Thorium is a more abundant material than Uranium. It has a half life or 14 billion years, older than the universe.  It is regarded as fertile, not fissile. That means it needs to be hit with a stream of neutrons to transmute it from Thorium 232 then into Uranium 233 which is fissile.Once the U233 has built up it can supply most of the neutrons to keep the reaction going.

It becomes a breeder reactor where the non fissile Thorium or Uranium 238 is transmuted to  fissile atoms

The Neutrons for this reaction can come from bomb grade or enriched Uranium or Plutonium, or the neutron stream can come from a spallation source. This is an accelerator bombarding a pool of mercury. Many research reactors use spallation sources and use no Uranium.

Arguments for Thorium fission

Thorium is more abundant than Uranium.  There are large known deposits in Australia, India,  Idaho in the U.S, and Norway.
No need to separate an isotope as all the same.

Higher MP and chemically more stable than Uranium
Proposed as a way of destroying bomb grade Plutonium or Uranium, and nuclear waste.

Thorium is a byproduct of most “rare-earth” mining around the world and with a 14-billion year half life, constitutes no danger when treated properly. Thorium has been stockpiled by US Dept of Energy in sufficient pure-metal quantities to meet all US energy needs for about a decade.
...the well-documented successes of Alvin Weinberg’s team at Oak Ridge over 40 years ago. They used liquid (molten-salt) fuel cycles, with Th232-U233.

..not only can Th232 be easily bred to U233 in molten salt, but the resulting U233 (which doesn’t occur in nature) fissions far more completely than other U or Pu isotopes, leading immediately to lower waste production.
...if a bomb is attempted with U233, enough U232 will naturally occur such that not only will workers be killed very soon, any successfully-constructed weapon would be so radioactive in penetrating gammas that its surrounding controls & delivery mechanisms would be ruined. And, it would be extremely hard to hide & easy to detect. U233 is in no way a military or terrorist weapon, rather it would eliminate any terrorists foolish enough to try to use it.

There would never be any “enriched fuel” sitting around for theft – it would all exist as, say, Fluoride salts, dissolved into the simple MSR chemistry. The Th232-U233 transmutation can even be started with a medical proton-accelerator, as the Japanese have done.
..there’s no “spent fuel” in an MSR (Molten Salt Reactor) – all Th232 & U233 are consumed, and there’s never a scheduled shutdown for refueling, because of the very nature of the design – an unpressurized,liquid. ThF4 or UF4 (or even higher U & Pu isotopes as salts) are simply added into the molten mix as it’s pumped around the reactor & heat-exchanger plumbing. It’s what every chemist understands & loves: liquid, unpressurized chemistry. And, since all fuel is consumed, an MSR can be used to reduce nuclear wastes down to any level desired, even on the site of a de-commissioned U/Pu reactor. This is exactly the kind of ability responsible scientists, engineers, doctors, politicians and citizens care about. PSR/IEER proliferation of this paper hides what is perhaps the most important knowledge we need today to pursue a weapons-free world — MSRs can consume them all.

Dr. Alexander Cannara
Menlo Park, Calif.
650-400-3071
Source: Clean Technica (http://s.tt/1nb7Q)​

Thorium deposits in Australia

Spallation

Spallation can be used to supply the neutrons to start a fission reactioni. It is a process in which fragments of material (spall) are ejected from a body due to impact or stress.

In nuclear physics, spallation is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic weight.