The molten salt reactor is designed to have a negative temperature coefficient of reactivity. This means the reactor’s power quickly drops if its temperature rises above the operating point, which is an important and necessary safety feature. The molten salt reactor is especially good in this respect—it has little excess reactivity because it is refueled frequently online and has a high conversion rate that automatically replaces fuel consumed. Failure to provide makeup fuel is failsafe as the reactivity is self-limiting by the burnup of available fuel. A small amount of excess reactivity would be compensated by a temporary interruption of adding makeup fuel online. Present reactors have ;20% excess reactivity. Control rods and burnable poisons are used not only in accident control but also to barely maintain criticality. In the molten salt reactor, control rods are used to control excess reactivity of perhaps only 2%, which is necessary to warm the salt from the cooler start-up temperature to the operating temperature ~i.e., overcome the negative temperature coefficient!. That is, only enough fissile fuel is in the core to maintain a chain reaction and little more. Gaseous fission products are continually removed and stored separately from the reactor in pressurized storage tanks. By contrast, in conventional reactors the gaseous fission products build up in the Zr-clad fuel tubes to a high pressure that presents a hazard and can cause trouble. If an unforeseen accident were to occur, the constant fission-product removal means the molten salt reactor has much less radioactivity to potentially spread. The usual requirement of containing fission products within three barriers is enhanced by adding a fourth barrier. The primary vessel and piping boundary, including drain tanks, constitute one barrier. These components are located in a room that is lined with a second barrier, including an emergency drain or storage tank for spills. The third barrier is achieved by surrounding the entire reactor building in a confinement vessel. A fourth safety measure is locating the reactor underground, which itself is one extra “gravity barrier” aiding confinement. A leakage of material would have to move against gravity for 10 m before reaching the atmosphere. In case of accidents or spills of radioactive material, the rooms underground would remain isolated. However, the residual decay heat that continues to be generated at a low rate would be transferred through heat exchangers that passively carry the heat to the environment aboveground, while retaining the radioactive material belowground. This passive heat removal concept perhaps using heat pipes will be used to cool the stored fission products as well. The initial fuel needed including the amount circulating outside the core is considerably less than half that of other breeding reactors such as the liquid metal–cooled fast reactor. This is a consequence of fast reactors having much larger critical mass than thermal reactors and for the molten salt case, avoiding the need for extra fuel at beginning of life to account for burnup of fuel.s the pug is frozen, it blocks further drainage to the tankThe Negative Temperature Coefficient of Reactivity is a natural safety mechanism, that strictly obeys the laws of nature. A second ORNL safety feature, which Moir and Teller do not mention here is the frozen salt plug. The plug is designed to drain the MSR core if a frozen salt plug melts. The salt is frozen by blowing cool air over a section of the drain pipe that is designed to serve as the location of the freeze plug. An electrical shutdown will automatically shutdown the fan that keeps the freeze plug frozen, the tank will begin to melt. Once the plug stops blocking the drainage of the core, thew fuel carrier salt drains out of the core, into a tanke which has a geometry that prevents a chain reaction.
Thus natural processes in the MSR slow and then stop its chain reaction as the core temperature reaches a maximum permutable temperature. Once electrical generation using the MSR's heat stops, the freeze plug fan stops, and the freeze plug melts. At that point the nuclear fuel flows to a storage tank which is designed to inhibit a chain reaction. Decay heat from the fuel storage tank, can be transferred, again using a process that is natural, to transfer heat from the fuel storage tank to the surface where it can be radiated away. Another safety feature componant is the capture of noble gasses that are radioactive fission products. The continuous capture of these gases, and their transportation to a storage system, prevents their release in the event of an accident. In addition the Noble gases pose problems for the opperations of all reactors. A system of noble gas control increases MSR stability, and hence safety.
It should be noted that Moir and Teller did not mention every possible MSR safety feature proposed by ORNL. For eexample Uri Gat and H.L. Dodds had plenty more potential safety features to add.( See also Uri Gat, The Ultimately Rafe Reactor) We thus have a plethora of safety system choices for the Molten Salt Reactor. These are ORNL safety Plus Gat, and ORNL safety plus the Moir-Teller Barriers.
We are fortunate that a nearly 2 billion year event gave rise to anportant information about the safety of underground reactors. That advewnt was the emergance of natural nuclear reactors in a Uranium ore deposit, now located in Gabon, Africa. This deposit was identified by the lesser U-235 pressence in a Uranium deposit at Oklo in Gabon. Investigation showed that in multiple areas of the deposit spontanious chain reactions occured due to the presence of ground water in the deposit. Despite the flow of ground water into the fractured ore body, there had been very little movement of radioactive eliments away from the natural reactors. The reactors appeared to have operated for a period of around 100,000 tears, after which the U-235 content of the ore no longer supported chain reactions. Eventually the nuclear process stopped, and the Okla natural reactors remained dormant and harmless for two billion years..
The Oklo experience suggest that underground could solve a variety of problems, but I favor it because it provides protection against terrorists attempts to harm the reactor. An underground reactor cannot be damaged by aircraft crashes into a structure, and it would provide excellent containment of radioactive isotopes in the event of a nuclear accident. The underground location
Underground housing of reactors, in the absence of ground water intrusions, can provide very effective barriers to environmental release of radioisotopes, but there are some issues for MSRs, if ground water.
I identified Ground water as a safety issue when I did a case study involving the placement of an underground MSR on the site of the TVA Bull Run coal fired power plant. Bull run is located in a bally between two ridges near Oak Ridge, Tennessee. Like much of East east tennessee the geology is complex, but dominated by limestone. Bull Run fronts on the Clinch River, which is impounded below Bull Run by Milton Hill Dam. Thus the water level of the Miltonb Hill lack is higher than that of the historic water level of the Clinch River. Because the limestone which underlies the vally on which Bull Run is built is composed of pours limestone, the water table beneath Bull Run is
probably unacceptably high. As a consequence, Underground placement of a MSR at Bull Run might involve an unacceptable risk for underground placement.
This does not create intractable problems. Salt mines typically are relaticely safe, and they offer long term evidence of the absence of water table issues, since water turns underground salt to brine. There are literly thousands of old salt mines in the United States, and they sould be explored to identify suitable candidates to MSR housing.
The safety advantages of underground placement include protection form truck bombs and attacks by aircraft, as well as presenting barriers posed by the overburden of rock and soil, to the transport of fissionable materials. In addition to the materials barrier, the force of gravity would also tend to prevent radioisotope release. The evidence from the Oklo reactor tends to support that given placement of radio-isotopes in geologically stable environments, radio-isotope transport may not occurre for periods of time as long as two billion years. Thus underground placement of MSRs depends on geological factors, which in some cases, might lead to unsafe underground placement. In those cases the choice will be between identification of safe alternative placements, and the building of safe above ground facilities. The later choice is clearly achievable, given the evidence of safety found in the operation history of conventional reactpors, which do not posses the inherant safety advantages of the Our expection should be a superior form of nuclear safety in the design and operation of the ThorCon reactor, a safety that could be described as super safety.