Someone recently posted a comment to an article I had posted about a particular nuclear battery being so much bunk asking why I hadn’t mentioned nuclear salt reactors.

This is an interesting technology that properly configured has the potential to make nuclear power viable in the long term without so many of the problems present in pressurized and boiling water reactors.

There are four major problems with the nuclear industry as it exists.

1. Conventional pressurized water and boiling water reactors are prone to meltdown and the release and widespread dispersion of radioactive materials.
2. Fissile materials, especially plutonium 239, can readily be diverted from the fuel cycle of conventional reactors which breed an amount of Pu-239 from the absorption of a neutron by U-238.
3. Because processing is done off-site, there exists the possibility of diversion of radioactive materials to terrorist uses such as dirty bombs.
4. The radioactive waste produced by conventional reactors is a 100,000 year problem.  No civilization exists for long enough to contain that, nor is any repository technology sufficient.

Molten salt reactors address the first problem by having a plug in the bottom of the reactor that melts when temperatures exceed a safe threshold allowing the fuel to drain into a tank that is sufficiently large that the fuel disperses to the point where the chain reaction can’t be sustained, the molten salts solidify, and the radioactive elements are contained.

There is also an inherent safety factor in that molten salt reactors operate with a negative temperature coefficient and there are actually two factors that contribute to this.  First is that the thermal expansion of the salts takes the fissile nuclei farther apart from each other reducing reaction rates.  The other thing that contributes to a reduction in reaction rates is Doppler shifting of the neutron energies due to the thermal motions broadening the neutron energy spectrum.  Only neutrons of the right energy are efficiently absorbed maintaining the chain reaction.

Unlike pressurized water reactors that have to be constantly maintained at the right reaction rates with the use of neutron absorbing control rods, these reactors have a negative temperature coefficient, that is to say as the temperature increases the reaction rate naturally decreases, that tends to stabilize them.

Molten salt reactors can be operated using fuels such as thorium (actually uranium-233 bread from thorium) or the transuranics (transuranics can only make up a fraction of the fuel because of neutron budget issues) from the waste of conventional reactors producing a mix of isotopes not suitable for the production of bombs.

On-site reprocessing eliminates the need to transport high-level waste and the potential for terrorist diversions or accidents in the process of transporting those wastes.

Because molten-salt reactors are capable of burning transuranics, they leave only the much shorter term fission products as wastes, turning a 100,000 year problem into a couple of hundred year problem, and even there, there are only a couple of long-lived isotopes which could be destroyed in an accelerator turning even that couple hundred year problem into a couple decade problem.

Conventional boiling water reactors effectively extract less than .7% of the Uranium fuels energy potential, molten-salt reactor systems can extract up to 98%, and they can use thorium fuel cycles which have little proliferation potential and thorium is about 4x as plentiful as uranium in the Earth’s crust.

The original Oak Ridge design used graphite as a moderator and that was bad because hot graphite exposed to air equals Chernobyl.  Newer designs have eliminated the need for graphite in their design.

All in all, I believe this is a technology that could make nuclear not only viable for the long term but would allow us to clean up some of the mess we’ve created with existing reactors.