents, but it might have been, ya know, the library.

Been a long time since I looked at it.

[floss]

Thanks with recent developments in thorium I was thinking of a dual mode reactor with 2 radiators one in the h2 fuel thank to skyrocket the pressure  and the other to provide power.

[sdsds]

Nuclear reactors of some type (and LFTRs are definitely front-runners) are going to be super-important for electricity-intensive, long-duration missions,  operating where solar power is insufficient. One kind of electricity-intensive mission is the kind that uses some form of electric propulsion, like ion engines. But if the LFTR has a useful life of 30 years ... what type of mission needs ion propulsion for that long? Shuttling main-belt asteroids closer to the Earth?

[john smith 19]

I was wondering what are the implications of a Liquid fluoride thorium reactor   to future spaceflight  are they useable in a engine or only as a traditional nuclear plant ?

Minimal as an NTR. Possibly a bit better for on site power if you're talking settlement.

[IanO]

Thanks with recent developments in thorium I was thinking of a dual mode reactor with 2 radiators one in the h2 fuel thank to skyrocket the pressure  and the other to provide power.

What recent developments? The only news on thorium in the last five years has been futile attempts to raise interest and funding in the west. As far as I can tell no actual research or development programs have been initiated except in India and China.  India has large thorium reserves, but is more interested in mixed-oxide solid fuels for their existing CANDU technology.  Only China is going after LFTR with any real support, because they are frantically pursuing all non-polluting energy sources.

Nuclear reactors are massive, and the LFTR concept only had a lunar application because it didn't require high pressure water cooling. I don't think even LFTR has the power-to-weight ratios that make it attractive for spaceflight applications, and the many moving parts are not good for long-term reliability compared to solar and nuclear-thermal.

Hopefully, Kirk Sorenson himself will chime in on this; I think he still monitors this site.

[butters]

As I understand it, the principal operating advantages of the LFTR are efficient consumption of long-lived isotopes and the primary fluid loop runs at atmospheric pressure. These advantages are very relevant to terrestrial reactors, but not so much for off-world applications. That molten salt may plug a leak on contact with the Earth's atmosphere, but it will behave differently in deep space.

[ArbitraryConstant]

I was wondering what are the implications of a Liquid fluoride thorium reactor   to future spaceflight  are they useable in a engine or only as a traditional nuclear plant ?

The alleged advantages, even if they can be demonstrated, aren't really helpful ones for spaceflight. The disadvantages like having to manage the fluid would be worse.

Like naval reactors, space reactors will likely prefer HEU.

[Vultur]

Nuclear reactors of some type (and LFTRs are definitely front-runners) are going to be super-important for electricity-intensive, long-duration missions,  operating where solar power is insufficient.

Yeah. Though as lighter solar panels are proven in space that area may shift outwards significantly. I think IKAROS had 25 um thick solar panels... if they only have to stand up to the tiny acceleration of electric propulsion and so don't need much of a support structure, you could get quite a bit of power per mass even out at Jupiter, maybe even Saturn.

(I don't know the density of IKAROS' solar panels but silicon is about 2 g/cm^3, so by that assumption about 20,000 square meters would mass 1 tonne. At 5 AU sunlight should be about 4% of at 1 AU or something like 54 W/m^2 ... if the panel is 12% efficient that's about 6.5 W/m^2 or 130 kW for that 20,000 square meters. 10 AU near Saturn would be about a fourth of that....)

I'm sure it would be more complicated than that in real life, power cables would have mass and even supported on a solar sail like IKAROS the support structure would have some mass, but still....

[Adaptation]

Every design I've seen intended for flight has terrible power/weight. i.e., to the point that they don't even make sense for powering an aircraft.

Same could be said about the solar panels we use in space flight.  OK yes some ultra light weight drones use solar but comparing aircraft use to space use doesn't make much sense to me.

[cordwainer]

Fuel cells and metal-air batteries, you already have to carry chemical fuels and ionizates for propellant so it makes more sense to get some energy directly from the propellant for use on a spaceship.(unless you have some very energy intensive onboard task) For terrestrial applications in space solar, nuclear and fuel cells all have certain advantages. For propulsion though unless your building an NTR there is not as much need to carry an onboard nuclear reactor, even in the case of a large scale electric propulsion system beta-emitting thermocouples and RTG's can provide sufficient power for most applications with a smaller weight and safety penalty.

[Adaptation]

Yes but soon the only one with any plutonium-238 will be Iran.   

[hellofu]

LFTR would be better than any other active nuclear fission reactor then maybe a traveling wave reactor. the real advantages are in it safety, ease of fueling and power/ temperature control. the basic principles that make a LFTR great apply to space except the gravity based safety system.

[vulture4]

For space I think the conventional HEU reactor is simple, power dense, and remains a good choice, at least until the fluoride salt system has a lot of ground service experience. Coupled with the 50Kw hall effect thrusters planned for the asteroid retrieval mission an HEU reactor would greatly reduce transit time for planetary missions beyond Mars.

[john smith 19]

So what exactly  do people plan to use this reactor for?

Keep in mind some of it's key benefits, like purging off Xe135 (a major reactor poison) doesn't work so well without a gravity field.

It's standard operating temp is tiny next to an NTR, as is it's P/W ratio.

BTW if you're looking to run a sustainable self expanding colony you'll need to be able to do it with natural Uranium or Thorium, because enrichment is a major PITA, needing very specialized ITAR controlled stuff in large amounts. 

[ArbitraryConstant]

LFTR would be better than any other active nuclear fission reactor then maybe a traveling wave reactor. the real advantages are in it safety, ease of fueling and power/ temperature control. the basic principles that make a LFTR great apply to space except the gravity based safety system.

Not really, and doubly incorrect for traveling wave.

I think there's a lot of misconceptions going around about exactly what makes different reactor designs good or desirable for one thing versus another, and I think the people advancing thorium or various fast breeder concepts for spaceflight are straight up missing that information and coming to conclusions that don't make sense. Watching a youtube video on LFTR just isn't going to give you that information.

What about current reactors does LFTR seek to improve on? It wants reduced waste volume and half life. It wants improved safety. It wants lower cost. It wants higher temperatures. It claims to want more fuel availability, but that's a non-issue and therefore a red herring.

Why can't light water reactors deliver this? Well, the relatively low enrichment of the fuel means they're mostly U-238, so you have the nasty stuff mixed in with a huge volume of a benign material. Additionally, U-238 gets bred into higher actinides which have long half lives, so it makes the waste problem much worse, and this adds to the heat load in shutdown, which is a safety flaw (see Fukushima). Being water cooled also limits temperature.

Ok, so why is LFTR bad for spaceflight? As John points out the technology needs gravity for a number of reasons. You have to do stuff to the fluid, when solid fuel elements would be better.

So can we get those advantages with solid fuel? As it turns out we can. HEU doesn't have the U-238 to deal with. This means it doesn't have all those actinides to deal with. Proliferation concerns prevent us from using this in civilian reactors on Earth, but not really for specialized space stuff any more than naval reactors. Typical naval reactors use water as moderator and coolant, but there's no reason a space reactor couldn't be graphite moderated and use molten metal or gas as coolant, with commensurately higher temperatures.

It seems to me LFTR wants to solve problems space reactors don't have, and it'll cause problems HEU reactors don't have.

[Adaptation]

AFIK thorium is supposed to have a very good fuel to energy conversion ratio and very low quantity of dangerous byproducts.  Thorium reactors could be a good powersource for a generational starship or an alternate to solar for an Aldrin Cycler. 

[mmeijeri]

Isn't a LFTR a more likely choice for a surface reactor?

[john smith 19]

Just a reminder that the US does have experience of a space reactor in the form of the SNAP 10a design.

http://en.wikipedia.org/wiki/SNAP-10A

With ZrH fuel and a NaK coolant it was a thermal spectrum reactor (important as fast reactors need higher enrichment to work). It produced 500W. With modern thermoelectric elements that would probably be 1Kw. Shifting to a Stirling generator that goes up to maybe 4Kw.

OTOH you could tap the heat directly and use it for things like cracking CO2 into CO and O2, base heating, cracking the Martian rust into Iron and O2, making "Marscrete" etc.

[QuantumG]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

[ArbitraryConstant]

AFIK thorium is supposed to have a very good fuel to energy conversion ratio and very low quantity of wait/byproducts.

Just use HEU, same advantages. I guess this is news people don't want to hear.

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

More or less. I think China's building a 2 MW technology demonstrator. They're pursuing several new fuel cycles.

Isn't a LFTR a more likely choice for a surface reactor?

More likely than space anyway.

If you were building a colony on Mars and you sent a reactor from Earth, it would probably be HEU. Where you'd explore other technologies would be a reactor actually built on Mars.

[Vultur]

BTW if you're looking to run a sustainable self expanding colony you'll need to be able to do it with natural Uranium or Thorium, because enrichment is a major PITA, needing very specialized ITAR controlled stuff in large amounts. 

(my italics)

I don't see any reason why US technology transfer regulations would necessarily apply to a colony on another planet or asteroid, especially if the colony was not funded or mostly not funded by the US. Presumably Russia, China, and others have enrichment technology too...

(And I suppose there would actually be not much point in restricting such technology in space; if you can hit the Earth with a nuclear weapon from, say, an asteroid, you can hit it with a big chunk of asteroid material...)

EDIT: bold

[john smith 19]

I don't see any reason why US technology transfer regulations would necessarily apply to a colony on another planet or asteroid, especially if the colony was not funded or mostly not funded by the US. Presumably Russia, China, and others have enrichment technology too...

You are mistaken if you think ITAR is solely a US thing. The I is for International as it's an international agreement. Other countries may apply it differently. That does not mean they don't apply it at all.

(And I suppose there would actually be not much point in restricting such technology in space; if you can hit the Earth with a nuclear weapon from, say, an asteroid, you can hit it with a big chunk of asteroid material...)

That's logical.

But the history of the US ITAR law (and the efforts to reform it) show that "logical" has rarely been a guiding principle.   

Personally I wonder if this thread would have been better starting with the question "What isthe best reactor for space or planetary use?"

[ArbitraryConstant]

I don't see any reason why US technology transfer regulations would necessarily apply to a colony on another planet or asteroid, especially if the colony was not funded or mostly not funded by the US. Presumably Russia, China, and others have enrichment technology too...

You are mistaken if you think ITAR is solely a US thing. The I is for International as it's an international agreement. Other countries may apply it differently. That does not mean they don't apply it at all.

It's not an international agreement, it's US export restrictions, and it doesn't unilaterally ban nuclear technology.

The applicable international agreement would be the nuclear non-proliferation treaty, and it doesn't ban sales outright either. Countries like India that build weapons without being recognized as one of the weapons states are out, but under both international and US law, sales are possible. For example China is building AP-1000 reactors, an American design.

I don't think a Mars colony would have any issues getting any nuclear technology that might be useful. That's where we'd start talking about what nuclear technology if any was the best choice given the choices and given the limits of the Mars industrial base.

[Hanelyp]

A centrifuge should be able to substitute for a gravity field in most industrial processes, including a molten salt reactor.

Many of the qualities commonly attributed to thorium are more directly associated with a molten salt reactor.  For example, high fuel burn up comes from being able to continually remove reactor poisons and replace fissioned fuel.

The liabilities I see for a thorium molten salt reactor for a spacecraft are large minimum mass, and lower power density.  Also, natural thorium232 is a fertile fuel, like U238, and must be converted in a breeder before being used for fuel.  Minor issues for a fixed planetary power station, problems for a spacecraft.

[Adaptation]

ITAR  is export restrictions on information from the US to international parties.  In principle its a good thing in practice its a huge barrier to entry and massive money pit for any small US business who falls under its umbrella.  Even stuff like spacesuits and space toilets are restricted.  Companies who want to work with international partners have to pay us government minders to flow them around while over seas to make sure they are not divulging sensitive toilet technology at an incredible expense. 

This is some bat scat crazy north korean type of implantation.




Itar was also used to shut down defense distributed even though things in the public domain are exempt from itar and everything DD made was in the public domain. 

[john smith 19]

ITAR  is export restrictions on information from the US to international parties.  In principle its a good thing in practice its a huge barrier to entry and massive money pit for any small US business who falls under its umbrella.  Even stuff like spacesuits and space toilets are restricted.  Companies who want to work with international partners have to pay us government minders to flow them around while over seas to make sure they are not divulging sensitive toilet technology at an incredible expense. 

This is some bat scat crazy north korean type of implantation.

Itar was also used to shut down defense distributed even though things in the public domain are exempt from itar and everything DD made was in the public domain.

Well they were arms and they their plans could (theoretically) be distributed outside the US --> ITAR.

Nuclear technology (which this definitely is) makes governments very twitchy. 

Personally I think if people want fission power systems to become accepted they have to operate unenriched,  and that's tough because so many nuclear engineers are trained only in light water reactor technology (whose primary design criteria was "will this work well to power a US Navy submarine," whereas a system designed from day one to support utilities would have made different choices ) which just does not work without enrichment. 

Returning to topic my instinct is this tech will only be relevant off Earth when people need the kind of power stations you see on Earth, IE GW sized. That's 10s of 1000s of people to support (or some serious industrial mfg hardware).

Until them most (all ?) space reactors will run with HEU (or "bomb grade" Uranium as I like to think of it) for compactness, and I don't see that being left in the hands of the private sector any time soon.

[sheltonjr]

Here is a real reactor that actually ran, and ran well with great load following characteristics. That shows the possibilities with a Molten Salt Reactor (MSR). Thorium only makes things more complicated requiring possibly two fluids and salt processing.

A MSR running on Highly Enriched Uranium (HEU) or even plutonium would make a great space reactor. Small size, High temperature, Load following. The only fuel processing required is to allow the Xenon and Krypton and other gases escape from the system as they are major neutron absorbers.

The reactor would only have enough fuel in the system to maintain criticality. NO EXCESS CRITICALITY. Due to the strong negative temperature coefficient of reactivity, No dump tanks are required. (While still prudent for an Earth based reactor). 50 gram pellets would be added every week to replace the fuel used.

The high temperature of the reactor and the cooling loop make the radiators a lot more efficient, though it will still require pretty big radiators.

Once loaded with fuel,  1 Kg will provide almost 2 years of power at full power generating 2.5 thermal MWatts and 1.25 MW of electrical power if required. It will not normally be at full power. This electrical power can be used anywhere in the solar system.

Space is hard, Trying to do space with limited power only makes it harder.  Having lots of power makes electric propulsion a lot more viable. HiPEP, VASIMR, Hall Effect thrusters etc…. More importantly, life support becomes easier to provide the air we breathe, food we eat and to keep our cabin comfortable.  High power sensor systems such as surface penetrating radars, and high power high bandwidth laser communications. Electromagnetic shielding to reduce radiation effects.

The US and UK have an overabundance of Plutonium with 99 & 112 MT each!!! I cannot think of a better use for it.

After 30 years of use, a space tug could dock with the power plant and drain the salt and replace it with a fresh salt or from another reactor that had been processed to removed the remaining fission products.

I would also develop supercritical CO2 turbines and power conversion technology to reduce the size and weight of converting all that heat to electricity.

Aircraft Reactor experiment Statistics:
Size:        3ft diameter, 3ft tall
Power:    2.5 MWatts Thermal
Fuel Temp:   1600F, 870C
Fuel:       177 lb of U235, 25 lb/ft3, 93.4% Enrichment
Fluid:      1153 lbs.
Fuel Usage:   1.5g/day at max power
Core Mass:   Approximately 6000 lbs, 2700 Kg

http://web.ornl.gov/info/reports/1955/3445603498221.pdf

[sdsds]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

I somewhat trust this source, as Baroness Worthington is patron and trustee of the Weinberg Foundation.
http://www.the-weinberg-foundation.org/wp-content/uploads/2013/06/Thorium-Fuelled-Molten-Salt-Reactors-Weinberg-Foundation.pdf

Regarding molten salt reactors it says:

Current international research and development efforts are led by China, where a $350
million MSR programme has recently been launched, with a 2MW test MSR scheduled
for completion by around 2020.

[sdsds]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

I should partially retract what I wrote previously. The same source describes (in section 6.1.1) the molten salt reactor being prioritized in China. As QuantumG indicates, the fuel for that is in a pebble bed; the salt is used for cooling.

I'm unsure of the mechanics involved: apparently the pebbles move through the salt in the core. For zero g applications it isn't clear (to me at least) that approach will work....

The source indicates the Chinese are also pursuing a reactor design in which the fuel is dissolved in the molten salt, but that is being moved forward at a pace which puts it several years behind the pebble-based design.

[JasonAW3]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

I should partially retract what I wrote previously. The same source describes (in section 6.1.1) the molten salt reactor being prioritized in China. As QuantumG indicates, the fuel for that is in a pebble bed; the salt is used for cooling.

I'm unsure of the mechanics involved: apparently the pebbles move through the salt in the core. For zero g applications it isn't clear (to me at least) that approach will work....

The source indicates the Chinese are also pursuing a reactor design in which the fuel is dissolved in the molten salt, but that is being moved forward at a pace which puts it several years behind the pebble-based design.

I imagine that a slow centrifuge of the pebble bed, with the cooler moltant salts being injected into the center of the centrifuge and being withdrawn from the edge of the centrifuge, would likely be the best and most efficent way to do a Pebble Bed reactor in microgravity.

[Robotbeat]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

Yeah, but that's because putting some Thorium in an existing design is obviously way easier than designing a new reactor entirely. A molten salt reactor makes a lot of sense (i.e. isn't mere internet amazing peopleism), but to get there you have to do actual hard engineering work.

[Rhyshaelkan]

Necromancy!!

Liquid Flouride Thorium Reactors were well on their way till Nixon killed it for political favors. There is nothing standing in the way of LFTR except funding and regulations. Especially on another world like the Moon or Mars. Far away from regulations.

LFTR is everything Fusion wishes it could be.

[Rei]

Why HEU and not 239Pu?  239Pu has:

 * A higher energy release in easily captured fission fragments per fission event (175.8 vs. 169.1 MeV)
 * More energy in prompt neutrons (5.8 vs. 4.8 MeV) - although about the same energy difference in reverse for beta- (5.3 vs. 6.5 MeV). We'll ignore harder to capture energy like gamma, neutrinos, etc.
 * More neutrons per fission (2.88 vs. 2.43 for slow fission, 2.94 vs. 2.45 for fast fission), and thus significantly smaller mass required for criticality
 * The other main criticality aspect, cross sections, are similar - 235U is better at some energies, 239Pu at others (http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15337&mf=3&mt=18&nsub=10 http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15321&mf=3&mt=18&nsub=10)
 * Higher metal density (19.8 vs. 19.1 g/cm^3) and higher oxide density (11,5 vs 10,97 g/cm^3) at the same oxidation state.

Uranium is cheaper, but for space applications I'd think this nearly irrelevant; 239Pu isn't crazy expensive like 238Pu.  Plutonium (both metal and oxide) has a lower melting point that uranium, but not enough that I'd think it relevant, they're both very high.  Uranium has better thermal conductivity, but that's mostly an engineering design issue.
 

[Hanelyp]

I have this crazy idea of a liquid fission core in a centrifuge, hydrogen propellant bubbling up through the liquid then out through a nozzle for a rocket.  One question is how much fuel gets picked up by the propellant flow.

[randomly]

HEU is preferred because of Launch failure risks and ease of handling. The minor energy differences are outweighed by other aspects of the reactor design. Breach of an unused HEU core presents little risk as radiation off of it is so low. Pu239 on the other hand is very nasty stuff. If you introduce a fragment of Pu into a glove box the decay events are so energetic that it blasts bits of Pu off the fragment and in a relatively short time the entire interior of the glove box is covered in highly radioactive Pu.

Minimum critical mass may not be that much of an advantage when your design is limited by the maximum energy density of the core that you can deal with, other factors are defining core dimensions. HEU also has larger margins of control-ability than Pu. Besides the safety factors an HEU reactor is probably a lot more cost effective than developing a Pu reactor given the current state of nuclear technology.

[randomly]

I read something recently which indicated everyone who is actually doing something with Thorium as a nuclear fuel is doing so with pebble-bed reactors and even liquid water reactors. Liquid salt reactors are more an Internet amazing people thing. Is that about right?

It's probably best to separate the two technologies.
 Molten salts can be use with any Nuclear fuel. Molten salts can be used as coolants in solid fuel reactors, replacing the water coolant loops. More interestingly molten salts can also be used in liquid fueled reactors in which the nuclear fuel is dissolved in the molten salt. There are numerous research and development  projects on different designs using molten salt fuels in US, Canada, Russia, France, Japan, and China.
 Some of these use Uranium fuels (U235,U238), Some are using spent light water fuel (U235,U238,Pu239)(Russia), and some are working on U233 designs (bred from Thorium). China is by far the leader in Thorium (U233) based MSR, with at least 700 engineers on the project last I heard.

Molten salt reactors are a pretty broad category, with a lot of very different designs and approaches, different fuel cycles, different salts, thermal neutron designs, epithermal designs, fast designs. Different approaches to removing fission fragments (or not).
Thorium has become heavily associated with MSRs, probably because it's the best suited reactor type to run a thorium based fuel cycle. Thorium is not itself a fuel, but a fertile isotope that can be converted to a usuable nuclear fuel (Uranium 233) through neutron absorption in a reactor. A thorium MSR is essentially a breeder reactor, making it's own fuel from spare neutrons in the core. It's actually a lot more complicated than that as you want the thorium to absorb one neutron and then it takes about a month (half life) for it to decay into U233. You don't want it to absorb a second neutron, so you have to remove it from the core and wait for it to decay. All this adds complications to the design. The advantages to thorium as a fuel source are several. One is availability, it's 3 times more common on earth than Uranium. On the other hand it is not geologically concentrated like Uranium, and there is no real shortage of uranium either so no real cost advantage there. The other thorium advantage is the actinides generated in a thorium reactor all have relatively short half lifes of 30 years or less, so the radioactivity of the spent fuel diminishes much more rapidly than U235/U238 fueled reactors and reach safe levels in only hundreds of years instead of hundreds of thousands.

The downsides are that U233 is a proliferation hazard like U239. It can be used in nuclear weapons. So you run into problems of how to safeguard that, or denature the material with U238 (which then negates the rapid decay of the waste products because of the actinides generated by the U238).

There are no clean all purpose solutions. Molten Salt Reactors are not tied to Thorium. But there are many large advantages to be had in the constellation of options they enable. Much better passive reactor safety, higher fuel efficiency, use of light water spent fuel as fuel, higher operating temperatures for chemical process use and higher efficiency electric power generation, lower costs, more scalable reactor sizes, better waste management, etc.

[Rei]

HEU is preferred because of Launch failure risks and ease of handling. The minor energy differences are outweighed by other aspects of the reactor design. Breach of an unused HEU core presents little risk as radiation off of it is so low. Pu239 on the other hand is very nasty stuff. If you introduce a fragment of Pu into a glove box the decay events are so energetic that it blasts bits of Pu off the fragment and in a relatively short time the entire interior of the glove box is covered in highly radioactive Pu. Minimum critical mass may not be that much of an advantage when your design is limited by the maximum energy density of the core that you can deal with, other factors are defining core dimensions. HEU also has larger margins of control-ability than Pu. Besides the safety factors an HEU reactor is probably a lot more cost effective than developing a Pu reactor given the current state of nuclear technology.

The main benefit above is not so much the energy difference but the much smaller quantity required to reach critical mass, aka you can make an efficient core at a smaller size.  Poor neutron efficiency within the core means greater need for reflector mass to compensate.   For most space applications, unless you're talking something like manned-scale VASIMR missions, you don't need GW-scale reactors.  Also, minimum core size for efficient operation additionally comes into play when you're talking very long/high delta-V missions (Oort cloud and whatnot), which would call for a small core with a large fuel supply to be cycled through it, so as to maximize the fuel to core mass ratio. 

But I understand very well about how handling issues of nuclear fuels can make otherwise suboptimal choices into optimal ones.  If handling wasn't an issue we'd surely use 232U as a radioisotope fuel rather than 238Pu.  It undergoes a far longer, far more energetic decay chain over practical (~70yr) timeperiods, all the way down to lead, and is also cheaper to acquire (it's an unwanted contaminant in the thorium cycle). But it puts off some nasty gamma in its decay chain.  So it might be nice for using for something like Starshot where only miniscule quantities are required, but you wouldn't want to use it in multi-kilogram quantities to power a traditional probe.

[randomly]

The main benefit above is not so much the energy difference but the much smaller quantity required to reach critical mass, aka you can make an efficient core at a smaller size.

Not much weight benefit to be had there. You need less than 8Kg of HEU to achieve critical mass in a reactor.

[Rei]

The main benefit above is not so much the energy difference but the much smaller quantity required to reach critical mass, aka you can make an efficient core at a smaller size.

Not much weight benefit to be had there. You need less than 8Kg of HEU to achieve critical mass in a reactor.

I believe you're thinking of bombs, including reflectors, compression and the like. My reading says that the bare sphere critical mass of 100% 235U is ~50kg, vs. ~10kg for 100% 239Pu.  Practical reactor geometries lead you to significantly worse than the bare sphere minimum before neutron reflection comes into play.  Neutron reflector effectiveness is proportional to thickness, wherein mass for a constant thickness reflector grows proportional to the core radius squared.  Hence my comments about scaledown.

[DLK]

My two cents...

The choice of a reactor cycle could be important in the long run, particularly if intentions of becoming a space-fairing civilization are being seriously considered. This would include a supply of unlimited fissile material that doesn't require a lot of effort.

Thorium has a number of advantages:
1. There's apparently lots of it distributed around the solar system. It has such a long half-life (~13Gy) that not much decay has occurred yet.
2. It gives you a way to replenish your fissile inventory. The only natural fissile isotope, U-235, takes a bunch of resources to separate it from U-238.
3. It's the only fertile material that can breed fissile material (U-233) in a thermal neutron spectrum, which limits the amount of fissile inventory you have to maintain and makes smaller reactors possible.

The molten salt reactor concept is a good fit for breeding thorium into U-233. The liquid fuel allows relatively easy recovery of potentially-useful fission products. High temperatures at low pressures reduce loss-of-containment risks.

[randomly]

I believe you're thinking of bombs, including reflectors, compression and the like. My reading says that the bare sphere critical mass of 100% 235U is ~50kg, vs. ~10kg for 100% 239Pu.  Practical reactor geometries lead you to significantly worse than the bare sphere minimum before neutron reflection comes into play.  Neutron reflector effectiveness is proportional to thickness, wherein mass for a constant thickness reflector grows proportional to the core radius squared.  Hence my comments about scaledown.

No, not thinking of bombs. You are thinking of bombs, prompt neutrons, and fast reactors.

 Fully functional HEU reactors with around 8kg core material. The SNAP-10A reactor launched into orbit had a core load in that range.

You are missing the large increase in fission cross section from thermal neutrons. A moderated reactor requires about 1/10 the fuel load of a fast reactor.

In any case weight of the fissile material only accounts for a few percent of the total weight of the reactor and power system. The tiny weight advantage can easily be 'outweighed' by safety, handling, and design impacts from accommodating a highly radioactive core. Given the current state of the art, and economics, I think it highly likely that an HEU reactor would be the most cost effective, by far.

[JulesVerneATV]

A new memorandum has firmed up China and Russia's intent to lead the construction of a new lunar base to be completed by 2036, as NASA talks about scaling back its own lunar ambitions.
https://www.livescience.com/space/the-moon/china-signs-deal-with-russia-to-build-a-power-plant-on-the-moon-potentially-leaving-the-us-in-the-dust
Russia has signed a deal with China to build a nuclear power plant on the moon.

[JulesVerneATV]

UPSC Key: Operation Rising Lion, India-France relationship, and Nuclear reactors
https://indianexpress.com/article/upsc-current-affairs/upsc-key-operation-rising-lion-india-france-nuclear-reactors-10066709/


they closed the discussion?

Most users ever online was 7940 on Mar, 2025
https://energyfromthorium.com/forum/index.php

[Asteroza]

UPSC Key: Operation Rising Lion, India-France relationship, and Nuclear reactors
https://indianexpress.com/article/upsc-current-affairs/upsc-key-operation-rising-lion-india-france-nuclear-reactors-10066709/


they closed the discussion?

Most users ever online was 7940 on Mar, 2025
https://energyfromthorium.com/forum/index.php

The Energy from Thorium forum was getting assaulted by bots, so Kirk was forced to implement draconian blocks via Cloudflare, but it appears he wholesale blocked asia, maybe europe too. If he was better able to use the more nuanced blocking Cloudflare provides he wouldn't have to exclude the world outside the USA, but he's paying for the forum, not us, so it's his call.