In Which I Use Scientific Reasoning to Doubt the Thorium-Powered Car

While I was away at Dragon*Con, stories about a possible thorium-powered car popped up in the news. From the write-up at Txchnologist:

Charles Stevens, an inventor and entrepreneur, recently revealed that his Massachusetts-based R&D firm, Laser Power Systems (LPS), is working on a turbine/electric generator system that is powered by “an accelerator-driven thorium-based laser.” The thorium laser does not produce a beam of coherent light like conventional lasers, but instead merely heats up and gives off energy.

There’s a whole lot of science word salad in that paragraph — what does it even mean to have a laser that isn’t actually lasing and producing coherent light? So let’s detangle it and see if the proposed thorium-powered car makes sense.

The first thing to know is that there’s been a lot of work done on using thorium for fuel in a nuclear reactor. Thorium-232, the kind you dig up out of the ground, is only weakly radioactive and won’t undergo fission by itself. What you can do, however, is bombard it with slow neutrons from uranium or plutonium. That turns thorium into uranium-233, which is fissile and can be used in a nuclear power plant. That means you can use thorium as breeder fuel to produce the fissionable material you really want.

Thorium has several benefits over uranium-235, the usual nuclear reactor fuel. For one, thorium ore is about three or four times as prevalent in the Earth’s crust as uranium ore. For another, you can use all of the thorium you mine. Only 0.7% of uranium is 235U. The rest of it is 238U, which isn’t useful for nuclear reactors. That makes thorium far more abundant for nuclear power purposes.

There are downsides, of course. As the World Nuclear Associate fact page dryly puts it, “Despite the thorium fuel cycle having a number of attractive features, development has always run into difficulties.” There aren’t any commercial thorium reactors yet.

However, if you read carefully what Charles Stevens is saying, he’s not claiming to be using thorium in a nuclear reactor. The WardsAuto article on Stevens states, “Stevens agrees, emphasizing his system is ‘sub-critical,’ which means no self-sustaining nuclear reaction within the thorium creating significant amounts of radioactivity.” So what is he doing?

Unfortunately his two websites currently have very little information. To find out more we have to look at the 2009 version of his webpage and exerpts from that same page. Back then he talked about laser-driven cars in which a “Hybrid Solid state Free Electron laser” heats up thorium, which releases even more heat to turn water into steam and drive a turbine. Also an accelerator may be involved. And according to what he told WardsAuto, “1 gm of thorium equals the energy of 7,500 gallons (28,391 L) of gasoline Stevens says. So, using just 8 gm of thorium in a car should mean it would never need refueling.”

Right, let’s try and make sense of this, starting with one gram of thorium equaling the energy of 7,500 gallons of gas. This a question of energy density: how much energy can you extract from a given amount of fuel?

I have no clue what process Stevens is claiming to use that lets you bombard thorium with a laser and get energy out, especially since he said it’s sub-critical and thus not a nuclear fission reaction. Instead I’ll pretend he is doing nuclear fission, since that’s one of the more energy-productive reactions we can do and will set a good estimated upper bound on how much energy Stevens could extract from thorium. I’ll also assume 235U fission, since we don’t have a thorium reactor yet.

Uranium in a reactor produces about 20 terajoules of energy per kilogram. For comparison, gasoline gives you about 48 megajoules per kilogram. That means uranium gives us about 425,000 more power per kilogram than gas. Let’s assume thorium will give us roughly the same ratio. That means one gram of thorium would be like 425 kg of gas. Gas has a density of about 2.7 kg per US gallon, so that 425 kg of gas is equivalent to 156 gallons.

That’s way short of Stevens’s claim of 1 gm of thorium being equivalent to 7,500 gallons of gas. For that to be true, his laser-induced power output has to be fifty times more energy efficient than nuclear fission. That is an extraordinary claim, to put it mildly, and he’s offered no proof and precious few details.

Looking through his other claims, it sounds as if he glued together actual science together as if making a collage for kindergarten, regardless of whether the results made sense or not. You could in theory make an actual thorium laser, though that’s not what he’s doing. You can use a particle accelerator to drive a nuclear reaction by knocking neutrons out of other particles, though again that’s not what Stevens is doing despite him adding “accelerator-driven” to the description of his process. You can even induce nuclear reactions using super-powerful lasers, but Stevens says he’s not inducing fission.

So to sum up: Stevens isn’t claiming to have made a nuclear-powered car. He’s claiming to have made a steam-powered car where the steam is heated up when he shines a laser on thorium. I don’t know of any physical process that would let you get more heat energy out of the thorium than you’d spend on making the laser go. For his process to be so awesome that it would power a car for some 200,000 miles on a single gram of thorium, he’d have had to come up with something that’s fifty times more powerful than a nuclear reactor. And he hasn’t released any papers, only press releases. That’s 3 out of 3 red flags for the research not being real.

Sorry, world. If we’re going to have a laser-powered car, it sounds like this isn’t it.

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