Molten Salt Reactors

While IFFY now covers all types of reactors, the original focus of the project was Molten Salt Reactors (MSR).  In an MSR, the fuel is dissolved in liquid, rather than fixed in the solid fuel rods used in today’s reactors.  In addition, the liquid is at atmospheric pressure, which has major advantages over today’s technology.


Nuclear waste has two major components,

  • Fission products: elements from zinc to the rear earths.
    • The big problem is the medium-range isotopes, 137-Cs and 90-Sr.  These have half-lives of thirty years.  In 300 years, these will be 99.9% stable.
  • Actinides: elements heavier than lead.
    • Some of these have half-lives of thousands of years, and constitute the main long‑term waste issue.

The fission product composition, volume, and residual radioactivity does not depend (much) on reactor type and/or fuel choice.  The heavy element waste does.  The longer the actinides stay in the reactor, the more likely they get burned as fuel.  This is one promise of MSRs.

The solid fuel in today’s reactors needs to be replaced after about four years in the reactor.  This is due to the buildup of fission products.  Some fission products absorb neutrons, which are needed to keep the fire burning.  The hungriest neutron absorber is a gas, 135-Xe, which is trapped inside the fuel pellets in today’s reactors.  In an MSR, this gas can bubble out of the liquid and be removed before it eats too many neutrons.

In addition, other neutron absorbers, primarily rare earths, can be extracted from the liquid (though not as easily as a gas).  With these gone, the actinides can remain longer in the core, where they can add to the energy produced instead of the waste generated.


Our focus is on waste, but unless a reactor costs less than today’s, it won’t get built.  There are several reasons MSRs should be cheaper.  First, an MSR operates at atmospheric pressure.  Todays’ pressurized water reactors need heavy pipes, and a core vessel so thick only a few foundries in the world can make them.  An MSRs pipes can be far cheaper.  Also, the high pressure necessitates a large, strong containment structure.  The containment for an MSR can be much less massive.

There are also cost savings if Thorium is the fuel.  Enriching Uranium is expensive.  In addition, Thorium is now a by-product of rare-earth mining, so it’s basically free.  However, this is a minor advantage because fuel costs are not very large in any reactor.



The reason meltdowns occur is that the hot fuel becomes separated from the coolant.  Solid fuel rods are cooled by water, which can drain off or boil away.  In an MSR, the fuel is dissolved in the coolant salt.  This makes meltdowns nearly impossible.


The water in today’s reactors is pressurized, which is inherently dangerous.  Steam explosions are one danger.  Another is that high pressure steam will try to escape the building, carrying radioactive material with it.  MSRs operate at atmospheric pressure, avoiding both hazards.  If a pipe breaks, radioactive materials drain to the basement rather than rushing outside.

Also, the explosions at Fukushima were due to the release of hydrogen.  There is no apparent path for this to occur in a MSR.

Weapon proliferation

MSRs are more capable of using Thorium than most reactor designs.  Thorium becomes 233-U fuel, which has not been used successfully in nuclear weapons.  Moreover, a Thorium economy will eliminate the need for enriching Uranium (concentrating 235-U).  When the rest of the world is using Thorium, a nation that is enriching Uranium is obviously intent on building bombs.


MSR use fewer resources, both fuel and construction materials such as concrete and steel.  Less obviously, MSRs can also provide new resources not available today.


An MSR can use uranium far more effectively than today’s reactors, simply by keeping the fuel in the reactor longer.  But the real winner is Thorium, which is perhaps 500 times more common than 235-U.  This means Thorium could power a modern economy for thousands of years.

New materials

The salt stream in an MSR can be tapped and processed to yield radioactive isotopes not available in quantity today.  One of these is 213-Bismuth, which holds great promise for treating cancer.

New processes

An MSR runs hotter than today’s reactors.  High temperatures are valuable for processes other than power generation.  For instance, one Canadian MSR is being developed to heat tar sands, rather than using the tar itself for this purpose.


According to one pundit, MSR technology can achieve everything promised by fusion power.  Molten salt reactors seem to cover all the bases: Faster/Cheaper/Better.