The problem with Thorium reactors is proliferation. The way Thorium advocates address this problem is to simply deny it: Thorium reactors, they say, are inherently proliferation-safe. Not only they don't admit the problem, they claim that Thorium reactors are superior to Uranium reactor exactly on this point.

Who is right?

You can read [1], but here's a summary. Uranium reactors have a proliferation problem like this: up to 20% enrichment uranium cannot be weaponized. Most likely an aspiring proliferator can't weaponize even at 50% or 60% (look at Iran), they need to go to 80% or more. But once you get to 50%, going to 80% is not that difficult. The fundamental problem is that it is very difficult to enrich Uranium. Civilian uranium reactors generally run at 5% or lower enrichment levels. Taking the fuel out and enriching it to 80% is not an easy path to making a bomb, or at least not significantly easier than digging Uranium out of a mine, where it is at 0.7% level, and enriching it.

But Uranium reactors produce Plutonium. And Plutonium is a different element, so it can be separated chemically from Uranium, and one can make a bomb, just like the Trinity and Nagasaki bombs were made. The problem with that is that Plutonium made in a reactor generally contains a blend of Pu-239 and Pu-240. The first is the best bomb-making material out there, the second is the worst. To only make the first and not the second, you need to burn Uranium in a reactor for a very short period of time, remove it, extract the tiny quantity of Pu-239 that was produced, reconstitute the Uranium fuel, and repeat the process. And use lots of chemicals while at it, and lots of remote-operated machines, and get a lot of radioactive waste all over the place, etc. This is such a messy process that the US has produced more, much more nuclear waste manufacturing plutonium for bombs than running its civilian reactor fleet.

So, that's the story of Uranium. Proliferation is possible, but difficult.

Thorium reactors run like this: Thorium is bombarded with neutrons, it absorbs some and transmute into Protactinium, or one of 3 isotopes: 232, 233, and some other isotopes that we can ignore. Pa-233 is the "good one", and Pa-232 the "bad one". Both for running a reactor, and unfortunately, for making a bomb.

You see, if you could get your hands on pure Pa-233, that would decay into Uranium-233 with a half life of one month. In two years it would be essentially pure U-233. Which you can take and make a splendid bomb.

Thorium advocates reply that Pa-233 comes mixed with Pa-232, and that one decays into U-232, which is nasty stuff. Its decay chain releases some hard gamma rays, and those are not good for those handling bombs, and are not good for the electronics inside those bombs either.

The problem with this argument is that Pa-232 has a very short half-life: only 1.3 days. If you start with a 50-50 blend of Pa-232/Pa-233, after one month the Pa-232 has fully decayed, while only 50% of the Pa-233 has decayed. You can separate the Pa-233 chemically from whatever else is there, and then wait 2 years as explained above and get U-233, which is nuke-maker's best friend.

What is the solution? Maybe a regime where thorium reactors are allowed only if operated directly by the Department of Energy, or a similar agency in other countries.

But denial is not a solution.

[1] https://thebulletin.org/2018/08/thorium-power-has-a-protacti...