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XYLENE POWER LTD.

MOLTEN SALT REACTORS

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

INTRODUCTION:
A Molten Salt Reactor (MSR) is a fast neutron 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 advantages of a Molten Salt Reactor (MSR) over a liquid sodium cooled fast neutron reactor (FNR) are:
1. A 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 coolant is less of fire hazard than liquid sodium;
 
3. A MSR avoids the hundreds of thousands of steel fuel tubes that comprise a FNR.
 
4. A MSR uses liquid fuel which is relatively uniform and hence is less complex to control than a FNR due to absence of potential "hot spots".
 

The disadvantages of a MSR as compared to a FNR are:
1. The high temperature melting point of the salt triggers a host of material problems;
 
2. Salts that have lower melting points have major nuclear waste disposal and/or isotope separation problems;
 
3. Mitigating the nuclear waste disposal problems involves very expensive isotope separation steps;
 
4. The molten salt containment vessel has a limited life due to on-going fast neutron damage;
 
5. The intermediate heat exchanger tubes have a limited life due to on-going fast neutron damage;
 
6. The intermediate heat exchanger tubes have a limited life due to formation of low thermal conductivity deposits on the salt side of the tubes;
 
7. The more complex fuel recycling chemistry triggers additional problems and costs;
 
8. The required shutdown time for fuel exchange and reactor maintenance is much longer with a MSR than with at FNR.
 
9. If the cost of a MSR per kWt is comparable to the cost of a FNR per kWe the high temperature heat produced by a MSR can easily be replaced by electricity from a FNR and the whole concept of a MSR becomes financially unviable.
 

MOLTEN SALT REACTOR POWER CONTROL:
A Molten Salt Reactor (MSR) has flowing liquid fuel rather than fixed position solid fuel as in a FNR. The reliance on delayed neutrons for power stability places an additional contraint on MSR design. If the liquid fuel residence time in the critical region of the core in a MSR is less than the delayed neutron arrival time (approximately 3 seconds) the fraction of delayed neutrons available for power stabilization decreases causing a MSR to operate closer to prompt neutron criticality than does a FNR.

Consequently at high liquid fuel flow rates a MSR is more sensitive to small step changes in core reactivity than a FNR. A reactivity increase can cause the fission power to rapidly grow before the lattice temperature has time to suppress the power rise. This issue is of particular concern because in a MSR the fraction of delayed neutrons participating in criticality can be reduced to dangerously low levels in an effort to increase the reeactor's power density for economic reasons.
 

MSR SALT CONSIDERATIONS:
MSRs are fast neutron nuclear reactors in which the fuel is dissolved in a pool of molten salt. In theory the 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 half life of 308,000 years. From a nuclear waste perspective the best salt is NaF. However, NaF has a melting point of 993 degrees C. At the melting point of NaF there is rapid thermal expansion. A practical NaF MSR operates with the salt pool at close to 1100 degrees C. This temperature is so high that it triggers a lot of material problems.
 

LiF/BeF2
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.

The LiF/BeF2 salt mixture will age as the Li-7 gradually converts to Li-6. Practical processing of this salt to selectively extract the Li-6 may be a challenge. If the Li-6 is not removed it will likely absorb too many neutrons to support fission breeding. It may support fusion breeding but that assumes the existence of a practical fusion technology.

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. This radioactive salt could easily become one of the worst environmental toxin problems known to man. In the view of this author the parties that are seriously considering use of this salt mixture in MSRs have no concept of the long term environmental problems the Be-10 may cause.
 

OTHER MSR ISSUES:
A practical way to remove heat from the molten salt pool is to float a pool of lower density liquid sodium on top of the molten salt. At 993 degrees C = 1266 degrees K sodium boils and has a saturated vapor pressure of about 2.6 atmospheres.

For safety the reactor enclosure must be designed to withstand an internal pressure of at least 6 atmospheres.

This physical configuration minimizes deterioration of the intermediate heat excahnger by keeping this heat exchanger out of the neutron flux and by almost eliminating fission product contact with the heat exchanger. The heat exchanger secondary fluid can be non-radioactive sodium at a high pressure. Then a minor heat exchanger leak is of little consequence.

Neutron irradiation of the reactor pressure vessel walls can be minimized by use of a B4C neutron reflector and a neutron absorber between the salt and the pressure vessel wall. High level neutron activation of The B4C will form C-14 which with a half life of 5730 years will become another major environmental problem.

An MSR is characterized by rapid thermal power output changes near the reactor's critical point. For safe power control MSRs rely on thermal expansion of the molten salt to reduce the salt pool reactivity and hence the power output as the pool temperature increases. There are vertical control rods to adjust the critical point. These control rods also enable a reactor cold shutdown.

A provision must be made for molten salt transfer into smaller sub-critical tanks to allow maintenance and repair of the reactor pressure vessel.

One of the issues in MSR design is ensuring that no matter what adverse circumstances occur the control rods will always fall into a safe cold shutdown position.

The major advantages of an MSR over water moderated reactors are:
1. MSRs output heat at a sufficient temperature to drive important chemical reactions such as formation of ammonia and energy dense liquid hydrocarbon fuels;
2. MSRs can potentially yield about 100 fold more energy per kg of mined uranium than a CANDU reactor;
3. MSRs can potentially reduce nuclear waste storage time about 1000 fold as compared to a CANDU reactor;
4. MSRs thermal output can easily track rapid changes in electricity grid load:
5. The molten salt in a NaF MSR typically runs at a temperature of over 993 C. The intermediate loop will operate at 700 C to 800 C. These temperatures compare to the 260 degree C to 300 degree C primary coolant temperature in a CANDU reactor and 320 C to 445 C in a FNR. The higher temperatures cause a range of metalurgical problems but allow efficient use of evaporative cooling and hence require less cooling water per kWhe generated than does a direct lake water cooled CANDU reactor;
6.The reduced cooling water requirement of an MSR reduces impact on marine ecology.
7. Due to the reduced requirement for cooling water an 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, hurricanes, meteorite strikes, etc. However, in this respect a MSR has no advantages over a FNR
8. The molten salt primary coolant in a MSR operates at a relatively low pressure which simplifies many reactor design, construction, operation and maintenance issues;
9. The components of a MSR that are exposed to a high neutron flux are routinely replaced and recycled along with the reactor fuel.
10.Unlike water cooled and moderated reactors a MSR does not produce high pressure radioactive steam. Hence a MSR design does not need to be concerned about radioactive steam containment. If the electricity generation steam pressures gets too high the steam can be directly vented to the atmosphere because it is not radio active.
11. The molten salt of a MSR does not pose the combustability and water incompatibility problems of a liquid sodium cooled FNR.

The biggest issue in safe MSR operation is complete exclusion of water and air. When the reactor is cool the cover gas is argon. At high metal temperatures atmospheric corrosion occurs very quickly.

A second major issue with MSRs is safe 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 MSRs is that they operate with a large plutonium inventory. At every stage of reactor fuel handling appropriate security measures must be taken to ensure that the ratio of Pu-239 to Pu-240 is sufficiently small to prevent the plutonium being suitable for building an atom bomb.

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

The scientific issues related to MSRs are well understood. However, there is relatively little practical MSR operating experience. North American electricity utilities presently have no financial motivation to adopt MSRs. The near term opportunity for MSRs lies in the chemical and hydrocarbon fuel reformation industries, but those industries need a high fossil carbon emissions tax to make MSRs financially viable. Even so, MSRs will likely not be financially viable for electricity generation due to material cost and nuclear waste issues.

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

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

1) The main component of a MSR power reactor is a large double wall high temperature rated tank (like a deep swimming pool) that contains the molten salt near the bottom and lower density liquid sodium near the top.

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

3) The molten salt is denser than the sodium, so the sodium floats on top of the molten salt. The molten salt density stratifies so that the hottest salt is at the liquid sodium-molten salt interface;

4) A molten salt power reactor contains a large number of vertical control rods;

5) The differential pressure created by Na vapor condensation pushes liquid sodium or sodium vapor through the primary to secondary heat exchanger;

6) In a MSR fast neutrons travel much further through the molten salt than they travel through water before they lose their kinetic energy. Hence outside the reactor core there should be a thick subcritical blanket of NaF or Na containing no fuel. This extra salt or sodium, in addition to absorbing neutron kinetic energy, provides thermal mass that limits the overall rate of change of pool temperature. This blanket of molten salt or sodium protects the reactor enclosure from cumulative neutron damage;

7) The reactor core control rods are almost completely withdrawn in normal reactor operation but are completely inserted to achieve a cold reactor shutdown;

8) The positions of the control rods should be adjusted to keep the thermal power output uniform across the reactor;

9) As the molten salt warms up and thermally expands the pool reactivity decreases, which reduces the reactor thermal power output. Similarly when the molten salt cools and contracts the thermal power increases. Hence, in normal reactor operation when the salt temperature is higher than its melting point the reactor thermal power output is low and as the salt temperature goes below its melting point the reactor thermal power output increases;

10) As the salt temperature decreases the sodium vapor pressure and hence the rate of sodium flow through the intermediate heat exchanger also decreases. Thus the MSR spontaneously seeks an operating temperature at which the rate of heat generation equals the rate of heat removal;

11) As long as there is at least the specified minimum thermal load and there is sufficient fuel residency time in the reactor coe the MSR is passively stable;

12) 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 at all control rod positions;

13) For safety there must be available sub-critical tanks into which the salt can easily be easily transferred;

14) For safety and simplicity the intermediate fluid flow rate should be nearly constant. Then the reactor power output is proportional to the intermediate loop differential temperature;

15) For safety on a turbo/generator fault trip there should be sufficient steam bypass to maintain the specified minimum thermal load to prevent the salt pool overheating. The molten salt pool enclosure and the heat exchange elements must be temperature rated to safely accommodate a worst case load transient;

16) In normal operation overall reactor thermal power is nearly constant. Power reduction is achieved via control rod insertion.

17) A MSR with fully withdrawn control rods inherently runs too hot and hence at too high a sodium vapor pressure under low load conditions.

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

19) Normal cold shutdown of a MSR is achieved by fully inserting the control rods;

20) Emergency hot shutdown of a MSR is achieved by thermal expansion of the molten salt pool;

21) After either a hot or cold reactor shutdown there must be sufficient coolant circulation to safely remove fission product decay heat.
 

This web page last updated May 13, 2017

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