At the beginning of the 1900s, the Doble Steam Motors Company built steam cars. Their engineers solved many of the problems steam cars had until then by using superheated steam and a condenser to recirculate the steam instead of just expelling it as exhaust.

Their cars ran almost completely silently and made 1000 (yes, a thousand) ft-lbs of torque.

Here's a video with Jay Leno where he explains the inner workings of one in more detail and drives it around. Here's a diagram explaining how they work, and for flavor, a picture of thorium salt crystals

The way I'm thinking it'd work is

  • you'd have a tiny version of the thorium reactor "bulb" circulate molten salt into a heat exchanger (like the furnace in the Doble)...
  • ...to create superheated steam...
  • ... then use that to drive a steam engine or turbine...
  • ... hooked directly to the back wheels (with that much torque, you don't need a gear box)...
  • ... then recirculate the steam thru a condenser to turn it back into water.

I can think of several pros and one big con:


  1. Your fuel is literally glowy blue-green crystals.
  2. Silent running.
  3. Nuclear-powered AND steam powered.
  4. Tons of torque.
  5. No exhaust pollution — you'd run your car until the thorium salt was expended then dispose the waste at a processing facility.
  6. Thorium has 1000s of times the energy density of fossil fuels. You could run your car for months on a single "tank."
  7. Thorium is about as abundant as lead in the ground, so fuel cost wouldn't be unreasonable.


Is it practical to combine this "advanced" retro steam technology with a mini liquid thorium salt reactor?

  • 2
    $\begingroup$ I guess no one would be willing to spend a single cent on these type of vehicles as long as there are suicide bombers out there. Who would want to be remembered as "s/he who made terrorism so much easier"... $\endgroup$
    – Mrkvička
    Jan 30, 2017 at 0:09
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    $\begingroup$ We recycle more than 98% of our lead. Without that, costs of car batteries would already be unreasonable. This part should be corrected and moved to cons. $\endgroup$
    – Mołot
    Jan 30, 2017 at 10:58
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    $\begingroup$ See en.wikipedia.org/wiki/Ford_Nucleon $\endgroup$
    – pjc50
    Jan 30, 2017 at 12:15
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    $\begingroup$ It's worth noting that torque figures by themselves are fairly meaningless, as engines will generally run through a gearbox, where the RPM figure matters a lot. Whilst torque is OK for roughly comparing similar engines, it's hopeless when comparing engines that are vastly different - for example, a Toyota Prius has more torque than a Ferrari 355. But because the torque is at very low RPM, it's not comparable. Put through the right gearbox, 100nm at 10,000 rpm will generate just as much motive force as 1,000nm at 1,000 rpm. $\endgroup$ Jan 30, 2017 at 13:22
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    $\begingroup$ @Mrkvička Thorium is not weaponize-able in the same fashion as plutonium or uranium, according to my sources. $\endgroup$
    – nijineko
    Jan 30, 2017 at 20:41

5 Answers 5


I have some interest in nuclear powered… well, spaceships mainly, but some of the problems apply to cars too.

Radiation is your enemy. It's not just "Oh we need some shielding, slap some lead around it!" kind of enemy, it's "If your body absorbs ten joules per kilogram of body weight, you die. Your modern car engine produces around fifteen thousand joules per second (and that's when it's cruising, multiply that by ten if you're accelerating as hard as you can). If you are in a crash and the shielding is breached, you die and so does everyone else nearby."

This is true regardless of the specific nuclear technology being used; RTG, reactor (Uranium, Plutonium, Thorium, fusion…), induced gamma emission, betavoltaic batteries, anything.

Other problems are technology-specific; as JDługosz has mentioned, throttling the power of a reactor up or down is far too slow for a car. You can compensate for that with batteries — drain them for a boost, let them recharge when you're not accelerating hard. But then, you'd have a hybrid car, or electric car that just happens to be getting recharged by an on-board nuclear reactor, not a steam car. Your torque number would be totally different.

  • 3
    $\begingroup$ Right, an RTG charging a large battery would be a nice idea for, say, a Mars rover. But a reactor is something you’d use on an aircraft carrier, not a car. $\endgroup$
    – JDługosz
    Jan 30, 2017 at 5:33
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    $\begingroup$ You don't throttle the boiler, you throttle the steam flow, so that's a minor detail. $\endgroup$
    – Separatrix
    Jan 30, 2017 at 10:17
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    $\begingroup$ @Separatrix It's more important than you think; the thermal output is constant, and that power has to go somewhere. You'd need to keep replenishing your water supply or have an enormous heat sink. The alternative is your car will become too hot to touch. (xkcd approximation) $\endgroup$
    – BenRW
    Jan 30, 2017 at 11:17
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    $\begingroup$ @BenRW, you need a heat sink to run a closed cycle engine anyway, all that superheated steam needs to be converted back into water to complete the rankine cycle. If you're having problems you just waste it back into the condenser, if you're really having problems, vent it. $\endgroup$
    – Separatrix
    Jan 30, 2017 at 11:24
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    $\begingroup$ Ultimately, you go down the following logical route - if you just use the batteries then you can reduce the weight of all that shielding, water and turbine and replace it with more batteries. That way you also get rid of the safety aspects. Instead, keep the reactor at home and charge in between journeys. But instead of having one reactor per home, you could make things far more efficient by having one reactor and sharing it amongst lots of homes... and you realise that you've ended up with an electric vehicle and a nuclear power plant. $\endgroup$ Jan 30, 2017 at 13:26

See this debunking of the one in the news. Thorium is very slow to change the output on: you can’t just throttle it, but it would be putting out full power full time. For this reason alone, making it the primary power supply of a vehicle is impractical. See the video for more points that might still apply to your more realistic design.

Also, will your “bulb” work, or is there a minimum size for a sustained chain reaction? If it’s just a RTG you won’t have nearly enough power as you require.

If you’re set on using a reactor (not an RTG) in a cool story, do it on Mars. Surviving expedition members jury-rig a drive to move the entire habitat, using the same reactor normally used for general power. Without parts for huge electric motors or drive train of any kind, they come up with the 19th century locomotive design. Nuclear steam train on Mars! Steampunk solutions to problems at hand, with existing computers and such they can use, but can’t make anything high-tech. They can use local iron from the soil to make parts from cast iron and wrought iron!

It could be a literal train, with multiple hab modules and cargo cars hooked up to the locomotive engine.

  • $\begingroup$ Power is a gearing/clutching problem. $\endgroup$
    – fectin
    Jan 30, 2017 at 3:52
  • $\begingroup$ @fectin, gearing lets you swap power for speed or vice-versa. You may be able to move a one-ton car with a hundred-watt RTG with suitable gearing, but it's not much use if it has a top speed best measured in centimeters per second. $\endgroup$
    – Mark
    Jan 30, 2017 at 4:54
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    $\begingroup$ You don't throttle the boiler though, you throttle the steam flow. The boiler runs constantly. I'm not sure why that video is wittering on about density though rather than just correcting it to energy density which is what matters here. I gave up at that point. $\endgroup$
    – Separatrix
    Jan 30, 2017 at 10:14
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    $\begingroup$ Nuclear Steam Trains on Mars is going to be the theme of my next Shadowrun campaign, for sure. What an awesome concept! I can even use that as the tittle without too much work! It has the same vibe that "Teenage Mutant Ninja Turtles" does, just fifty-two times more awesome. $\endgroup$
    – Mermaker
    Jan 30, 2017 at 11:42
  • $\begingroup$ Dammit, now I really need some nuclear steam trains sci-fi! Welp, time to hit the bookstore... $\endgroup$
    – Mermaker
    Jan 30, 2017 at 11:48

The first problem here is scale. Reactors are big. Thorium salt reactors are small, for reactors. That doesn't mean they're small, it's just a relative term.

Assuming you've successfully miniaturised and sealed the reactor to make a lightweight, small, plug and play unit that can be replaced in its entirety should it have a problem. Then the next issue is contaminated water. Ideally it's a closed system, the water never gets released but runs through a full rankine cycle, but unfortunately it probably will on a regular basis. Your fallback system for the condenser is to vent steam and take in fresh water. You're not going to be making friends by venting reactor water into the open air.

Speaking of the condenser, this is one of the great problems with steam cars. Most of them didn't have a condenser at all, one of the key factors that makes the Doble steam cars so much better is that they did. This means that you can recycle your water and not have to carry so much or fill it up as often. Ultimately this depends on external temperature. If you're running around in Scandinavia in mid-winter, no problem, you'll probably be successfully condensing all your water. In Dubai in high summer, you'll be boiling through your water at a steady rate and probably condensing almost none of it.

Reactor and throttle control is an interesting game. The reactor will run at a set temperature and pressure, throttle control on a steam power plant is by steam control not reactor control.

I'm assuming you can actually control this reactor, it's not just running at a steady rate all the time because that leads to disaster. You'll also need the ability to completely shut it down and restart it at will, this is a deal breaker if not possible.

Given a computer controlled reactor to maintain proper temperature and pressure at all times, you'll probably have to set it for current driving conditions, performance, cruise, traffic.

  • Running in traffic you'll need to turn the whole thing down to use as little water as possible, you're almost certainly not condensing much and you can't afford to vent too much or you'll be stopping all the time for more water.

  • Driving steadily at speed will get you highest condensing rate, but you don't need a vast amount of torque so the system again can be tuned.

  • Performance is the Clarkson option, assuming you have plenty of water and only want more power, condensing is a nicety.

If you run on performance mode in traffic you probably won't get to the end of your journey, but if you try to race in traffic mode you won't have the steam pressure to keep your speed up.

Assuming the reactor is small enough and good enough: The condenser is the weak point.


It is technically feasible. However, technology is not the biggest problem. The biggest problem is public perception of nuclear power. There's no way the public will allow a nuclear-powered car. It will be perceived as an atomic bomb on a car. If you say that you can't make a bomb from a Thorium-powered car, you are correct. The general public will not believe it. There's no way a nuclear-powered car will appear on the road anytime in the future.

  • 1
    $\begingroup$ If you have a bunch of thorium reactors sitting around on your showroom floor, I suppose you could always threaten to blow them up in a dirty bomb unless people from across the city come buy your cars... first rule of marketing: turn your liability into an asset! $\endgroup$
    – SRM
    Jan 30, 2017 at 4:33
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    $\begingroup$ Weaponise it, that'll sell. Just tell people that if they push this big red button it'll give the driver in front a lethal dose of radiation. $\endgroup$
    – Separatrix
    Jan 30, 2017 at 11:18
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    $\begingroup$ "Honk if you love cancer" $\endgroup$
    – kat0r
    Jan 30, 2017 at 16:07
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    $\begingroup$ It's essentially the same problem as a hypothetical anthrax-powered car. Due to public perception of substances that can kill you in imperceptible quantities, the gummint just isn't going to let you sell it to the general public nor hurtle about the streets with a box of it. Anthrax isn't explosive either. $\endgroup$ Jan 31, 2017 at 11:49

see the "Statement on Thorium-fueled Reactors" of the 'Union of Concerned Scientists'. http://www.ucsusa.org/sites/default/files/legacy/assets/documents/nuclear_power/thorium-reactors-statement.pdf

Hmm, "thorium-232 has a half-life of 14 billion years and its decay products will build up over time..." - Peggy Conte, Fairewinds in http://lists.csbs.utah.edu/pipermail/energy/2013-January/002514.html

Even if nuclear powered cars become theoretically possible, they would be disastrous in reality.

  • $\begingroup$ The first link presumably explains that such reactors are not practical? Link-only answers are frowned upon here. The second link does nit seem relevant, as leaving it laying around in low concentration (to exhibit the stated half life) is what is happening when you don’t have a reactor. $\endgroup$
    – JDługosz
    Jan 31, 2017 at 8:45
  • $\begingroup$ @JDługosz The first link explains thorium reactors don't have advantages over uranium reactors, but that molten salt reactors have major safety issues. The second link is an interesting read, but its title says it almost all "Thorium reactors are not the solution". The second link is actually relevant after all. $\endgroup$
    – a4android
    Jan 31, 2017 at 11:04
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    $\begingroup$ Welcome to Worldbuilding, user3038776, answers on this site prefer facts, information & reasons to support their assertions. Link heavy answers are frowned upon. You can improve your answer by condensing the content in your links. Links tend to go awy, answers don't. Please click HELP above at the top of webpage and take the tour to find out more about how WB SE works. $\endgroup$
    – a4android
    Jan 31, 2017 at 11:08
  • $\begingroup$ A recent episode of Nova had good things to say about molten salt, and modern designs are much safer than the legacy water-cooled reactors. $\endgroup$
    – JDługosz
    Jan 31, 2017 at 20:08

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