Electrochemically modulated molten salt reactor

An electrochemically modulated molten salt reactor (EMMSR) that contains a vessel and a power source. The vessel houses a fuel salt, at least a portion of a neutron moderator, and at least a portion of an insulator. The fuel salt includes enough dissolved fissile isotopes to cause continued self-sustaining fission reactions during the operation of the EMMSR. The neutron moderator is configured to slow down fast neutrons produced by the dissolved fissile isotopes. The insulator is configured to electrically isolate the neutron moderator from the vessel. The power source has a positive potential and a negative potential. The positive potential is received by the neutron moderator and the negative potential is received by the vessel.

FIELD OF THE INVENTION

The present invention relates to an electrochemically modulated molten salt reactor.

BACKGROUND OF THE INVENTION

Molten salt reactors (MSRs) have been considered a front-runner among currently developed next-generation nuclear technologies because they potentially offer a safer, more efficient, and sustainable form of nuclear power associated with on-line fuel processing. MSRs run at a much higher temperature, up to approximately 750° C., than traditional light-water-reactors and operate at near atmospheric pressure. MSRs are not cooled by water, which minimize the chances of a steam explosion. Several conceptual MSRs have been proposed and studied using different fuel and salt compositions, mainly chlorides and fluorides in the past few decades. However, the implementation of the proposed designs is hindered by serious technological challenges associated with the complexity of molten fuel salt chemistry, and a shortened lifetime of structural materials.

The composition of molten fuel salt can change dramatically during operation of the MSR. The molten fuel salt becomes unstable as it is greatly affected by fission products and operating conditions, causing instability, corrosion of MSR components, and unsafe operating conditions. Therefore, there is an urgent need to develop transformational MSR technologies to address these challenges and further improve reactor performance.

SUMMARY OF THE INVENTION

In one aspect, an electrochemically modulated molten salt reactor (EMMSR) that contains a vessel and a power source. The vessel houses a fuel salt, a neutron moderator, and an insulator. The fuel salt includes enough dissolved fissile isotopes to cause continued self-sustaining fission reactions during the operation of the EMMSR. The neutron moderator is configured to slow down fast neutrons produced by the dissolved fissile isotopes, the insulator is configured to electrically isolate the neutron moderator from the vessel. The power source has a positive potential and a negative potential. The positive potential is received the neutron moderator, and the negative potential is received by the vessel.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description provides illustrations for embodiments of the present invention. Each example is provided by way of explanation of the present invention, not in limitation of the present invention. Those skilled in the art rill recognize that other embodiments for carrying out or practicing the present invention are also possible. Therefore, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Referring toFIG. 1, a schematic illustration of an embodiment of this invention is shown. In the electrochemically moderated molten salt reactor (EMMSR or reactor)100, the neutron moderator102is electrically isolated from the vessel104by an insulator103. During the electrochemical modulation, the reactor is utilized as an electrochemical cell, and appropriate electrical signals are applied between the neutron moderator102and the vessel104from the power source101.

The power source101is any source of power that is capable of generating an electrical current sufficient to operate the EMMSR100. For example, the power source101can be a digital or analog direct-current power source that can provide various electrical signals to the EMMSR100. The signals can include constant or pulse voltages and currents. The power source101can include other elements to help control one or more electrical variables, including a potentiostat, galvanostat, or other instruments for similar purposes. The power source101enables the electrochemical modulation of the neutron moderator102, the vessel104, or both the neutron moderator102and vessel104. As discussed in reference toFIG. 4, a positive potential can be applied to the neutron moderator102and a negative potential can be applied to the vessel104. In this configuration the electrochemical modulation drives salt anions303, such as F−, and UF73−and UF74−, to move to the surfaces of the neutron moderator102, and salt cations304, such as Li+, to move to the inner surfaces of the vessel104.

The fuel salt109is a melt containing dissolved fissile isotopes that produce fast neutrons. The fuel salt109is compatible with high-temperature alloys, graphite, and ceramics. The fuel salt109contains dissolved fissile isotopes in a concentration sufficient to cause continued self-sustaining fission reactions. The fuel salt109can be any chloride, fluoride, or combination of chlorides and fluorides that enable the operation of the EMMSR100in the temperature range of approximately 500° C. and 800° C. If the fuel salt109is a fluoride, it may be LiF—BeF2, NaF—BeF2, LiF—NaF—KF, NaF—Zr4, or LiF—NaF—ZrF4. If the fuel salt109is a chloride, it may be LiCl—KCl, NaCl—MgCl2, KCl—MgCl2, LiCl—RbCl, and LiCl—KCl—MgCl2. Beyond the fluoride and chloride salts, familiar oxygen-containing salts (nitrates, sulfates and carbonates) are less suitable because they do not possess the necessary thermochemical stability at high temperatures relevant to the EMMSR100operation and they are also incompatible with the use of carbon materials. The fuel salt109is made from fluorides or chlorides because they are good heat transfer media with large specific heats, large thermal-expansion co-efficients, have low viscosity. The fuel salt109can be introduced into the EMMSR100through a pump from a fuel tank. If the EMMSR undergoes continuous refueling, the refueling rate is determined by the operation parameters, such as power density, inlet106and outlet107temperatures, of the reactor, the capabilities of materials, and safety control. The flow rate can be adjusted, such as from approximately 0.1 m/s to approximately 10 m/s, to control the power output. An emergency shutdown is enabled by conventional control valves that can be secured electromagnetically in the event of power failure or other situation requiring immediate shut down of the EMMSR100.

The dissolved fissile isotopes can include an actinide element that will split when it absorbs a neutron such as Uranium, Plutonium, or Thorium. The fissile isotopes are present within the fuel salt109up to approximately 15 wt %, and preferably between approximately 1 and 10 wt %. The fuel salt109can also act, indirectly, as a moderator, negative thermal reactivity feedback mechanism, heat transfer medium, and natural drive mechanism for decay heat removal.

The neutron moderator102is any component capable of slowing down fast neutrons produced by the dissolved fissile isotopes from the fuel salt109. For example, the neutron moderator102can be conductive carbons, metals, alloys, or a combination of conductive carbons, metals, and alloys. Carbons could include nuclear-grade graphite, amorphous carbon, and their composites. Metals or alloys could include zirconium, beryllium, or their oxides with low cross-section of neutron adsorption. The neutron moderator102can be composites of conductive carbons, conductive metals, or oxides of Zr and Be. At least a portion of the neutron moderator102is within the vessel104, but enough of the neutron moderator102must be within the vessel104to moderate fast neutrons.

The insulator103is any component capable of electrically isolating the neutron moderator102from the vessel104. The insulator103is made from high temperature inorganic materials such as mica, ceramics, and concretes. The insulator103must have a low neutron-adsorption cross-section and good chemical and mechanical stability at the operating temperature of the EMMSR100. Moreover, insulator103can also tolerate the irradiation of neutron, alpha, and beta rays, and the attack by fission products. Preferable ceramic materials are alumina, stabilized zirconia, magnesia, silicon carbon, and zirconium carbide. The shape of the insulator is determined by the design of the neutron moderator102and the connection between the neutron moderator102, the insulator103, and the vessel104. A portion of the insulator103may be within the vessel104. A rod holder110can be used to stabilize the neutron moderator102and the insulator103. The rod holder110can be made from high-strength ceramic, graphite, metal alloys, or their combinations. The insulator103can be made from ceramic-based shoulder sleeves, sleeve bushings, flange bushings, ceramic O-rings, short tubing, piping, or fittings. For example, if the insulator103surrounds the neutron moderator102from the vessel104, the center opening of the insulator103can be in the shape of the neutron moderator102. For example, if the neutron moderator102is circular, cubic, or hexagonal, the insulator103center opening would be the same. However, in all geometries, the insulator103electrically isolates the neutron moderator102from the vessel104.

The vessel104is any container capable of housing the components of the EMMSR100. The vessel104can be substantially cylindrical or a rectangular prism. The vessel104is electrically isolated from the neutron moderator102. The vessel104is made from metal alloys that are stable in the fuel salt109. Metal alloys can be easily attached to by molten fluorides or chlorides because their surface oxide films, which normally protect the metals from corrosion by water or air, dissolve in the fuel salt109. The metal alloy itself must be chemically stable in terms of dissolution of its constituent metal into the fuel salt109through an oxidation process by oxidative compounds such as UF73−and ICl73−. The main alloy component vulnerable to oxidation is chromium. Based on the control of the concentration of the oxidative compounds, few nickel alloys exhibit good stability in the salt. In this invention, the electrochemical modulation can considerably expand the choice of the alloys. Besides a range of nickel alloys, stainless steels and other alloys can also be employed as the vessel104material.

The gas phase105is a space within the vessel104designed to hold the gaseous fission products, such as Xe and Kr, and harmful volatiles such as HF that are produced from the fuel salt109. The gas phase105mitigates the corrosion of the vessel104and other EMMSR100components by fission products and the salt released from the fuel salt109. The gas phase105improves the safety and control of the reactor by providing a space for the corrosive elements to go, instead of allowing the corrosive elements to fester in the fuel salt109or otherwise attack the vessel104or other EMMSR100components. The gas phase105can be purged to empty the vessel104of the gaseous fission products using an inert gas such as helium or argon. The gas phase105can facilitate the direct detection of gas phase105composition, temperature, and pressure through the introduction of in-reactor sensors within the gas phase105.

The vessel104receives the fuel salt109from at least one inlet106. The inlet106is designed to allow the addition of other components for adjusting the fuel salt109chemistry as necessary during operation and the refueling of the fuel salt109. During operation of the EMMSR100, the fuel salt109is heated through the electrochemical reactions and power generation. After the fuel salt109is heated, the fuel salt109is released through at least one outlet107. For example, the outlet107can be connected to the primary heat-exchanger. Or, the fuel salt109exiting through the outlet107can be fed back to the EMMSR100through the inlet106with or without chemical treatments. The flow rate of the fuel salt109entering inlet106and exiting outlet107can be controlled by circulation pumps, not pictured, configured to adjust the flow rate of the fuel salt109.

The EMMSR100has a negative fuel salt109void coefficient Therefore, the fuel salt109is pushed through the at least one outlet107, and the EMMSR100has a negative thermal reactivity feedback. This configuration avoids a set of major design constraints in prior art solid-fuel reactors. A passive core drain system activated by a melt plug enables draining the radioactive inventory into geometrically subcritical drain tanks.

The control rods108are used to control the reactivity of the EMMSR100core individually or in combination with the refueling system. The control rods108are made from materials with strong neutron adsorption capabilities, such as silver-indium-cadmium alloys, high-boron steel, boron carbide, and hafnium alloys and borides. To avoid the contamination of the fuel salt109by the control rods108, the control rods108must be stable in the fuel salt109or stable with a cladding protecting the control rod108from the fuel salt109. If a cladding is used, the cladding surrounds the control rod108. The cladding can be a ceramic or nickel-based alloy based material.

Regarding electrical signals controlled by the power source101shown inFIG. 1and employed for electrochemical modulation, they can be digital, analog, or their combinations. Typical DC signals are shown inFIGS. 2A-2F.FIG. 2Aillustrates constant voltages or currents, from a battery or regulated power sourceFIG. 2Billustrates a smoothed power source.FIG. 2Cillustrates a power source without smoothingFIG. 2Dillustrates a positive pulsed voltage or current.FIG. 2Eillustrates mixed positive-negative pulsed voltage or current.FIG. 2Fillustrates a linearly varying voltage or current.

Referring toFIG. 3, a simplified illustration of an embodiment of the EMMSR100ofFIG. 1is shown to highlight the reactions that occur during operation. The neutron moderator102is connected to the positive end of the power source101, not pictured, acting as the anode301. The vessel104is connected to the negative end of the power source101, not shown, acting as the cathode302. During operation, a positive potential is applied to the neutron moderator102where anode301reactions occur during electrochemical modulation and, likewise, a negative potential is applied to the vessel104where cathode302reactions occur during electrochemical modulation. The voltage, which is the difference of the positive and negative potentials, time period, or frequency can be adjusted for optimized electrochemical modulation. The potential difference across the neutron moderator102and the vessel104, the voltage, can be in a range between approximately 1-10 millivolts to 1-25 volts. Modulation is advantageous and effectively addresses challenges which have plagued traditional molten salt reactors such as neutron poisoning, excessive flux and power peaking, and vessel104and other component degradation. Advantages over the prior art include, but are not limited to, additional neutron moderation, decreased oxidation of the vessel104, decreased degradation of the vessel104from harmful fission products.

The enrichment of anions303adjacent to the neutron moderator102and the enrichment of cations304adjacent to the vessel104, driven by applying a positive potential to the neutron moderator102and a negative potential to the vessel104, promote additional neutron moderation through the combination of the neutron moderator102and the fuel salt109because the fuel salt109provides additional neutron moderation. For example, referring toFIG. 4, in an embodiment of the EMMSR100using a fuel salt109made of LiF—BeF2—UF4, the salt anions303, such as F−, BeF42−and UF73−, are rich around the neutron moderator102and the cations304, such as Li+, are rich close to the vessel104. Because the elements, Be, F and C, have lower cross-sections for neutron adsorption than the element6Li, the potential-induced charge separation and ion-enrichment can promote the moderation of neutrons in the EMMSR100through the combination of Be, F and C, and mitigate the neutron adsorption by the impurity6Li which is normally present in highly-purified7Li salt. Furthermore, the above neutron moderation is a gradient along the distance away from the neutron moderator102. In other words, moderation becomes weaker with increasing distance away from the neutron moderator102.

The movement of UF73−from the zone adjacent to the vessel104inner surfaces to the neutron moderator102decreases its oxidative attack on the vessel104through the reactions represented in Equations 1, 2 and 3, also mitigating the vessel104corrosion induced by the U(IV) species.
Cr+2UF73−+2F−→CrF2+2UF74−(1)
Ni+2UF73−+2F−→NiF2+2UF74−(2)
Fe+2UF73−+2F−→FeF2+2UF74−(3)

When the voltage across the anode301and the cathode302is high enough to sufficiently trigger the reaction of Equation 11, the reductive conversion of U(IV) to U(III) can mitigate the oxidation of the vessel104by U(IV).
UF73−+e−(electron)→UF74−(4)

Therefore, the redox corrosion of the vessel104in the EMMSR can be significantly reduced through the potential-modulated charge separation and electrochemical reactions. Therefore, the vessel104does not need metallic Be, U and Zr, or the salt of U(III) which have been used in the traditional MSRs for mitigating the corrosion of the vessel104by the strongly oxidative U(IV) species. Because electrons are much “cleaner” and “safer” than the above reductive metals or salts, the EMMSR100is also a cleaner and safer technology than the traditional MSRs.

In the EMMSR, a potential modulation can also mitigate vessel104degradation by two harmful fission products, Te and3T, through their oxidation at the anode301as shown in Equations 5 and 6.
Te−xe−+xF−→TeFx(5)
3T−e−+F−→3TF  (6)

Besides the reactions shown in Equations 5 and 6, Te and3T are likely to be oxidized by the U(IV) species. Their corresponding oxidized products TeFxand3TF can be removed from the salt by purging He or Ar through the salt. Therefore, the embrittlement of nickel-based vessel104induced by Te and3T (Equations 7 and 8) can be significantly mitigated in the EMMSR100.
3F−+UF4↔UF73−(7)
Cr+2HF→CrF2+H2(8)

Because the EMMSR100is capable of reducing the formation of3T through the potential-induced charge separation described previously, and remove3T through the reaction shown in Equation 6, it could decrease the demand of ultra-high purity salt of7Li which is also a challenge for prior art molten salt reactors. As a result, the cost of the fuel salt109can be reduced and its purification can be simplified.

In the EMMSR100, the corrosion of the vessel104can be also mitigated through electrochemical reduction of corrosion products such as CrF2and FeF2based upon the reactions shown in Equations 10 to 12.
CrF2+2e−→Cr+2F−(10)
FeF2+2e−→Fe+2F−(11)
NiF2+2e−→Ni+2F−(12)

One further advantage of the potential modulation is to control the release of elemental fission products such as Cs and I through their anode and cathode reactions respectively as shown in Equations 13 and 14.
Cs−e−+F−→CsF  (13)
3I+e−→I3−(14)

The use of mixed pulsed voltage or current, as shown inFIG. 2E, or linearly varying signal,FIG. 2F, can drive the anode reactions to occur on the vessel104and the cathode reaction to occur on the neutron moderator102, when a negative potential is applied to the neutron moderator102. This can be done to remove reductive U deposited on the vessel104bottom as shown in Equation 15. The re-dissolution of U deposit from the vessel104solution into the fuel salt109can prevent local over-heating caused by deposited U and increase the lifetime of the vessel.
U−3e−−7F−→UF74−(15)

Turning toFIG. 5, an embodiment of the EMMSR100is shown. In addition to the components discussed in reference toFIG. 1, at least one supporting electrode501is introduced into the EMMSR100. The supporting electrode501is configured to catalyze particular anode reactions that improve the power generation efficiency of the EMMSR100. The supporting electrode501would have chemical stability in the fuel salt109and low neutron-adsorption cross-sections. For example, the supporting electrode501could be made from metals and alloys of nickel, platinum, ruthenium, molybdenum, and palladium. A supporting electrode501can also be coated onto a carbon substrate such as a graphite rod. For example, a supporting electrode501made from palladium coated onto a graphite rod would catalyze the reactions shown in Equations 12, 13, 17, and 18. Because the supporting electrode501is a catalyst for these reactions, and by comparison has a higher activity for these reactions than the neutron moderator102, the overall power efficiency of the reactor is improved. More than one supporting electrode501can be used in the EMMSR100, each supporting electrode501meant to catalyze different selected reactions.

InFIG. 6, an embodiment of the EMMSR100is shown. In addition to the components discussed in reference toFIG. 1, at least one reference electrode601is introduced into the EMMSR100. The reference electrode601allows for more accurate modulation of electrical signals, the tuning of electrochemical reactions at the neutron moderator102and vessel104, the monitoring of the fuel salt109chemistry, and better control of the EMMSR100. The electric potential of the reference electrode601facilitates the measurement of the voltage between the vessel104and the reference electrode601using a voltmeter603, or a potentiostat or galvanostat602. The potentiostat or galvanostat602tunes the electrochemical reactions occurring at the neutron moderator102or on the vessel104. The at least one reference electrode601can be ordinary reference electrodes, dynamic reference electrodes, or combinations of ordinary and dynamic reference electrodes can be used. Examples of ordinary reference electrodes include Ag/AgCl, Ni/NiO, and Ni/Ni/ZrO2. A dynamic reference electrode refers to electrodes that can build up stable interfacial potentials after the dynamic electrode is in contact with the fuel salt109. Examples of dynamic reference electrodes include Be. When the reference electrode601is in the fuel salt109, a porous ceramic filter is used to separate the internal electrolyte used inside the reference electrode601from the fuel salt109. In a fuel salt109made from molten fluorides, the filter could be as simple as Ni, Co and W wires for a reference electrode601made from carbon.

Because the response of the potential of a reference electrode601to the chemistry of the fuel salt109, or other operating parameters such as temperature, is unique, the reference electrode601provides a mechanism to monitor the fuel salt109chemistry and even the safety of the EMMSR100based on changes of the reference electrode601potential. An array of reference electrodes601can be used to improve the reliability of the monitoring of the EMMSR100. The potential differences between the reference electrodes601provide statistical information about the fuel slat chemistry and the operation conditions.

InFIG. 7another embodiment of the present invention is shown. In addition to the components discussed in reference toFIG. 1, a multi-channel potentiostat or galvonostat701is introduced into the EMMSR100. A multi-channel potentiostat or galvonostat701allows for independent control of each neutron moderator102and supporting electrode501. The reference electrode601can be an array of electrodes. During modulation of the EMMSR100, each channel of the potentiostat or galvonostat701can be connected to the neutron moderator102, reference electrode601, supporting electrode501, or electrochemical gas sensor702. The vessel104can also be used as a common electrode connected to more than one channel. To monitor the fission products and other volatiles in the gas phase105that are released from the fuel salt109, at least one electrochemical gas sensor702can be introduced into the gas phase105and controlled by the multi-channel potentiostat or galvonostat701. The electrochemical gas sensor702can be any sensor capable of withstanding the environment conditions within the gas phase105and provide the necessary data to be acquired. For example, the electrochemical gas sensor702can be an I2sensor, a BF sensor, a Sr sensor, or an O2sensor. The electrochemical gas sensor702can considerably improve the monitoring and control of the EMMSR100, resulting in improved performance and safety.