I'm very pro-nuclear, but I have huge doubts about this design. It may work in theory, but I don't think it will have a chance to get an approval from the Nuclear Regulatory Commission, and for good reasons.

In a reactor that uses slow (thermal) neutrons, like the vast majority of current reactors, once a neutron hits a nucleus, it can either trigger fission, or get absorbed. For U-235 the probability to get absorbed is 18%, and you get U-236, which is a very long lived radioisotope.

The idea of the traveling wave reactor is that U-238 will absorb a neutron and become (after a few short steps) Pu-239, which then can undergo fission. The problem is that Pu-239, just like U-235, has a good chance to just absorb a neutron, and become Pu-240. Bottom line, the more the reaction continues, the more transuranic elements you get. Some of them are good fissile radioisotopes, some are nuclear poisons. After a while you end up with a lot of possible mixtures.

The NRC's job is to try to analyze all possible corner cases and make sure that the probability of failure is extremely low. For example, for the NuScale reactor approval that they issued recently, they asked so many questions, that NuScale had to spend two million man-hours of work, and produce a 2 million pages of documentation. Of course, it was not the writing that was hard, it was running the experiments to produce the results. All in all, they spent half a billion dollars [1] to get an approval for a classical pressurized water reactor, which shares the majority of the corner cases with the reactors in production today.

In the case of the traveling wave reactors, how can the NRC gain enough confidence to give an approval. Nothing close to this technology exists, so they can never be sure they found all the corner cases. NRC is a very conservative agency, a lot of pro-nuclear people think it's too conservative. I'm not one of them. I think they are doing a good job. It's a tough job, with an immense responsibility.

If NuScale's approval took 6 years for a classic design, I expect the approval for the traveling wave technology to take at least 20 years. And many, many billion dollars. How can Terrapower's investors tolerate pumping billions and billions in an unproven technology for decades without selling a kwh of electricity and with no certainty that they'll ever get an approval?

One can say that one of the investors is Gates, who has an infinite amount of money. But he's not the only one, and even he is probably a good businessman who makes decisions based on an analysis of risks and returns.

There is a way. Terrapower is working on a sodium-cooled fast reactor (Natrium). While radically different from the reactors running in the US nowadays, there are precedents in history. NRC is probably reasonably familiar with the design, so the approval process has a chance to work. It will most likely take more than a decade, and billions of dollars of experiments. But once it is approved, maybe Terrapower will become a fabulously profitable company, that will have enough money to try new designs, like the traveling wave or the molten chloride reactor.

My bet though is that if they get their fast sodium-cooled reactor approved, they'll stick to it, and keep producing and improving it. If they will invest in anything else, it will be the molten chloride reactor. Fast reactors are just so much better than thermal reactors, because they produce much less waste. The traveling wave technology was a cool sounding technology when they came up with it, about 10 years ago, but I think it has no future.

[1] https://www.nuscalepower.com/newsletter/nucleus-fall-2020/pr...