The two primary drivers of Uranium fission reactors are the need for fissile material for nuclear weapons, and the ability to make compact reactors for powering nuclear submarines. Point two also explains why light water reactors are the primary design of existing reactors in the United States; they all evolved from the engineering knowledge and experience gained by building nuclear submarines in the early 1960's.

If WWII had ended with both Nazi Germany and the USSR having collapsed (perhaps the Germans actually won the invasion of the USSR in 1941 to the extent that the Communist government and organizing structures had been destroyed), then the United States would not feel an overwhelming need to push the production of fissile material for bombs and triggers (in practice, Plutonium bred from fission reactors), nor the desire for compact, high energy reactors for submarines.

With fewer incentives to follow a particular path, US nuclear engineers could have worked on a much broader range of reactor designs. Thorium was an obvious choice for civilian power reactors since Thorium is cheap and abundant, and there would be no obvious pressures, pre existing pools of resources and talent or sunk costs to swing the balance towards light water reactors.

Don't have a Cold-War.

The United States had to choose a direction in which it would go in terms of nuclear research: Thorium, or Uranium.

Only one of these had the corollary benefit of making bigger bombs. Guess which one got funding from the Defense budget to keep ahead of those pesky Communist Russians?

Thorium people have not stopped their activity, but they suffer from financing issues.

If the question is - "what if history turned another way" - then Reagan could possibly be your point of divergence, as it was a political decision to shut down the process. It had probably bunch of aspects behind the scene, having plutonium sure one of them. Efficient burning was not the goal, but getting specific types of isotopes was.

So a high energy demand, successful breakthroughs in problems, the possibility to make Plutonium, some accidents with solid reactors at early stages like Chernobyl to build public resistance for those types of reactors (so it means actually some serious errors in process of developing nuclear reactors, to be too optimistic and go big too soon)

So significant negative pressure could limit solid reactors to military goals only. And military played significant role in developing those reactor for their purposes.

World's First NUCLEAR SALT REACTOR - Documentary Films pretty interesting history kind video. (they blame Reagan, saying that if he would be more familiar with technology he could decide other way)

Positive factors which could keep a molten salt reactor afloat could include space program like moon base, mars base.

Salt reactor, one of reason to start that program was wish of military to use it in aircraft, and conventional reactor was not capable to do that, but a molten salt reactor could in theory (even if the creator did not believe that it would work in a plane) to be used there.

For use in space, it has advantages too, one advantage being the more efficient use of fuel without expensive and technology intensive processes to recover fuel, to reassemble fuel rods. A lower reactor mass and simpler management are also very significant advantages for use in space.

It is also much easier to deliver the initial fuel - because of how it is packed. There is much less danger in the case of a mishap during launch, because of how it can be packed. It is easier to work with thorium as a main fuel source. There are also pretty significant amounts of it on the surface on the moon (12 ppm is about 70 tonnes per million tonnes of dust, thorium is ca 230 atomic mass, average for moon ca 30 atomic mass, so it is 12 * 7 tonnes per million, my rough estimation, even in the unlikely case that there are no concentrated ores)

According to fast link

• Thorium reactors deliver 3.6 billion kWh of heat per ton of thorium, which implies that a 1 GW reactor requires about 6 tons of thorium per year, assuming its generators are 40% efficient.

So it would be enough for a 10GW reactor, which will generate 315 GJ of electricity per tonne of ore. On the moon efficiency can be higher overnight, just as a note. Glass production uses 9GJ per tonne of product (mostly for melting), Aluminum production uses 54 GJ per tonne (because of the energy requirements of reduction of Al₂O₃ into Al). So you basically can melt it (ore) all down, reduce all stuff to pure metals and gases by electricity and it will be something like 20% of what you will produce after extracting the thorium. Not saying it is the way to do, but why not if you need Iron, Aluminum, and Titanium - which are present on the moon: some NASA thoughts about Al on the moon

So a strong lunar program could be one of the factors in favor of molten salt reactors.

One of the factors involved in solid reactors being preferred is investments from the military, they wanted them for a bunch of reasons.

Thorium reactors for a lunar base are an excellent choice, so they may have happened during the Appolo era, if there were other factors enabling moon base. This will aid progress in other areas, not just nuclear related problems, and could be one of the factors that could open perspectives for this technology to be more attractive, or enough attractive to pour money into it, and that is actually all it needed at that time.

So it may be not a big stretch, that history of molton salt reactors could be different.

P.S. as people have mentioned the Russians, so will I. Greater successes for their moon program could be also one of the factors supporting an expanded US moon program, without collapsing it. So moon base race could be an enabling factor too.

Note

read in case factual incorrectness feeling. disclaimer^TM, do not build it at home^TM, make your own research before building^TM, call me if u did^TM

In this answer I rather project words of other, who are optimistic about thorium reactors and who thinks that problems can be solved.

Also it is not mention explicitly, answer keep in mind one particular type of possible types of Thorium reactors, molten salt with in situ utilization.

Packing benefits assumes packing fuel pellets, like in Pebble bed reactor, and relays on possible advantages dividing fuel in to small packages, encapsulating them in robust shell which can withstand fall from orbit if needed, where shell may be actually non radioactive part of future fuel(same thorium actually). It may be non radioactive (as Thorium non radioactive, not as completely non radioactive) in case of some kind of ADS(accelerator-driven system) as starter for whole process. There is difference with uranium rods, which I think can be exploited to our advantage.

Each reactor development includes lot of work behind the scene. This is complex work, which includes solving non obvious for the public or outsiders set of problems. If everything would be easy and simple, then it would be solved in the past, and we would not have question may or may not it to be solved. It wasn't, and question is valid.

Simplicity of main concept, does not mean that solution will be simple, it does not mean it is possible at all, at least at some technological level, which looks sufficient for solving that problem, but in fact, it is not sufficient and needs involving of more advanced technologies, to make it working or just working.

Some examples for better illustration:

Examples of such, which comes in my mind right now, as example knife. It is very old design, stone age old, but really good results where achieved after work with metal became more usual practice.

Another relatively simple idea as Space Elevator, with less obvious problem not exactly but like - to have easy access to space, u have lift huge amount of stuff or build that in space - both looks like easy access to space help here. Obscure/contrast it a bit more - to get easy access, u better have easy access. Even it almost technically possible to build such cable as just rope, u need more advanced rocket systems, or another and better launch technology it is possible, but would u need elevator then?

³He as fuel in thermonuclear reactor there are objections based on speed of reactions which would have place in such reactor.

Launch loop where main problem is reliability at current level of tech.

So there are interesting and promising things which looking good, but do not work at current tech level, for understandable reasons. How advanced we should be to solve them, who knows? Those who knows about thorium dividing in 2 categories believers and non believers, but none of them can prove that other side is wrong. One side not failed yet, but also do not have great success. Other side can point those 2 facts, but can't state it is not possible, exactly because of those 2 facts, but they try to make their reactors better, just in case(for reasons of waste). So this race is not ended yet.

Interesting links

play scientists, investigate isotopes chains - periodictable.com, isotopes, Th

Thorium optimists site energyfromthorium.com, they recommend this video 2h video "NASA" - Thorium remix 2016 to watch, I expect it to be at least interesting, but have not watched it yet.

Thorium fuel cycle, and specially "List of thorium-fuelled reactors"

Molten salt reactor concepts in general, https://en.wikipedia.org/wiki/Molten_salt_reactor

Just a practical way of harnessing its power.

Thorium power-plants need to go a long way it before being commercially interesting. Not because Thorium is expensive or hard to use, but because we don't really have a easy way to tap on its power — yet.

People are driven by necessities. If coal and oil (the "easy" power sources) are getting rarer and more expensive, people will turn to other, "harder to use" energy sources, like nuclear power, hydroelectric or wind-based.

However, as you use something, you learn its limitations. This provides a good base to future research, which in turn makes it easier to use as new, more efficient stuff is discovered about that something. It was the same with computers, with the Architecture change — people didn't swap to x64 right away because it was easier to keep using x86, even if x64 was way better. Nowadays, you only find x86 computers on some really outdated offices and on a few grandfather's houses. Heck, it's HARD to find x86 machines anymore!

So, yeah. It's mostly time. Once you find a way to make use of Thorium, just keep up researching and making it easier to use, and eventually people will embrace it.

The requirements for developed theorium reactors are not that tough: easy access to thorium, limited access to uranium and other sources of power and no incentive to develop a nuclear arsenal (and this last point is perhaps not even true; there are some that claim that the uranium 233 that is the actual fuel in a thorium-based cycle is viable as bomb material).

Thorium is today seen as a less viable choice because it would need different reactor technology than what is used in most of the world to be most effective. However, this is not in itself a major problem. The Soviet union, Canada and the UK uses or has used reactor designs that significantly differ from the US-designed LWRs.

I will give you one possible scenario: Sweden had it's own program for building heavy water reactors, with uranium mined in Sweden (Sweden has a lot of uranium, but not very concentrated). Had the large thorium deposits found in Norway been on the other side of the border, it is possible it would have choosen designs based on thorium instead, and gotten so far before the non-proliferation treaty that it would have decided to not buy american LWR designs but instead continue with the domestic design. At the peak, Sweden's reactors produced slightly less than half of the electricity used, so that requirement is not much of a stretch.

Thorium is not a practical nuclear fuel

Uranium contains 0.72% by mass of U-235, which is capable of sustaining a fission chain reaction, yet it does not produce high radiation by itself. So a block of Uranium can be refined to 3-4% U-235 by mass. At this point, it can sustain a fission chain reaction, and can be used for fuel.

The process by which uranium is enriched is by attaching Uranium atoms to Florine atoms to create Uranium Hexaflouride gas. This is then spun in a centrifuge; the heavier U-238 atoms are separated from the lighter U-235 atoms. Once enough of the U-238 is removed, the UH$_6$ gas is converted to UO$_2$ for use in reactors. The excess U-238 becomes depleted uranium. While the UH$_6$ gas is highly toxic, the radiation exposure of people and equipment from this process is negligible. Almost all reactors in the world (~90%) use this process to generate power.

The remaining 10% are Pressurized Heavy Water Reactors. These reactors do not use enriched Uranium, but rely on heavy water (D%_2$O). Heavy water does not absorb neutrons as readily as regular water; it thus reflects more neutrons back into uranium fuel as it moderates and allows natural Uranium to become fissile. The trade off for the reduced fuel cost of natural Uranium is the high costs of heavy water and reduced energy density of the fuel.

A Thorium reactor would work on a different principle altogether. It would 'breed' productive U-233 fuel from Th-232. The U-233 must then be removed from the thorium and concentrated to use as fuel for the reactor. Once a U-233 reactor is set up, Th-232 can be inserted into it to breed more U-233.

The problem with U-233 comes with separating it from the Thorium. This can be done chemically; since Th and U are different elememnts, they react with different chemicals making it easy to dissolve one but not the other. However, a byproduct of breeding U-233 is U-232, which is extremely radioactive, having an extended decay chain that releases multiple MeV gammas per atom that decays. That is the showstopper.

Remember, that enrichment of U-235 takes place from a material whose radiation level is barely above background. Instead, when trying to separate fissile material from Thorium, you are trying to remove fissile material from highly radioactive nuclear waste, and within the usable product is more highly radioactive nuclear waste. High energy gammas require the separation process be done in a sealed, shielded room; but they also will disrupt electronics in that room.

The damage to electronics is key; if electronics (i.e. robots) are damaged by gamma radiation, the entire process must be stopped and radiation levels allowed to drop until equipment can be replaced or repairs made. These work stoppages would make the Thorium fuel cycle uneconomic. The only alternative is to expose workers to high radiation levels to limit reliance on robots. Radiation exposure of that level is obviously not feasible in Western countries, as the liability to lawsuits of nuclear power companies would be prohibitive.

Fissile material cannot be economically refined from Thorium due to high radiation levels of the 'bred' Thorium and the mixture of highly radioactive U-232 into the U-233 fuel. An engineering solution to this may eventually be possible, but currently there is no way to use Thorium as a fuel without accepting high levels of radiation to workers.