Molten salt reactor

Figure 1. The MSR used in the Oak Ridge Molten salt reactor experiment in the 1960's.

Molten salt reactors (MSRs) are a Generation IV nuclear reactor that use molten salts (high temperature liquid salts) as their nuclear fuel in place of the conventional solid fuels used in the world's current reactors. The use of fluids allows for it to act both as their fuel (producing the heat) and coolant (transferring the heat).

These reactors have been designed in many different ways using different fuels. All of these reactors initially have their fuel chemically bonded to fluoride, which is then dissolved into a molten carrier salt. The most commonly proposed carrier salt is a mixture of LiF (Lithium Fluoride) and BeF₂ (Beryllium Fluoride) commonly referred to as FLiBe. MSRs have not been implemented since the shut down of the Molten Salt Reactor Experiment (MSRE) in 1969. This is primarily due to technical issues associated with the high temperature and corrosive nature of the salts.

Many countries around the world are actively pursuing research and development of MSRs.

How do they work?

Figure 2. Molten FLiBe.

The goal with any reactor is to produce thermal energy through the use of nuclear chain reactions. The way this is done varies drastically between reactors, and molten salt reactors are perhaps one of the most unique. Modern reactors currently use solid fuels in their operation, with uranium being the dominant fuel for these. MSRs however dissolve their fuel in a molten salt mixture, allowing for many interesting benefits which will be discussed in the section below. First it is important to understand the reactor's operation.

In a basic molten salt reactor, enriched uranium (Uranium-235 or -233) is dissolved in a single molten salt solution. The core consisting of a neutron moderator allows the salt solution to flow at high temperatures - 700°C or higher - while remaining at fairly low pressures. The use of low pressures is an important safety feature, as the risk of an equipment malfunction is greatly diminished. The heat generated by the nuclear reactions in the salt would be transferred to a secondary circuit, which would heat up water to steam and from there produce electricity.

The concept of this basic MSR could be expanded to various other operating features, with perhaps the most promising being its use as a breeder reactor. This means it would produce more fissile fuel than it required in the first place!

Figure 3. Diagram of a Molten Salt Reactor.

Use as a breeder reactor

The molten salt breeder reactor (MSBR) expands on the basic MSR operating principle. Instead of a single fluid system as described above, a second molten salt fluid is introduced for the breeding of fissile isotopes. The first fluid would contain a fissile fuel (Uranium-235, or other) which is the "driver" of the nuclear reaction - the fission of it provides neutrons to the second loop, moderated to intermediate to low speeds, along with its normal chain reaction providing useful energy. The second fuel loop would contain a fertile fuel, which could absorb these neutrons and eventually transmute into a fissile fuel. It would breed more of this new fissile fuel than would be used to do so, hence the name "breeding".

The operation of the MSBR is promising for the use of thorium as a nuclear fuel, since it has lots of potential in nuclear reactor technology but is currently not in use. A type of MSBR that would use thorium is the Liquid Fluoride Thorium Reactor (LFTR). In this reactor, thorium would absorb neutrons from the fissile loop, and would produce uranium-233 by a series of beta decays. The uranium-233 can be chemically extracted from this loop, and injected into the fissile loop, thereby extending fuel life of the reactor and reducing nuclear waste.

Benefits and Drawbacks

MSRs have many great benefits, however benefits cannot come without some problems. For a more complete story of the pros and cons, visit "What is nuclear?".

Benefits

• Fission products can be removed or added while the plant is operational. Allows for removal of neutron absorbing materials that are produced during fission and for on-line refueling.
• Use of thorium is promising since it is more abundant than uranium, and in combination with on-line refueling its use can be optimized.
• Fuel fabrication is limited to chemical processes, rather than the need to manufacture fuel rods, assemblies, tubes, etc.
• High temperatures increase efficiency of heat transfer, and low pressures ensure safer operation while reducing the size and costs of the reactor building.
• Improved safety features such as a drain tank mechanism where the fuel can be passively cooled in the event of overheating (see Figure 3), lower pressures, and negative temperature coefficient of reactivity meaning if the temperature increases the fuel expands and becomes less radioactive.

Drawbacks

• Material degradation can be a problem due to the corrosive nature of the chemicals present in the fluid.
• Production of radioactive Tritium is unavoidable if lithium is used, and it is capable of escaping to the environment because it is so small.
• Complex chemical plant operation and maintenance would increase costs and introduce many complications in operation.
• Proliferation concerns are present due to the difficulty of fissile materials tracking inside of the core, and if thorium is used as the fertile fuel in breeding its byproduct proactinium-233 could be harnessed to make weapons since it decays to weapons-grade uranium-233.

Completed Designs

• Aircraft Reactor Experiment (ARE) (1947-1961)
• Molten Salt Reactor Experiment (MSRE) (1965-1969)

Proposed Designs

• Liquid Fluoride Thorium Reactor (LFTR)
• Molten Salt Breeder Reactor
• Fuji MSR