Part 2 of 2 Parts (Please read Part 1 first)
The main political problem that causes concern for the adoption of MSRs is the fact that they are perceived pose a nuclear weapons proliferation problem. Non-proliferation experts state that as soon as the cladding of a nuclear fuel rod is cut open, it poses a proliferation danger. Obviously, because the fuel in a MSR is already exposed in a vat of molten salt, it is already part way to a useful material for making nuclear bombs.
1. Protactinium-233 decays to pure, weapons-grade U-233 – Many thorium-cycle MSRs have to capture protactinium as it is produced, remove it from the reactor while it decays to U-233 and then mix it back in with the molten salt. This is necessary because Pa-233 absorbs so many neutrons that it prevents the maintenance of a fuel breeding cycle. The problem with this process is that U-233 is pure weapons grade uranium which could be used to make a bomb. The U-233 is usually mixed with zirconium but the zirconium is easy to remove. Most common power reactors do not require such a proliferative stage in their fuel cycle. In addition, many types of MSRs do not carry out such a process. Liquid fluoride thorium reactors (LFTR) do require this process. This means that anyone who is operating a LFTR could be producing nuclear bombs. There are many suggestions of how to mitigate this problem such as deliberately using U-232 to contaminate and denature the U-233 would only serve to prevent diversion by criminal or terrorist third parties. However, for the owners of a LFTR nuclear plant, it would be easy to deal with most of the fixes suggested.
2. Inventory tracking is difficult – Because a lot of the materials in the fuel will plate out in the fuel vat and the chemical plant, it can be very difficult to keep precise records of all of the actinides. The International Atomic Energy Agency (IAEA) requires the installation of safeguards in reactors to ensure that all the actinides are accounted for. This is done to prevent proliferation. However, it can be difficult for the IAEA to separate plate out losses from proliferation diversion.
There are a few other problems but they will probably have practical solutions.
1. Unknown waste form – Since MSRs are a new technology, it is not yet clear exactly what the waste from MSRs will look like. The molten salt is not stable enough to be put in a repository. There will have to work on the development of a stable waste form for the molten salt.
2. Electrical heaters are required to stay liquid – During a long power outage, the colder parts of the heat transfer loop in the MSR could solidify. This could result in temperatures to rise in the core. This could be a problem.
In order for MSRs to breed in a thermal spectrum, if there is lithium in the mixture, it must be enriched to very pure Li-7. Li-6 is a very strong neutron poison which becomes tritium. FliBe is a molten salt of beryllium fluoride and lithium fluoride. FliBe is a controlled substance and it has some weapons applications. It is also a very dangerous materials when inhaled. In chloride salts, the chloride must be enriched to pure Cl-37. Cl-35 is a strong neutron poison which will prevent nuclear fission. The activation product of Cl-36 is long-lived, water soluble, and a hard beta particle emitter which complicates waste disposal. These enrichments increase the cost and complexity of the MSR fuel cycles.
There are more specific problems with each specific type of MSRs in addition to the problems mentioned above. MSRs are underdeveloped and will require a great deal more research, especially with respect to corrosion issues before they will be ready to be added to the world fleet of commercial nuclear fission power reactors. They may ultimately be viable sources of electricity but that remains to be seen.