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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 molten salt liquid fuel reactors. It is important to distinguish between these two different types of reactors.

On this web page the acronym MSR is used to refer to both types of molten salt reactor. If a comment applies to only one type of MSR that type is clearly specified.

A molten salt solid fuel FNR is similar to a liquid sodium cooled FNR except that the liquid sodium primary coolant is replaced by 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 more complex metallurgy and a requirement for a higher operating temperature to prevent the molten salt becoming solid. There are also potential problems with neutron activation of the molten salt 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.

The theoretical advantages of a molten salt liquid fuel reactor over a liquid sodium cooled FNR are:
1. A molten salt liquid fuel FNR 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 molten salt liquid fuel reactor avoids the hundreds of thousands of steel fuel tubes that are required in a molten salt solid fuel FNR or a liquid sodium cooled FNR.
4. In a molten salt liquid fuel reactor 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 molten salt liquid fuel reactors the issues of wave and votex formation in the liquid fuel are of extreme importance.

The disadvantages of a molten salt liquid fuel reactor as compared to a liquid sodium cooled FNR are:
1. The high melting point temperature of the salt triggers a host of metallurgy 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 fast neutron damage;
4. The intermediate heat exchanger tubes have only a short working life due to on-going fast neutron damage and 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 complex side arm fuel recycling chemistry of a molten salt liquid fuel reactor triggers additional problems and costs;
6. The required shutdown time for intermediate heat exchanger maintenance is much longer for a molten salt liquid fuel reactor than for either a liquid sodium cooled FNR or a molten salt solid fuel FNR.
7. The high temperature heat produced by a molten salt liquid fuel FNR can potentially be replaced by electricity supplied by liquid sodium cooled FNRs. This potential for high temperature heat replacement places a cap on allowable price of molten salt liquid fuel FNRs.
8. All fission reactors rely on 0.2% delayed neutrons for power control. Normally a reactor operates with 99.8% prompt neutrons but relies on delayed neutrons for power control. In a molten salt liquid fuel reactor the fuel must have a residency time of 3 seconds in the core region of the reactor for thermal power control stability. This residency time requirement limits the reactor thermal power output.
9. In a solid fuel reactor the core zone fuel geometry is stable. In a molten salt liquid fuel reactor 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 99.8% to 100+% critical on prompt neutrons the reactor will explode. The potential for core zone geometry changes that lead to a reactor explosion increases as the size of a molten salt liquid fuel reactor increases.

A molten salt liquid fuel reactor 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 reactor design. If the liquid fuel residence time in the reactor core zone is less than the delayed neutron arrival time (approximately 3 seconds) the fraction of delayed neutrons available for thermal power regulation decreases causing a molten salt liquid fuel reactor to operate 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 molten salt liquid fuel reactor is more sensitive to small changes in core zone geometry than is a liquid sodium cooled FNR or a molten salt solid fuel FNR. A reactivity increase can cause the fission power to rapidly grow before the corresponding increase in lattice temperature has time to suppress the power rise. This issue is of major concern because in a molten salt liquid fuel reactor 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.

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 must be monoisotopic Li-7.

If the Li-6 is not removed from LiF/BeF2 the molten salt may absorb too many neutrons to support 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 concept of the scope of the long term environmental problems that the Be-10 might cause.

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.

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

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.

The major theoretical advantages of a molten salt liquid fuel reactor over water moderated reactors are:
1. Molten salt liquid fuel 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.
6. Due to the reduced requirement for cooling water 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.
7. The primary coolant of a FNR and a MSR operates at a relatively low pressure which simplifies many reactor design, construction, operation and maintenance issues;
8. The components of a FNR that are exposed to a high neutron flux are routinely replaced and recycled along with the reactor fuel.
9. Unlike water cooled and moderated reactors FNRs and a MSRs do not produce high pressure radioactive steam. Hence FNR and MSR designs do not need to be concerned about radioactive steam containment. If the electricity generation steam pressures become too high the steam can be directly vented to the atmosphere because it is not radio active.
10. The molten salt of a MSR does not pose the combustability and chemical water incompatibility problems of a liquid sodium cooled FNR.

A major issue in practical FNR operation is complete exclusion of water and air. The reactor cover gas is argon. At high metal temperatures exposure to the atmosphere will cause rapid metal corrosion.

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

A significant security issue with FNRs is that they operate with a large plutonium inventory. At every stage of reactor fuel handling appropriate measures must be taken to ensure that the ratio of Pu-240 to Pu-239 is sufficiently large to prevent the plutonium being suitable for use in atom bombs.

A significant limitation on the rate of deployment of FNRs is the time required for one FNR to breed enough plutonium to start another identical FNR. This period is known as the plutonium doubling time.

The scientific issues related to FNRs are well understood. However, there is relatively little practical power FNR operating experience. North American electricity utilities presently have no pressing financial motivation to adopt FNRs. The near term opportunity for molten salt liquid fuel reactors lies in the chemical and hydrocarbon fuel reformation industries, but those industries need a high fossil carbon emissions tax to make conversion to molten salt liquid fuel FNRs financially viable. Liquid sodium cooled FNRs will likely be more economic for electricity generation than molten salt liquid fuel reactors.

It may be that molten salt liquid fuel reactors 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.

FNR design concepts have been tested in small research reactors. However, the power output of these research reactors is small as compared to full size modern electricity utility power reactors. The design concepts of a FNR are significantly different from a water moderated nuclear power reactor. These design concepts are reviewed below:

1) There is a large swimming pool size double wall primary coolant containment tank.

2) For safety there are no penetrations through the tank bottom or the vertical tank side walls;

3) The primary coolant density stratifies so that the hottest coolant is at the top;

4) A FNR contains fuel bundle control portions that actuators move in and out of the reactor core zone to set the reactor operating temperature;

5) Natural circulation causes the primary coolant to circulate;

6) In a FNR fast neutrons travel much further through the primary coolant than they travel through water before they lose their kinetic energy. Hence the walls and floor of the primary coolant pool should be protected from fast neutron damage. The protective barrier, in addition to absorbing neutron kinetic energy, provides thermal mass that limits the overall rate of change of primary coolant pool temperature;

7) The positions of the local reactivity control devices should be adjusted to keep the thermal power output uniform across the reactor;

8) As the primary coolant warms up and thermally expands the primary pool reactivity decreases, which reduces the reactor thermal power output. Similarly when the primary coolant cools and contracts the reactor thermal power increases. Hence, in normal reactor operation when the primary coolant temperature is high the reactor thermal power output is low and as the primary coolant temperature decreases the reactor thermal power output increases;

9) A molten salt liquid fuel reactor automatically seeks an equilibrium temperature at which the rate of heat generation equals the rate of heat removal.

10) As long as there is at least the specified minimum thermal load and there is sufficient fuel residency time in the reactor core zone and the core zone is dimensionally stable then a molten salt liquid fuel reactor is potentially passively stable;

11) For safety the reactor cooling system should be designed so that natural fluid circulation is sufficient to remove fission product decay heat and to provide the minimum thermal load required for passive reactor stability;

12) For practical primary pool maintenance there must be available sub-critical size tanks into which the primary coolant can easily be transferred;

13) For safety on a turbo/generator fault trip there should be sufficient steam bypass to maintain the specified minimum thermal load to prevent the primary coolant pool overheating. The primary coolant pool enclosure and the intermediate heat exchange elements must be rated to safely accommodate worst case power transients;

14) In normal operation overall reactor thermal power is nearly constant. Power reduction is achieved by reducing the intermediate coolant flow rate.

15) A lightly loaded FNR or liquid fuel reactor inherently runs hot.

16) A distinct advantage of FNRs is that they are almost unaffected by slow neutron poisons. Hence the thermal power of a FNR can ramp quickly to follow a rapidly changing grid electricity load.

17) Normal cold shutdown of a molten salt liquid fuel reactor is achieved by fully inserting the control rods;

18) Emergency hot shutdown of a FNR or a molten salt liquid fuel reactor is achieved by thermal expansion of the primary coolant;

19) After either a hot or cold reactor shutdown there must be sufficient primary and intermediate coolant natural circulation to safely remove fission product decay heat.

This web page last updated January 13, 2018

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