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By Charles Rhodes, P.Eng., Ph.D.

The term Molten Salt Reactors (MSR) is used to refer to two different types of reactor. There are molten salt cooled solid fuel fast neutron reactors (FNRs) and there are liquid fuel molten salt thermal neutron reactors. It is important to distinguish between these two different reactor types.

A molten salt solid fuel FNR is similar to a liquid sodium cooled FNR except that the liquid sodium primary coolant is replaced by a molten salt. The main advantage of a molten salt solid fuel FNR over a liquid sodium cooled FNR is a reduced fire hazard. However, that advantage is offset by the disadvantages of much more complex fuel tube, intermediate heat exchanger and containment metallurgy and a requirement for a higher operating temperature to prevent the molten salt becoming solid. There are also potential problems with neutron activation and neutron absorption by the molten salt.

A molten salt liquid fuel reactor is a nuclear reactor in which the fissile material is dissolved in a molten salt instead of being formed into a solid rod inside a metal tube. Some of the advantages of a molten salt liquid fuel reactor are set out in the videos: Molten Salt Liquid Fuel Reactor Videos.

The theoretical advantages of a liquid fuel molten salt reactor (MSR) over a liquid sodium cooled FNR are:
1. A liquid fuel MSR delivers heat to the thermal load at a higher temperature. This high temperature heat is of particular importance in reforming of hydrocarbons;
2. The molten salt is less of a fire hazard than liquid sodium;
3. A liquid fuel MSR avoids the hundreds of thousands of metal fuel tubes that are required by a molten salt solid fuel FNR or by a liquid sodium cooled FNR.
4. In a liquid fuel MSR the fuel is relatively uniform and hence the reactor is theoretically is less complex to control than a molten salt solid fuel FNR or a liquid sodium cooled FNR due to absence of potential local "hot spots" as long as there are no liquid level gradiants, waves or vorticies. However, in real liquid fuel MSR the issues of wave and votex formation in the liquid fuel are of extreme importance at high power.
5. A liquid fuel MSR operates on thermal neutrons and relies on the presence of a graphite moderator on the core zone for criticality.

The disadvantages of a liquid fuel MSR as compared to a liquid sodium cooled FNR are:
1. The high melting point temperature of the salt triggers a host of metallurgy and corrosion problems;
2. Salts that have lower melting points generally involve beryllium, lithium or chlorine that have major isotope separation and/or nuclear waste disposal issues;
3. The molten salt containment pool walls require a thick porcelain shield to prevent cumulative neutron damage;
4. The intermediate heat exchanger tubes have only a short working life due due to formation of low thermal conductivity fission product deposits on the the heat exchange tube surfaces. These tubes are a major source of maintenance and decommissioning waste;
5. The required side arm fuel recycling chemistry of a liquid fuel MSR triggers additional problems and costs for distributed reactors;
6. The required shutdown time for intermediate heat exchanger replacement is much longer for a liquid fuel MSR than for either a liquid sodium cooled FNR or a molten salt solid fuel FNR.
7. The high temperature heat produced by a liquid fuel MSR can potentially be replaced by electricity supplied by liquid sodium cooled FNRs. This potential for high temperature heat replacement places a cap on allowable capital cost of liquid fuel MSRs.
8. All fission reactors rely on 0.8% delayed neutrons for power control. Normally a reactor operates with 99.2% prompt neutrons but relies on delayed neutrons for power control. In a molten salt liquid fuel reactor the fuel is in motion and must have a residency time of much longer than 3 seconds in the core region of the reactor for thermal power control stability. This residency time requirement indirectly limits the reactor thermal power output. This is an issue that is seldom adequately appreciated by liquid fuel MSR proponents. If in order to operate at high power the liquid fuel residency time in the core zone is reduced the reactor shifts toward a dangerous prompt critical state. If the residency time is increased to compensate for this problem the molten salt differential temperature rises causing increased corrosion of the metal components.
9. In a solid fuel reactor the core zone fuel geometry is stable. In a liquid fuel MSR the core zone geometry is potentially unstable. If an unforeseen event such as a hydraulic surface oscillation, accident or earthquake occurs which quickly changes the local liquid fuel core zone geometry from being 99.2% to 100+% critical on prompt neutrons the reactor will explode. The potential for core zone geometry changes that can lead to a reactor explosion increases as the size of a liquid fuel MSR increases.

A liquid fuel MSR has flowing liquid fuel rather than fixed position solid fuel. The reliance on delayed neutrons for power stability places an additional contraint on liquid fuel MSR design. If the liquid fuel residence time in the moderated reactor core zone is not much greater than the delayed neutron arrival time (approximately 3 seconds) the fraction of delayed neutrons available for thermal power regulation substantially decreases causing a liquid fuel MSR to operate much closer to prompt neutron criticality than does a liquid sodium cooled FNR or a molten salt solid fuel FNR.

Consequently at high liquid fuel flow rates a liquid fuel MSR is more sensitive to small changes in core zone geometry than is a liquid sodium cooled FNR or a molten salt cooled solid fuel FNR. A reactivity increase can cause the fission power to rapidly grow before the reactivity control system has time to suppress the power rise. This issue is of major concern because in a liquid fuel MSR the fraction of delayed neutrons participating in criticality can be accidentally reduced to dangerously low levels in an effort to increase the reactor's power output for economic reasons.

The physical viability of small liquid fuel MSRs was demonstrated at the Oak Ridge National Laboratory (ORNL) during the 1960s, as portrayed in the Molten Salt Reactor Experiment Video.

Liquid fuel MSRs fuelled by thorium have a wide range of potential applications if they can be made economic. Here is a video outlining the promise of molten salt reactors and another long video outlining the promise of molten salt reactors. However, major issues not addressed in these videos are power stability, economics, ongoing maintenance, inability to breed new fuel fast enough to expand the reactor fleet and nuclear waste disposal.

In theory the molten salt could be NaF, NaCl, LiF or LiCl. However, use of Li requires separation of Li-6 from Li-7 because the Li-6 absorbs neutrons to form H-3 and He-4. Use of Cl requires separation of Cl-35 from Cl-37 because Cl-35 absorbs neutrons to form Cl-36 which is a nuclear waste disposal nightmare due to its long half life of 308,000 years. From a nuclear waste perspective the most suitable salt is NaF. However, NaF has a very high melting point of 993 degrees C. At the melting point of NaF there is rapid thermal expansion. A practical NaF based MSR would have to operate with a maximum molten salt temperature of about 1200 degrees C. This temperature is so high that it triggers a host of material problems.

A molten salt mixture currently receiving attention for possible use in MSRs is LiF/BeF2, which is claimed to have a melting point in the range 360 C to 459 C and to have the best neutronic properties of all the fluoride salt combinations that are appropriate for reactor use. The Li component should be monoisotopic Li-7.

If the Li-6 is not removed from LiF/BeF2 the molten salt may absorb too many neutrons to support thermal neutron fission breeding. However, in the distant future LiF/BeF2 containing natural Li might support tritium breeding for fusion power generation.

The long term corrosiveness of LiF/BeF2 is presently not known to this author.

A major problem with LiF/BeF2 is that the stable isotope Be-9 absorbs neutrons to become Be-10 which is a beta emitter with a half life of 2.5 million years. Once formed Be-10 needs to be safely isolated from the environment for at least 25 million years. The isotope Be-10 could easily become one of the most persistent environmental toxin problems. In the view of this author the parties that are seriously considering use of MSRs have no appreciation of the scope of the long term environmental problems that the Be-10 might cause.

A provision must be made for molten salt transfer into holding tanks to allow replacement of the reactor intermediate heat exchanger.

1. Molten salt reactors output heat at a sufficient temperature to drive important chemical reactions such as formation of ammonia and energy dense liquid hydrocarbon fuels;

2.The reduced cooling water requirement of a molten salt reactor reduces impact on marine ecology.

A FNR is characterized by rapid thermal power output changes near the reactor's critical point. For safe power control FNRs rely on thermal expansion of reactor components to increase the fraction of neutrons that are produced in the reactor core zone that diffuse into the reactor blanket zone and hence reduce the thermal power output as the primary coolant pool temperature increases. The reactor core zone fuel geometry is modified to adjust the reactor operating temperature or cause a cold reactor shutdown.

One of the issues in FNR design is ensuring that no matter what adverse circumstances occur on loss of control power the reactor fails into a safe cold shutdown state.

1. Due to the reduced requirement for cooling water a FNR or a MSR can be sited much further above the local water table and surrounding bodies of water than the siting of a direct water cooled CANDU reactor, thus enhancing system safety in rare but severe events such as earthquakes, tsunamis, hurricanes, meteorite strikes, etc.

2. The primary coolant of both a FNR and a liquid fuel MSR operates at a relatively low pressure which simplifies many reactor design, construction, operation and maintenance issues;

3. Unlike water cooled and moderated reactors FNRs and liquid fuel MSRs do not produce high pressure radioactive steam. Hence FNR and liquid fuel MSR designs do not need to be concerned about radioactive steam containment. If the steam pressure becomes too high the steam can be directly vented to the atmosphere because it is not radio active.

4. The molten salt does not pose the combustability and chemical water incompatibility problems of a liquid sodium.

5. A major issue with molten salt liquid fuel reactors is ongoing on-site chemical processing of the salt-fuel mixture. That chemistry is complex and is beyond the scope of this web page.

6. The start fuel situation for liquid fuel MSRs is difficult because for practical purpooses liquid fuel MSRs cannot breed new fuel fast enough to expand the reactor fleet.

7. The near term opportunity for liquid fuel MSRs lies in the chemical and hydrocarbon fuel reformation industries, but those industries need a high fossil carbon emissions tax to make conversion to liquid fuel MSRs financially viable. Liquid sodium cooled FNRs will likely be more economic for electricity generation than liquid fuel MSRs.

8. It may be that liquid fuel MSRs will never be cost competitive because equivalent high temperatures can be attained by generating electricity with a liquid sodium cooled FNR and then using the electricity to make high temperature heat.

This web page last updated March 28, 2019

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