But after the ANP shifted to ORNL in 1950' the scientists involved began to understand that Molten Salt Reactors offered exceptional nuclear safety characteristics. First, reactors are made less safe by dangerous things that you put into their core. Things like water and sodium Water is dangerous because under heat and pressure it turns to steam and can explode. Steam explosions are well known problems with the use of water as a heat to work transfer medium. The literature of the early age of steam features dozens of accounts of boiler explosions. Boiler explosions are not just a curiosity from the past, witness the 2003 boiler explosion aboard the cruse ship Norway while in port in Miami. Light and heavy water cooled reactors are basically boilers heated by nuclear reactors. In order to operate efficiently core coolant water is superheated and kept liquid by high pressure. The presence of critically heated water in the core of water cooled reactors is a fundamental safety issue, that requires special design features to manage. Those features cost money to design and implement.
In contrast Molten Salt Reactors, even while operating at high temperatures, do not produce more than a single atmosphere's pressure, and thus will not produce anything like a steam explosion. Thus MSRs are at a significant advantage as far as nuclear safety costs.
The MSR was designed to cope with a number of safety problems associated with a liquid sodium cooled reactor. While liquid sodium cooled reactors including sodium cooled fast breeders do not operate under high pressures, the chemical nature of sodium, as well as some particularities of sodium flow inside reactor cores, create some safety issues. Sodium is extremely chemically active and will burn in contact with air and water. Thus special care must be taken with sodium cooled reactors to maintain separation between between coolant sodium and air, as well as structural materials which contain water such as concrete. This necessitates special design features which may increase the cost of sodium cooled reactors.
In contrast molten salts used in MSR cores do not burn. In addition the tend to freeze as some as their temperature is dropped by air contact. Thus MSR salt leaks can be expected to self seal. Since the reactor core salt is under only a one atmosphere pressure the frozen salt leak seal can be expected to hold. During the Oak Ridge Molten Salt Reactor Experiment researchers experienced reactor salt leaks on laboratory floors. These were cleaned up without difficulty.
In addition, research on fluid flow inside sodium cooled fast reactors has indicated the existence of flow problems called voids. A void can potentially cause a loss of operator control of a sodium cooled reactor, and lead to reactor run aways with potentially catastrophic consequences. Avoiding the void problem may lead to penalties such as limiting reactor performance and breeding capacity.
A simple Uranium cycle Denatured Molten Salt Reactor (DMSR) would not include such a potential for reactor reactor run away. Single fluid MSRs are extremely stable, and will shut down automatically if they overheat, due to fluid fuel expansion. For this reason there is no reason for control rods, or reactor monitoring by operators. ORNL researchers preparing for the 1960's Molten Salt Reactor Experiment determined that MSR operators would have nothing to do, and so would be board. They chose to design the MSRE without a control room, and ran the reactor without an operator present.
More complex MSRs such as 2 fluid LFTRs would have more complex control issues, but it seems possible through careful design that they can be made as safe as the DMSR.
Reactor safety problems are created by things that are put in reactor cores, as well as things that are created in reactor cores. Solid core reactors are stuck with dangerous fission products, and other materials like plutonium which are created in the reactor core. In the event of a catastrophic accident such materials are seen as a significant menace. Radioactive gases are of continuous concern. Most of the escaped fission products following the Three Mile Island accident were radio active gasses, and voluble fission products. It is possible to bubble radioactive gases out of the carrier salt of a molten salt reactor at low expenses, and the recovery of voluble fission products would not be too expensive. In addition the recovery of other fission products, called nobel metals would not be technically difficult or expensive. Thus many of the most dangerous fission products can be removed from a Molten Salt Reactor core as the reactor operates. By removing radioactive fission products either periodically or as they are produced, the worst case MSR can be rendered significantly less dangerous,
In addition actinides such as plutonium-239 can be simply left in the reactor core until they burn up. In might be desirable to periodically clean the salts of DMSRs or LFTRs, but dangerous materials called actinides can be automatically returned to the reactor core with out ever being accessible to people. This would prevent the diversion of nuclear materials by terrorists, or state based would be nuclear proliferators. The DMSR was designed to be proliferation resistant, and it has many features that would lead a would be nuclear proliferator to chose other routs to produce nuclear weapons.
Molten Salt Reactors, such as the DMSR offer very teal safety advantages over Light and Heavy water reactors as well as sodium cooled fast reactors. These MSR safety features can significantly lower reactor safety related costs, while increasing public confidence in the safety of nuclear generated electricity.