Molten salt reactors, MSR, are part of the generation IV development. They can be run as breeders or burners, or a balanced mixture of both.

They dissolve the uranium or thorium fluoride in molten salt at around 500^oC. If the salt gets too hot, the atoms separate and the reaction slows down. 

There were running in the 1950 and 60s but the MSR program was canceled in the early 1970s. In hindsight, this was a policy error.

There has been a resurgence of interest in MSRs, with programs in the USA, Japan, Russia, France, Czech Republic, and India. Most importantly, a large and well-funded effort in China has recently begun, with the Chinese Academy of Sciences planning to build two test reactors by the end of the decade. 

Ref Wikipedia

http://www.terrestrialenergyinc.com/msr-history.html

Breeding or burning?

The MSR can be run to breed or burn. 

Breeding is converting fertile atoms such as  Th or U-238 into fissile ones. It is caused by fast neutrons. Burning is causing fissile atoms to split, or fission. It is caused mainly by slow (thermal) neutrons.

Fission gives off fast neutrons.  

If the coolant salt doesn't slow the neutrons, then the reactor is a Fast Neutron MSR and is a breeder. 

If the coolant salt slows neutrons, then the reactor is a burner, or Thermal Neutron MSR.  

Fluoride salts tend to slow neutrons, whereas chloride salts don't. By adjusting the mix the reactor can be balanced to both breed and burn, to suit the fuel. As the fertile fuel is converted to fissile,  slow neutrons burn it at the same rate. So there is no build up of fissile material.

 By the late 1970s, it was recognized that a MSR-Burner could be superior to the breeder approach. This realization led to the conceptual development of what has become known as the DMSR or Denatured Molten Salt Reactor

The operating fuel requirement for the burner costs on the order of 0.1 cents/kWh of electricity produced, considerably less than the 0.6 cents/kWh that it costs for LWR fuel.(Normal nuclear power - Light Water Reactor)

Another advantage is that there is a lot of nuclear waste available, and this can be used as fuel.

Source: Terrestrial Energy inc

Advanced molten salt reactor

Transatomic Power is developing an advanced molten salt reactor that generates clean, passively safe, proliferation-resistant, and low-cost nuclear power. This reactor can consume the spent nuclear fuel (SNF) generated by commercial light water reactors or use freshly mined uranium at enrichment levels as low as 1.8% U-235. It achieves actinide burn-ups as high as 96%, and can generate up to 75 times more electricity per ton of mined uranium than a light-water reactor.

They have taken the MSR - Molten Salt Reactor developed at Oak Ridge in the 60s and 70s and changed the moderator from graphite to zirconium hydride and fuel salt used to a LiF-based fuel salt.

Previous molten salt reactors relied on high-enriched uranium, with 33% U-235. Enrichments that high would raise nuclear proliferation concerns if used in commercial nuclear power plants. 

Transatomic Power’s reactor can run for decades and slowly consume both the actinide waste in its initial fuel load and the actinides that are continuously generated from power operation. Furthermore, the neutron spectrum remains primarily in the thermal range used by existing commercial reactors . This avoids the more severe radiation damage effects faced by fast reactors, as thermal neutrons do comparatively less damage to structural materials .

Source

Advantages of MSR

The Oak Ridge National Labs developed a Denatured Molten Salt Reactor specifically because

(a) it can not melt down,

(b) it supports 30 year fuel/waste residency times for nuclear waste burnup,

(c) it does not and can not contain or generate substances that can form explosive mixtures, such as graphite/water (Chernobyl) or zirconium/water (Fukushima) induced hydrogen, high pressure (all three major incidents) or other chemical explosives (Liquid Sodium Fast Breeder Reactor), and

(d) it contains only LEU (Low Enriched Uranium) material throughout the lifetime of the reactor.

And an advanced Denatured Pebble Bed Moderated Molten Salt Cooled and Fueled reactor is one reactor design that offers the potential to operate in both the Thermal and Fast spectrums as per operational requirements such as accelerated waste burnup.

But in the end, the most ambitious program currently to commercialise Molten Salt reactors is that by the Chinese Academy of Sciences, and they are targeting 2030 for the first commercialised plant.  The UK's National Nuclear Laboratory believes a 10-15 year international research effort is still required to prove the technology around a common GenIV MSR design.

Australia isn't currently skilled to do much work in nuclear engineering,  UNSW has a Nuclear Engineering course.

Engel, J.R., et.al, (1980 July) "Conceptual Design Characteristics of a Denatured Molten Salt Reactor", ORNL/TM-7207

http://web.ornl.gov/info/reports/1980/3445603575931.pdf

OECD Nuclear Energy Agency's open tool JANIS (Java Nuclear Information System):
http://www.oecd-nea.org/janis

Why was development on MSRs stopped?

All of the following reasons:
1. Because an MSR in Tennessee wouldn't deliver jobs in California like a proposed Fast Breeder Reactor would have
2. Because Richard Nixon was from California, not Tennessee
3. Because MSRs wouldn't produce weapons grade plutonium like the Californian FBR (Fast Breeder Reactor)
4. Because all of the major manufacturers have focused on mechanical nuclear reactors and not chemical nuclear reactors which makes the transition to MSRs more difficult for them in terms of training to gain new knowledge
5. Because a nuclear navy would more preferably use a PWR (Pressurised Water Reactor) than an MSR (for sound technical reasons, such as because the presence of U-232 daughter Thallium-208 in a submarine reactor is more significantly a hazard than in a civilian reactor), and that means that civilian contractors like Westinghouse and Babcock&Wilcox are trained in mechanical and not chemical nuclear reactors (PWR not MSR), and because Submarine Nuclear Reactors operate on highly enriched fissile material and under conditions that maintain pressure and provide instant passive emergency cooling (ocean water).
6. Because the certification of nuclear reactors is a difficult and detailed process, and certification of similar reactors are more cheaply achieved.  An MSR would represent a departure and would involve increase certification costs.  Capital costs for nuclear reactors are the dominant lifetime costs, and saving in any area of capital cost is considered an advantage.
7. Because MSR chief designer Alvin Weinberg and Richard Nixon's Californian Congressman Chet Holifield didn't get along: Chet Holified suggested that Alvin Weinberg's concerns over nuclear safety at civilian facilities was a sign that Alvin should get out of nuclear energy.

Summary of nuclear energy - http://www.energy-without-carbon.org/NuclearSummary

Radio active decay - http://www.energy-without-carbon.org/RadioActiveDecay

Nuclear fusion - http://www.energy-without-carbon.org/NuclearFusion