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XYLENE POWER LTD.

MOLTEN SALT REACTORS

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

INTRODUCTION:
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.

Within the category of liquid fuel molten salt reactors is the potentially important subcategory of liquid fuel thorium reactors (LFTRs). These LFTRs offer the potenital benefit of an almost unlimited fuel supply. However, LFTRs have complications related to neutron conservation, corrosion, fuel chemistry and long lived waste generation. These complications can be partially circumvented by use of companion uranium fuelled FNRs.

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 intermediate heat exchanger and containment metallurgy and a requirement for a higher minimum operating temperature to prevent the molten salt becoming solid. There are also problems with neutron activation and neutron absorption by the molten salt. This neutron absorption process leads to corrosion of reactor metal components by fluorine for which there is no obvious complete solution other than frequent equipment replacement.

A liquid fuel molten salt reactor (MSR) 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 liquid fuel molten salt reactor are set out in the videos: Molten Salt Liquid Fuel Reactor Videos.
 

THEORETICAL ADVANTAGES OF A LIQUID FUEL MOLTEN SALT REACTOR:
The theoretical advantages of a liquid fuel molten salt reactor (MSR) over a liquid sodium cooled FNR are:
1. A liquid fuel MSR with good neutron conservation can be fuelled by thorium provided that the start fuel contains sufficient enriched uranium.
 
2. A liquid fuel MSR delivers heat to the thermal load at a higher temperature. This high temperature heat is of particular importance in production of ammonia and in reforming of hydrocarbons to make energy dense hydrocarbon fuels;
 
3. The high operating temperature of a liquid fuel MSR reduces the cooling water volume requirement per kWh for electricity generation which in turn reduces impact on marine ecology;
 
4. The molten salt is much less of a fire hazard than is liquid sodium;
 
5. A liquid fuel MSR avoids the hundreds of thousands of metal fuel tubes that are required by a solid fuel power MSR or by a liquid sodium cooled power FNR.
 
6. In a liquid fuel MSR the fuel is relatively uniform and hence the reactor is theoretically is less complex to control than a solid fuel MSR 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.
 
7. A liquid fuel MSR operates on thermal neutrons and relies on the presence of a graphite or other moderator in the core zone for criticality. The reaction can be controlled or stopped by transferring the liquid fuel to or from a companion dump tank that contains no moderator.
 

MSR DISADVANTAGES:
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 aggravates the already difficult metallurgy and corrosion problems inherent in all MSRs;
 
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. Neutron absorption by beryllium and lithium causes formation of tritium and He-4. The companion fluorine atoms in the salt then aggressively corrode metal heat exchange tubing and reactor walls. This corrosion limits both liquid fuel MSR and solid fuel MSR metal component working life;
 
4. The molten salt containment pool walls and intermediate heat exchange tubes require a thick porcelain shield and/or a thick molten salt layer in which neutrons are not released to prevent cumulative neutron damage to the reactor walls and intermediate heat exchange tubes. This shielding requirement limits the molten salt flow rate and increases reactor size for a specific thermal power output;
 
5. The intermediate heat exchange tubes have only a short working life due to formation of low thermal conductivity fission product deposits on the heat exchange tube surfaces. These tubes are a major source of maintenance and decommissioning waste;
 
6. The required side arm fuel recycling chemistry of a liquid fuel MSR is complex and causes additional problems and costs for distributed reactors;
 
7. The start fuel situation for liquid fuel thorium reactors (LFTRs) is difficult because for practical purposes LFTRs cannot breed new fuel fast enough to expand the reactor fleet.
 
8. The required shutdown time for intermediate heat exchanger and moderator replacement is much longer for a liquid fuel MSR than for a liquid sodium cooled FNR. In a practical nuclear power station typically two fully redundant MSR assembies are required to manage this issue.
 
9. 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.
 
10. 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 in proximity to the moderator (usually graphite) for thermal power control stability. This residency time requirement indirectly limits the molten salt flow rate and hence the reactor thermal power output.
 
11. The molten salt flow rate limit 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 increasing the corrosion rate of the metal components.
 
12. In a solid fuel reactor the core zone fuel geometry is stable. In a liquid fuel MSR the core zone fuel 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.
 
13) A provision must be made for molten salt transfer into a dump tank and then back again to allow replacement of the reactor intermediate heat exchanger and the graphite moderator.
 

LIQUID FUEL MSR POWER CONTROL:
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 fuel 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 reduced to dangerously low levels in an effort to increase the reactor's power output for economic reasons.
 

VIDEOS:
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, corrosion, ongoing maintenance, inability to breed new fuel fast enough to expand the reactor fleet and nuclear waste disposal.
 

MOLTEN SALT CHOICES:
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.

Other salts such as CsF have been considered for use in liquid fuel MSRs. However, for MSRs fuelled by thorium, which breeds to U-233, the selection of salts is further constrained by neutron absorption by the salt. The issue is that fissioning of U-233 yields only slightly more than two neutrons per fission. There is very little margin for neutron absorption in a fuel conserving breeding cycle.
 

LiF/BeF2
A molten salt mixture currently receiving attention for possible use in MSRs is LiF/BeF2, which in a suitable ratio 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 will likely absorb too many neutrons to support thermal neutron fission breeding.

The LiF/BeF2 is superficially chemically stable. However, when Li and Be absorb neutrons they change into He-4 and H-3. The companion F atoms in the salt mixture then become aggressively corrosive to the reactor intermediate heat exchange tubes and metal walls.

A major problem with LiF/BeF2 is that the stable isotope Be-9 on neutron absorption can 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.
 

MOLTEN SALT SIDE ARM CHEMISTRY:
An under appreciated issue relating to liquid fuel MSRs is the complexity of the side arm radio chemistry. This chemistry must on a continuous basis:
a) Selectively remove and safely store dissolved fission products;
b) Remove and safely capture xenon, krypton and other inert gases;
c) Add fuel as required to maintain core criticality;
d) Add caesium or a like substance to preferentially absorb fluorine released during neutron absorption by the molten salt;
e) Selectively remove the CsF or like substance resulting from fluorine capture;
f) Provide an electronic means of monitoring the state of the liquid fuel MSR.
Automatic implementation of this complex radio chemistry may be much more difficult than implementation of the entire balance of the reactor. It may turn out that the entire concept of distributed liquid fuel MSRs fails over the implementation complexity of the side arm radio chemistry.
 

MISCELLANEOUS MSR ISSUES:
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 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.

5. 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.
 

OTHER SOLID FUEL FNR-MSR ISSUES:
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.
 

This web page last updated May 4, 2019

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