CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/395,506, filed September 16, 2016, entitled "Reactor Control," the entirety of which is incorporated by reference.

BACKGROUND

[0002] The global demand for energy has largely been fed by fossil fuels. This typically involves taking reduced carbon out of the Earth and burning it. However, those hydrocarbons are in limited supply and burning the hydrocarbons can produce carbon dioxide. According to the U.S. Environmental Protection Agency, more than 9 trillion metric tons of carbon are released into the atmosphere each year. Nuclear power is an appealing alternative to fossil fuels due to relative abundance of nuclear fuel and carbon-neutral energy production.

[0003] Light water reactors (LWRs) are the predominant commercial nuclear reactor for electricity production. LWRs have significant drawbacks, however. In one example LWRs can use solid fuels that have long radioactive half-lives. In another example, LWRs can utilize fuels in a relatively inefficient manner. As a result, LWRs can produce dangerous and long-lived waste products. Nuclear fuel can also be vulnerable to extreme accidents or proliferation (e.g., plutonium) to make nuclear weapons.

[0004] Molten salt reactors (MSRs) have been researched since the 1950s to improve on LWR technologies. MSRs are a class of nuclear fission reactors in which the primary coolant, or even the fuel itself, can be a molten salt mixture. In general, MSRs can provide energy more safely and cheaply than LWRs. As an example, MSRs can operate at relatively low pressures and they can be potentially less expensive and passively safer than LWRs. Furthermore, compared to LWRs, MSRs can also provide advantages such as lower levelized cost on a per-kilowatt hour (kWh) basis, fuel and waste inventories of relatively benign composition, and more efficient fuel utilization. SUMMARY

[0005] Criticality in a molten salt reactor can be controlled using a control rod that displaces a molten fuel salt composition from a core of the reactor. When the control rod is removed from the reactor core and lifted up into a riser above the reactor core, the molten fuel salt composition can flows in to fill the void. In certain embodiments, the control rod can be a solid or hollow cylindrical volumetric displacement reactivity control rod for use with a molten salt reactor that is designed to be in a critical state given a certain fluid fuel volume inside its reactor core. During start-up of the molten salt reactor, the control rod can provides the ability to control the insertion of positive reactivity (increasing fluid volume) in the reactor core. The control rod also provides the ability to remove reactivity in a controlled manner when starting up or shutting down the reactor.

[0006] Embodiments of the control rod can be raised or lowered by a remote operator. Lowering the control rod into the reactor core can displace the volume of the molten fuel salt composition and thus inserts negative reactivity. Raising the control rod out of the reactor core allows the molten fuel salt composition to fill the volume and thus insert positive reactivity. The control rod can have an appropriate volumetric thickness to displace the appropriate amount of fluid such that inserting and removing the control rod crosses the threshold for criticality or otherwise gives the operator fine-tuned control over reactivity.

[0007] In some embodiments, the control rod has one or more channels through its cylindrical wall to allow fluid to flow through the walls when the control rod travels up the riser. Channels can be included up all or a portion of the rod depending on how far the rod will be pulled or if lubrication is needed between the rod and riser. A configuration of the control rod and any included channels can minimize flow restrictions up the riser. The rod may or may not need to be pulled to the top of the riser section.

[0008] In one embodiment, a molten salt reactor is provided and it can include a reactor vessel, a lifting mechanism, and a control rod. The reactor vessel can be configured to receive a molten fuel salt composition. The lifting mechanism can be operably coupled to the reactor vessel. The control rod can be operably coupled to the lifting mechanism. The lifting mechanism can be operable to lower the control rod into the reactor vessel. When the lifting mechanism lowers the control road into the reactor vessel, the control rod can displace a portion of the molten fuel salt composition from a reactor core of the reactor vessel and places the reactor in a sub-critical state.

[0009] In another embodiment, when the lifting mechanism raises the control rod out of the reactor vessel, the molten fuel salt composition can flow into the reactor core and place the reactor in a critical state.

[0010] In another embodiment, the reactor vessel can define a riser above the reactor core, such that when, the lifting mechanism raises the control rod out of the reactor vessel, the control rod enters the riser. In other embodiments, the riser can be omitted.

[0011] In another embodiment, the reactor can include an operator station that has a controller operably connected to the lifting mechanism. An operator can use the controller to cause the lifting mechanism to raise or lower the control road.

[0012] Embodiments of the control rod can have a variety of configurations. In one aspect, the control rod can be hollow. In another aspect, at least a portion of the control rod can include a neutron absorbing material. In another aspect, at least a portion of the control rod can be formed at least one of stainless steel, lead, tungsten, and indium. In another aspect, when the control rod is lowered into the reactor vessel, the molten fuel salt composition can fill the control rod and overflow a top of the control rod. In another aspect, at least a portion of the control rod is substantially cylindrical.

[0013] In another embodiment, the hollow control rod can include one or more channels defining passages between an inside and an outside of the control rod. In another aspect, the reactor can also include a heat exchanger disposed above the reactor core. When the lifting mechanism raises the control rod out of the reactor vessel, the molten fuel salt composition can flow out of the hollow control rod, through the channels, and into the heat exchanger. In another aspect, when the lifting mechanism raises the control rod out of the reactor vessel, the molten fuel salt composition can flow out of the hollow control rod through the channels. For example, the molten fuel salt composition can flow into the reactor core.

[0014] In another embodiment, the control rod can include a plurality of concentric cylinders. The lifting mechanism can be able to independently raise and lower individual ones of the plurality of concentric cylinders. For example, the lifting mechanism can raise and lower the concentric cylinders in a telescopic manner such that the cylinders are lowered serially from the innermost to the outermost. In another aspect, the lifting mechanism can raises and lower the concentric cylinders in a telescopic manner in which the cylinders are lowered serially from the innermost to the outermost.

[0015] In another embodiment, the control rod can include an upper cylindrical portion and a lower conical portion. The lower conical portion can taper from a wide base towards the cylindrical portion. The cylindrical portion can have a first diameter smaller than a second diameter characterizing the wide base.

[0016] In another embodiment, the control rod can be formed from a neutron poisoning material. In another embodiment, the control rod does not include any neutron poisoning materials. In another embodiment, at least a portion of the control rod can be formed from one or more of stainless steel, lead, tungsten, and indium.

[0017] In another embodiment, when the lifting mechanism lowers the control road into the reactor vessel, the control rod can displace a portion of the molten salt from a reactor core of the reactor vessel, decreasing a volume of the molten salt within the reactor core, and thereby placing the reactor in a sub-critical state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other features will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0019] FIG. 1 schematically illustrates a nuclear thermal generator plant (NTGP) system.

[0020] FIG. 2 is a schematic illustration of an exemplary reactor suitable for use with the system of FIG. 1.

[0021] FIG. 3 is a schematic illustration of an exemplary embodiment of a reactor including a hollow cylindrical control rod.

[0022] FIG. 4 is a top view of the control rod of FIG. 3.

[0023] FIG. 5 is a schematic illustration of an exemplary embodiment of a reactor including a tapered-end control rod. [0024] FIG. 6 is a schematic illustration of an exemplary embodiment of a reactor including a control rod with channels.

[0025] FIG. 7 is cross sectional view through a riser and the control rod of the control rod of FIG. 6 with channels.

[0026] FIG. 8 is a top view of illustrating an exemplary embodiment of telescoping control rod.

[0027] FIG. 9 is a cross-sectional view a shaped-channel control rod.

[0028] FIG. 10 is an a flow diagram illustrating an exemplary embodiment of a method of controlling criticality of a molten salt reactor.

[0029] FIG. 11 is a side cross-sectional view of an exemplary embodiment of a reactor system including one or more control rods.

[0030] FIGS 12A-12C illustrate embodiments of the control rods of FIG. 11 having different cross-sectional shapes.

[0031] FIG. 13 illustrates an embodiment of a control rod of FIG. 11 having a tapered shape.

[0032] FIG. 14 is a top cross-sectional view of the reactor system of FIG. 11 including multiple control rods.

[0033] FIG. 15A is a cross-sectional side view illustrating an exemplary embodiment of a reactor including one or more control rods positioned within the reactor core.

[0034] FIG. 15B is a top view of the reactor of FIG. 15 A.

[0035] FIG. 16A is a schematic diagram illustrating an exemplary embodiment of a reactor including one or more control objects withdrawn from a center of the reactor core.

[0036] FIG. 16B is a schematic diagram illustrating an exemplary embodiment of a reactor including one or more control objects positioned near a center of the reactor core.

[0037] FIG. 17 is a schematic illustration of a cylindrical model of the reactor core. [0038] FIG. 18 is a schematic illustration showing positions chosen for evaluating the reactivity worth of a 1 m ³ cylinder

[0039] FIG. 19 is schematic illustration showing positions chosen for evaluating the reactivity worth of four 0.25 m ³ cylinders.

[0040] It can be noted that the drawings can be not necessarily to scale. The drawings can be intended to depict only typical aspects of the subject matter disclosed herein, and therefore should not be considered as limiting the scope of the disclosure.

[0041] For a thorough understanding of the present disclosure, reference should be made to the following detailed description, including the appended claims, in connection with the above-described drawings. Although the present disclosure can be described in connection with exemplary embodiments, the disclosure can be not intended to be limited to the specific forms set forth herein. It can be understood that various omissions and substitutions of equivalents can be contemplated as circumstances can suggest or render expedient.

DETAILED DESCRIPTION

[0042] Volumetric displacement reactivity control rod can be used in a reactor that is designed to be in a critical state given a certain fluid fuel volume inside its reactor core. During start-up, use of the control rod allows control the insertion of positive reactivity (increasing fluid volume) in the reactor core. The control rod may also be used to remove reactivity in a controlled way when shutting down the reactor.

[0043] Startup and shutdown reactivity control involve processes that must be considered in any nuclear reactor design. Control of those processes is achieved with a control rod that may optionally include a drive mechanism and/or a poisoning quality. This poisoning quality can provide negative reactivity to prevent the reactor from achieving criticality based upon neutron absorption. In a molten salt reactor, reactivity is influenced by volume of fluid fuel present in the core. Reactivity can be controlled with a volume displacement mechanism alone. However, control rod poisoning can be used in combination with volumetric displacement. The volume displacement reactivity control mechanism can be constructed from one or more of stainless steel, lead, indium, tungsten, or other material. [0044] Reactivity can be controlled through the use of at one or more control rods. The control rods can be hollow or solid through their cross-section. In certain embodiments, the reactor can include a riser section above a reactor core, such that the control rod can be raised from the reactor core and into the riser section. The use of a hollow control rod can allow fluid to pass through the control rod and through the riser section during reactor operation. The use of a hollow control rod does not allow fluid to pass through the control rod and, for the same cross-sectional shape, provides greater volume displacement at a given immersion depth than a hollow control rod. Each are discussed in greater detail below.

[0045] In certain embodiments, normal operation of the reactor to produce heat energy can include the flow of fluid fuel up through the control rod and into a heat exchanger.

Embodiments of hollow control rods can include channels to allow that flowing fluid fuel to pass out from the center of the control rod to the outside of the control rod to reduce friction between the rod and riser section.

[0046] FIG. 1 is a schematic illustration of an exemplary embodiment of a nuclear thermal generator plant system 100. As shown, the system 100 includes a reactor system 102 and a secondary system 104. The reactor system 102 includes a primary heat exchanger 106 connected to a reactor 110 having a reactor core 112 configured to receive a fuel salt composition 114. The reactor 110 can be interchangeably referred to herein as a reactor vessel. The reactor system 102 also includes a reactivity control system 116 and a fuel conditioning system 120, each connected to the reactor 110.

[0047] The system 100 can be configured to generate electrical energy from fission of the fuel salt composition 114 in a molten state, where the fuel salt acts as a liquid fuel within the reactor system 102 to generate heat. In certain embodiments, the fuel salt composition 114 can include a carrier salt and a fuel salt. As an example, components of the fuel salt composition 114 can be in the form of one or more chloride salts, fluoride salts, and mixtures of one or more chloride and fluoride salts.

[0048] Embodiments of the fuel salt can include fissile materials, fertile materials, and combinations thereof. Examples of fissile materials can include, but are not limited to, thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), and any combination thereof. In certain embodiments, the fissile materials can include one or more of the following isotopes, in any combination: Th-225, Th-227, Th-229, Pa-228, Pa-230, Pa-232, U-231, U-233, U-235, Np-234, Np-236, Np-238, Pu-237, Pu-239, Pu-241, Am-240, Am-242, Am-244, Cm-243, Cm-245, and Cm-247.

Examples of fertile materials can include, but are not limited to, 232 ThC , 238 UC¾ and ²³⁸ UC1 ₄ .

[0049] In an embodiment, the fuel salt composition 114 can include one or more chloride salts and the system 100 can be referred to as a molten chloride fast reactor (MCFR). As an example, the carrier salt can include a chloride salt of an alkali or alkaline earth metal and the fuel salt can include a chloride salt of at least one actinide. In another non-limiting example, the fuel salt composition 114 can include a fuel salt containing one or more of ²³³ U ¾,

235 233 235 239

" ^J UC1 ₃ , ^{^} UCU, ^{^J} UC1 ₄ , and ^{^} TuCl ₃ ; and a carrier salt including one or more of sodium chloride (NaCl), potassium chloride (KC1), and calcium chloride (CaCl ₂ ). In another embodiment, the fuel salt composition 114 can include at least NaCl as the carrier salt and UCI ₃ as the fuel salt.

[0050] In an embodiment, fuel salt can have a concentration selected from about 1 mole % to about 90 mole % of the fuel salt composition 114. In further embodiments, the fuel salt composition 114 can have a melting temperature that is greater than or equal to about 300°C. In additional embodiments, the melting temperature of the fuel salt composition can be selected from about 325°C to about 475°C.

[0051] Further embodiments of fuel salt compositions suitable for use with the system 100 are discussed in greater detail in U.S. Provisional Patent Apphcation No. 62/340,754, filed on May 24, 2016, entitled "Chloride and Fluoride Salt Composition For Molten Salt Reactor," U.S. Provisional Application No. 62/340,762 filed on May 24, 2016, entitled "Salt Composition With Phase Modifiers For Molten Salt Reactor," U.S. Provisional Application No. 62/269,525, filed on December 18, 2015, entitled "Salt Composition for Molten Salt Reactor," and U.S.

Application No. 15/380,473, filed on December 15, 2016, entitled "Salt Compositions for Molten Salt Reactors," each of which is hereby incorporated by reference in its entirety.

[0052] Upon absorbing neutrons, nuclear fission can be initiated and sustained in the molten fuel salt composition 114 by chain-reaction within the reactor 110, generating heat that elevates the temperature of the fuel salt composition 114 (e.g., to about 650°C or about 1,200°F). The heated fuel salt composition 114 can be transported from the reactor core 112 to the primary heat exchanger 106 via a primary fluid loop 122 via a pump, discussed in greater detail below. The primary heat exchanger 106 can be configured to transfer heat generated by nuclear fission occurring in the fuel salt composition 114.

[0053] In general, fluids of three types can be contained in and/or circulated through the system 100, namely fuel, coolant, and moderator (e.g., any substance that slows neutrons). Various fluids can perform one or more of the fuel, coolant, and moderator functions simultaneously. One or more fluids, including more than one fluid of each functional type, can be contained within or circulated through the reactor core 112. Examples of fluids contained within or circulated through the reactor core 112 can include, but are not limited to, liquid metals, molten salts, supercritical H ₂ 0, supercritical C(¾, and supercritical ₂ O.

[0054] The transfer of heat from the fuel salt composition 114 can be realized in various ways. For example, the primary heat exchanger 106 can include a pipe 124 and a secondary fluid 126 (e.g., another molten salt). The molten fuel salt composition 114 can travel through the pipe 124, while the secondary fluid 126 (e.g., a coolant) can surrounds the pipe 124 and absorb heat from the fuel salt composition 114. Upon heat transfer, the temperature of the fuel salt composition 114 in the primary heat exchanger 106 can be reduced and fuel salt composition 114 can be subsequently transported from the primary heat exchanger 106 back to the reactor core 112.

[0055] The primary heat exchanger 106 can be provided in a variety of configurations. In various embodiments, the primary heat exchanger 106 can be either internal or external to a reactor vessel (not shown) that contains the reactor core 112. In additional embodiments, the system 100 can be configured such that primary heat exchange (e.g., heat exchange from the molten fuel salt composition 114 to a different fluid) can occur both internally and externally to the reactor vessel. In other embodiments, the system 100 can be provided such that the functions of nuclear fission and primary heat exchange can be integral to the reactor core 112. That is, heat exchange fluids can be passed through the reactor core 112.

[0056] The secondary system 104 can also include a secondary heat exchanger 130 configured to transfer heat from the secondary fluid 126 to a tertiary fluid 132 (e.g., water). As shown in FIG. 1, the secondary fluid 126 is received from primary heat exchanger 106 via fluid loop 134 and circulated through secondary heat exchanger 130 via a pipe 136. [0057] Additionally or alternatively, in another embodiment (not shown), heat exchange can occur within the reactor core 112 prior to heat exchange within the secondary heat exchanger 130. As an example, heat from the fuel salt composition can pass to a solid moderator, then to a liquid coolant circulating through the reactor. Subsequently, the liquid coolant circulating through the reactor can be transported to the secondary heat exchanger. As required by basic thermodynamics, after one or more stages of exchange, heat can be finally delivered to an ultimate heat sink, e.g., the overall environment (not shown).

[0058] Heat received from the fuel salt composition 114 can be used to generate power (e.g., electric power) using any suitable technology. For example, when the tertiary fluid 132 in the secondary heat exchanger 130 is water, it can be heated to a steam and transported to a turbine 140 by a fluid loop 142. The turbine 140 can be turned by the steam and drive an electrical generator 144 to produce electricity. Steam from the turbine 140 can be conditioned by an ancillary gear 146 (e.g., a compressor, a heat sink, a pre-cooler, and a recuperator) and it can be transported back to the secondary heat exchanger 130 through the fluid loop 142.

[0059] Additionally, or alternatively, the heat received from the fuel salt composition 114 can be used in other applications such as nuclear propulsion (e.g., marine propulsion), desalination, domestic or industrial heating, hydrogen production, or combinations thereof.

[0060] Embodiments of the reactivity control system 116 can include one or more fluid reservoirs in fluid communication with the reactor 110. The fluid reservoirs can contain an inert gas (e.g., argon) or a non-reactive liquid (e.g., a liquid metal). A selected amount of fluid can be transported from the fluid reservoirs to the reactor 110 to control reactivity.

[0061] The fuel conditioning system 120 can be configured to remove at least a portion of fission products generated in the fuel salt composition 114 during nuclear fission. In general, during the operation of the system 100 to generate power, fission products (e.g., radioactive noble metals and/or radioactive noble gases) can be generated in the fuel salt composition 114. Non-limiting embodiments of fission products can include, but are not limited to, one or more of the following, in any combination: rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), lanthanides, palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), technetium (Tc), xenon (Xe), and krypton (Kr). [0062] Buildup of fission products in the fuel salt composition 114 can impede or interfere with the nuclear fission in the reactor core 112 by poisoning the nuclear fission. For example, xenon-135 and samarium-149 can have a high neutron absorption capacity and they can lower the reactivity of the fuel salt composition 114. Fission products can also reduce the useful lifetime of the system 100 by clogging or corroding components, such as heat exchangers or piping. Therefore, it can be desirable to keep concentrations of fission products in the fuel salt composition 114 below certain thresholds to maintain proper functioning of the system 100.

[0063] This goal can be accomplished by the fuel conditioning system 120. As an example, the fuel salt composition 114 can be transported from the reactor core 112 to the fuel conditioning system 120, which can process the molten fuel salt composition 114 and allow the reactor 110 to function without loss of efficiency or degradation of components due to development of fission products. As shown in FIG. 3, the fuel conditioning system 120 can be contained within the reactor system 102 along with the reactor 110 and the primary heat exchanger 106. However, in alternative embodiments (not shown), at least one of the primary heat exchanger and the fuel-conditioning system can be located external to the reactor system.

[0064] FIG. 2 illustrates the fuel conditioning system 120 in greater detail. During normal operation of the reactor system 102, the fuel salt composition 114) can be circulated continuously or near-continuously from the reactor core 112 through one or more of functional sub-units of the fuel conditioning system 120 via fluid loop 146 by a pump 150. In certain embodiments, the fuel salt composition 114 can be in a state where at least a portion of the fuel salt component has been consumed in a nuclear fission process (e.g., partially or fully spent). As discussed below, examples of the sub-units can include, but are not limited to, a corrosion reduction unit 152, a mechanical separation unit 154, and a chemical exchange unit 156. The fuel conditioning system 120 can also include a tank 160 for storage (e.g., excess fuel salt composition 114 and/or substances removed from the fuel salt composition 114).

[0065] In an embodiment, the corrosion reduction unit 152 can be configured to inhibit or mitigate corrosion of components of the system 100 by the fuel salt composition 114. At least a portion of the reactor core 112 can be constructed of metallic alloy including one or more of the following elements: iron (Fe), nickel (Ni), chromium (Cr), manganese (Mn), carbon (C), silicon (Si), niobium (Nb), titanium (Ti), vanadium (V), phosphorus (P), sulfur (S), molybdenum (Mo), nitrogen (N), cermet alloys, stainless steels (austenitic stainless steels), zirconium alloys, or tungsten alloys, and variants thereof.

[0066] During operation of the system 100, the fuel salt composition 114 can be transported from the reactor core 112 to the corrosion reduction unit 152 and from the corrosion reduction unit 152 back to the reactor core 112. Transportation of the fuel salt composition 114 at a variably adjustable flow rate can be driven by the pump 150. The corrosion reduction unit 152 can be configured to process the fuel salt composition 114 to maintain an oxidation reduction (redox) ratio, E(o)/E(r), of the fuel salt composition 114 in the reactor core 112 (and elsewhere throughout the system 100) at a substantially constant level, where E(o) is an element (E) at a higher oxidation state (o) and E(r) is that element (E) at a lower oxidation state (r).

[0067] In one embodiment, the element (E) can be an actinide (e.g., uranium, U), E(o) can be U(IV) and E(r) can be U(III). In this embodiment, U(IV) can be in the form of uranium tetrachloride (UC ), U(III) can be in the form of uranium trichloride (UCI ₃ ), and the redox ratio can be a ratio E(o)/E(r) of UCI ₄ /UCI ₃ . Although UCI ₄ can corrode the reactor core 112 by oxidizing chromium according to:

Cr— > Cr ³⁺ + 3e ^~

Cr + 3UCl ₄ → CrCl ₃ + 3UCl ₃ the existence of UCI ₄ can reduce the melting point of the molten fuel salt composition 114. Therefore, the level of the redox ratio, UCI ₄ /UCI ₃ , can be selected based on at least one of a desired corrosion reduction and a desired melting point of the fuel salt composition 114. For example, the redox ratio can be substantially constant and selected between about 1/50 to about 1/2000. More specifically, the redox ratio can be at a substantially constant level of about 1/2000.

[0068] The mechanical separation unit 154 can be configured to remove at least part of insoluble fission products and/or dissolved gas fission products from the fuel salt composition 114. Examples of insoluble fission products can include one or more of the following in any combination: krypton (Kr), xenon (Xe), palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), and technetium (Tc). Examples of gas fission products can include one or more of xenon (Xe) and krypton (Kr). As an example, the mechanical separation unit 154 can generate a froth from the fuel salt composition 114 that includes the insoluble fission products and/or the dissolved gas fission products. The dissolved gas fission products can be removed from the froth, and at least a portion of the insoluble fission products can be removed by filtration.

[0069] The chemical exchange unit 156 can be configured to remove at least a portion of the soluble fission products dissolved in the fuel salt composition 114. Examples of the soluble fission products can include one or more of the following, in any combination: rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), and lanthanides. The removal of soluble fission products can be realized through various mechanisms.

[0070] Comprehensive lists of fission products applicable to various embodiments to the present disclosure are provided below. A person skilled in the art will appreciate that these lists are illustrative and not meant to be exhaustive.

[0071] Fission products sufficiently noble to maintain a reduced and insoluble state in the molten fuel salt composition can include, but are not limited to:

• Germanium - 72, 73, 74, 76

• Arsenic - 75

• Selenium - 77, 78, 79, 80, 82

• Yttrium - 89

• Zirconium - 90 to 96

• Niobium - 95

• Molybdenum - 95, 97, 98, 100

• Technetium - 99

• Ruthenium - 101 to 106

• Rhodium - 103

•Palladium - 105 to 110

• Silver - 109

• Cadmium - 111 to 116

• Indium - 115

•Tin - 117 to 126

• Antimony - 121, 123, 124, 125 •Tellurium - 125 to 132

[0072] Fission products that can form gaseous products at the typical operating temperature of can include, but are not limited to:

• Bromine - 81

•Iodine - 127, 129, 131

•Xenon - 131 to 136

• Krypton - 83, 84, 85, 86

[0073] Fission products that can remain as chloride compounds in the molten fuel salt, in addition to actinide chlorides (Th, Pa, U, Np, Pu, Am, Cm) and carrier salt chlorides (Na, K, Ca), can include, but are not limited to:

• Rubidium - 85, 87

• Strontium - 88, 89, 90

•Cesium - 133, 134, 135, 137

•Barium - 138, 139, 140

• Lanthanides

o Lanthanum - 139

o Cerium - 140 to 144

o Praseodymium - 141, 143

o Neodymium - 142 to 146, 148, 150

o Promethium - 147

o Samarium - 149, 151, 152, 154

o Europium - 153, 154, 155, 156

o Gadolinium - 155 to 160

o Terbium - 159, 161

o Dysprosium - 161

[0074] During operation of the system 100, the volume of molten fuel salt composition 114 in the reactor core 112 contributes to criticality of the reactor 110. In general, criticality refers to the balance of neutrons in a nuclear system. In nuclear systems where a loss rate of neutrons is approximately equal to a production rate of neutrons, the neutron population remains approximately constant. Under this condition, a nuclear system is referred to as critical. In nuclear systems where a loss rate of neutrons is greater than a production rate of neutrons, the neutron population decreases. Under this condition, a nuclear system is referred to as sub-critical. In nuclear systems where a loss rate of neutrons is less than a production rate of neutrons, the neutron population increases. Under this condition, a nuclear system is referred to as supercritical.

[0075] The power of a nuclear reactor is proportional to its neutron population. If there are more neutrons in the system, more fission will take place, producing more energy. When a reactor is starting up, the neutron population is increased slowly, in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical. This allows the neutron population to increase and more power to be produced. Subsequently, once a desired power level is achieved, the nuclear reactor can be placed into a critical configuration to keep the neutron population and power constant. Finally, during shutdown, the reactor is placed in a sub-critical configuration so that the neutron population and power decreases. Therefore, when a reactor is said to have "gone critical," it means the reactor is in a stable configuration producing power. As discussed in greater detail below, embodiments of the reactor system 102 can include a control rod that is configured to controlling its level of criticality (e.g., sub-critical, critical, or supercritical).

[0076] FIG. 3 is a schematic illustration of the reactor 110 in greater detail showing one exemplary embodiment of a control rod 301. As shown, the control rod 301 has a length L and is disposed within a riser 305 disposed above the reactor core 112. The reactor 110 can also include a downcomer 311, a region where the molten fuel salt composition 114 can flow down to a bottom portion of the reactor 110 to return to the reactor core 112. The molten fuel salt composition 114 can also flow up through the riser 305 and into a heat exchanger (not shown) disposed above the reactor core 112.

[0077] The reactor 110 can optionally include one or more reflectors 333. The reflectors 333 can be formed from a material that reflects incident neutrons. Thus, neutrons that are generated within the reactor core 112 and are incident upon an inward facing surface of the reflectors 333 can be reflected back towards the reactor core 112.

[0078] In some embodiments, the control rod 301 can be formed in a hollow and substantially cylindrical geometry. So configured, the molten fuel salt composition 114 can flow upward through a lumen 301a of the control rod 301. A non-limiting example of this geometry is illustrated in the cross-sectional, top view of FIG. 4. The control rod 301 includes a generally circular sidewall having a thickness defined by the difference between an inner radius η and an outer radius r ₀ . Assuming the cross-sectional geometry of the control rod 301 is approximately constant along its length L, a volume V of the control rod is given by the product of cross-sectional area and length, V = 7iL(r ₀ ² -ri ² ).

[0079] The reactor system 102 can also include a lifting mechanism 405 operably coupled to the reactor vessel 110 and to the control rod 301 and configured to raise and lower the control rod 301 with respect to the reactor vessel 110 and the molten fuel salt composition 114. As shown, the lifting mechanism 405 can include one or more hooks 461, load line 463, and hoist 467. In certain embodiments, the hoist 467 can be actuated by a motor (e.g., an electrical motor) sitting atop the reactor 110. The motor may be connected to the control rod 301 via the one or more load lines 463, which may be chains, rods, wire cables, or other suitable materials.

[0080] When the control rod 301 is raised by the lifting mechanism 405, at least a portion of the control rod 301 moves out of the reactor core 112 and into the riser 305. Conversely, when the control rod 301 is lowered by the lifting mechanism 405, at least a portion of the control rod 301 moves out of the riser 305 and into the reactor core 112.

[0081] In general, the control rod 301 displaces a volume of the molten fuel salt composition 114 from the reactor core 112 that is substantially equal to the volume of the control rod 301 immersed therein. Lowering the control rod 301 within the molten fuel salt composition 1 14 decreases the volume of the molten fuel salt composition 114 within the reactor core 112 and inserts negative reactivity. As an example, when the lifting mechanism 405 lowers the control rod 401 into the reactor vessel 110, the molten fuel salt composition 114 can be displaced from the reactor core 112 and place the reactor in a sub- critical state. Conversely, raising the control rod 301 within the molten fuel salt composition 114 increases the volume of the molten fuel salt 114 within the reactor core 110 and inserts positive reactivity. As an example, when the lifting mechanism 405 raises the control rod 401 out of the reactor vessel 110, the molten fuel salt composition 114 can flow into the reactor core 112 and place the reactor in a critical state.

[0082] The control rod 301 can be configured with an appropriate volume to control whether a given raising or lowering movement of the control rod 301 places the reactor 110 in a sub- critical, critical, or supercritical state. In one embodiment, the volume defined by the control rod 301 can provide enough fluid displacement (negative reactivity) such that, when fully inserted into the reactor core 112, places the reactor system 102 in a sub-critical state. In another embodiment, the volume defined by the control rod 301 can provide enough fluid displacement (negative reactivity) such that, when fully withdrawn from the reactor core 112, places the reactor system 102 in a supercritical state. In a further embodiment, the volume defined by the control rod 301 can allow enough fluid infill (positive reactivity) such that, when fully withdrawn from the reactor core 112, places the reactor system 102 in a supercritical state. In another embodiment, the volume defined by the control rod 301 can provide enough fluid displacement such that, when partially inserted the reactor core 112, places the reactor system 102 in a critical state.

[0083] In some embodiments, the position of the control rod 301 can be controlled by a remote operator from a control station (not shown). A control station can provide manual controls for the operator, a computer interface, or both. Additionally or alternatively, the control station can provide for the automatic control of the motor (e.g., via a programmed computer).

[0084] FIG. 5 shows another embodiment of the reactor system 102 including a control rod 501 having a non-uniform radius along its length. As shown, the control rod 501 includes an upper cylindrical portion 501a and a lower conical portion 501b. The lower conical portion 501b is tapered from a wide base towards the upper cylindrical portion 501a. The cylindrical portion 501a has a first diameter smaller than a second diameter characterizing the wide base.

[0085] The control rod 501 can also encourage optimal flow properties of the molten fuel salt composition 114 within the reactor 110. For example, upward flow of the molten fuel salt composition 114 can be guided towards the center of the reactor 110 and into the riser 305. Additionally, downward flow of the molten fuel salt composition 114 from the heat exchanger can be biased or guided towards an outer portion of the reactor 110, away from the reactor core 112.

[0086] In use, the control rod 501 can be raised or lowered by the lifting mechanism 405 to control reactivity within the reactor core 112, as discussed above with respect to the control rod 301. Similar to the control rod 301, the tapered-end control rod 501 displaces the molten fuel salt composition 114 from the reactor core 112 when immersed therein. When the lifting mechanism 405 is used to raise the tapered-end control rod 501 from the reactor core 112 and into the riser 305 above the reactor core 112, the molten fuel salt composition 114 flows into the reactor core 112. In certain embodiments, the tapered-end control rod 501 can be hollow and thus provides a hollow cylindrical volumetric displacement for control of criticality within the reactor 110.

[0087] FIGS. 6 illustrates another embodiment of the reactor system 102 including a control rod 601 with one or more channels 631 defining passages extending through a sidewall of the control rod 601. FIG. 7 is cross-sectional view through the riser 305 of the reactor 110, showing the control rod 601 therein. In certain embodiments, the geometry of the control rod 601 can be hollow and cylindrical. When the control rod 601 is fully inserted into the reactor core 112, the molten fuel salt composition 114 can flows up through and out of the top of the control rod 601. Once the control rod 601 is raised into the riser 305, the channels 631 can allow the molten fuel salt composition 114 to flow from the center of the hollow control rod 601, which can also be the center of the riser 305 to a heat exchanger 603. The one or more channels 631 can be positioned at any location within the control rod 601 and can have the same or different shapes and diameters.

[0088] In use, the control rod 501 can be raised or lowered by the lifting mechanism 405 to control reactivity within the reactor core 112, as discussed above with respect to the control rod 301. Similar to the control rods 301 and 501, the control rod 601 displaces molten salt 114 from the reactor core 112 when immersed therein. When the lifting mechanism 405 is used to raise the control rod 601 from the reactor core 112 and into the riser 305 above the reactor core 112, the molten fuel salt composition 114 flows into the reactor core 112. In certain embodiments, the control rod 601 can be hollow and thus provides a hollow cylindrical volumetric displacement for control of criticality within the reactor 110.

[0089] Optionally, functional relationships can be provided between the one or more channels 631 and components of the reactor 110, such as the riser 305 and/or the heat exchanger 603. In one embodiment, the channels 631 can allow the molten fuel salt composition 114 to easily provide lubrication between the control rod and the riser.

Additionally or alternatively, one or more of the channels 631 can be dimensioned to allow molten salt to flow from within the riser 305 into the heat exchanger 603. [0090] Further embodiments of the reactor system 102 can include control rods formed with two or more substantially concentric cylinders to provide improved control over criticality. That is, the volumetric displacement provided by the control rod can be adjusted in finer amounts by raising or lowering less than all of the concentric cylinders in a telescoping manner. For example, embodiments of the control rods 301or 601 and can include two or more substantially concentric cylinders.

[0091] FIG. 8 illustrates a top cross-sectional view of an exemplary embodiment of a telescoping control rod 801 which include a plurality of concentric cylinders. As shown, the control rod includes an inner cylinder 831, an intermediate cylinder 833, and an outer cylinder 835. However, it can be understood that more than one intermediate cylinder 833 can be present. In certain embodiments, the telescoping control rod 801 can be formed from a neutron poisoning material. In alternative embodiments, the telescoping control rod 801 does not include any neutron poisoning materials. As an example, any or all of the cylinders of the telescoping control rod 801 can be formed from one or more of stainless steel, lead, tungsten, and indium.

[0092] The concentric cylinders 831, 833, 835 can be coupled to the lifting mechanism 405 so that each can be raised or lowered independently of the others. In certain

embodiments, the lifting mechanism 405 can raise and lower the concentric cylinders 831, 833, 835 in a telescopic manner, serially from the innermost to the outermost. This mechanism provide for fine-tuned control over reactivity of the reactor 110 because the cylinder are moved in order from smallest to largest volume.

[0093] The size and shape of the concentric cylinders 831, 833, 835 can be configured to control criticality of the reactor 110 when starting up or shutting down. As an example, inserting the telescoping control rod 801 into the reactor 110 can provide sufficient negative reactivity for immediate shutdown. That is, when the lifting mechanism 405 lowers the telescoping control rod 801 into the reactor vessel 110 by a selected amount, the telescoping control rod 801 displaces enough volume of the molten fuel salt composition 114 from the reactor core 112 to place the reactor 110 in a sub-critical state.

[0094] FIG. 9 shows an embodiment of a shaped-channel control rod 901 for controlling criticality of a molten salt reactor. The shaped-channel control rod 901 can be hollow and substantially cylindrical and it can include one or more shaped channels 931. The shaped channels 931 can be disposed through a wall of the shaped-channel control rod 901 in order to guide flows of the molten fuel salt composition 114 during operation of the reactor system 102. For example, where the reactor system 102 includes a heat exchanger having a discrete number of sub-units disposed about the riser 305, the shaped channels 931 can be shaped and positioned such that the channels 931 guide the flow of molten salt into inlet ducts or ports on the heat exchanger sub-units when the shaped-channel control rod 901 is in a raised position and housed substantially within the riser 305.

[0095] Further embodiments of the reactor 110 can employ control rods or objects that are not withdrawn into the riser 305. As discussed above, the reactor 110 is designed to be in a critical state given a certain fluid fuel volume inside the reactor core 112. During start-up, it is important to control the insertion of positive reactivity (increasing fluid volume) in the reactor core. It is also necessary to remove reactivity in a controlled way when shutting down the reactor. The control rod(s) are designed to be motor operated where the motor will raise and lower the rod which either displaces fluid fuel volume (inserts negative reactivity) or allows fluid fuel to fill the volume (inserts positive reactivity). The control rod is designed with an appropriate volumetric thickness that allows this to happen. The control rod can also be made of a neutron poisoning material that absorbs neutrons so that when inserted, there is a negative reactivity insertion from absorbing neutrons. The control rod will then be able to insert/remove reactivity by volumetric displacement and by absorbing neutrons. In certain embodiments, the control rods or control objects can be composed of one or more materials such as steel, tungsten, lead, indium, or flux-trap combo materials like graphite/gad, or b4c/beryllium or similar.

[0096] FIG. 11 illustrates an exemplary embodiment of the reactor system 102 including one or more control rods 1100. The control rods 1100 can be raised or lowered within the reactor core 112 using the lifting mechanism 405, as discussed above, to control criticality. In further embodiments, the control rods 1100 can be hollow, as discussed above, or hollow.

[0097] In general, the cross-sectional shape of the control rods 1100 can be configured to provide appropriate fluid passage for upflow and downflow through the reactor 110. As an example, the control rods 1100 can be formed with star-shaped cross-sections, such as 10- sided stars (FIG. 12A) or 4-sided stars (FIG. 12B). Other shapes are also contemplated, such as squares (FIG. 12C), triangles, and other geometries having rounded edges. [0098] The shape of the control rods 1100 can also be configured to provide selected flow characteristics of the molten fuel salt composition 114. In one aspect, the control rods 1100 can include an end cap 1102 configured to minimize flow resistance from the reactor core 112 into the riser 305. In another aspect, the shape of the control rods 1100 to prevent flow resistance and turbulence in the riser 305. In a further aspect, the control rods 1100 can be shaped so that it prevents damage from vibration. In an additional embodiment, the control rods 1100 can be dimensioned such that, when undergoing thermal expansion, the they do not become stuck in the riser 305 or block fluid flow.

[0099] In certain embodiments, the control rods 1100 can include one or more protrusions configured to engage tracks in the riser. This engagement can keep the control rods 1100 in place upon raising and lowering.

[0100] The section of the control rods 1100 that is immersed into the reactor core 112 can contain materials that provide a longer lifetime in a fast neutron flux region.

[0101] The reactor 110 can include a plenum region 1104 that is dimensioned provide sufficient vertical space to accommodate the control rods 1100 when at least partially raised out of the reactor core 112.

[0102] In certain embodiments, the length of the control rods 1100 can be dimensioned such that the top of the control rods 1100 do not rise above the heat exchangers 603 when fully pulled from the reactor core 112.

[0103] In certain embodiments, the cross-sectional area of the control rods 1100 can vary along its length. FIG. 13 illustrates an exemplary embodiment of a control rod 1100 that includes a body 1300 that is tapered from a top portion to a bottom portion. An end cap 1302, similar to end cap 1102, can be mounted to the bottom portion of the body 1300 to minimize flow resistance from the reactor core 112 into the riser 305.

[0104] As shown in FIG. 13, two or more control rods 1100 can be provided within the reactor system 102. Each of the control rods 1100 can be independently raised or lowered using the lifting mechanism 405 to provide fine control of criticality of the reactor system 102. Using multiple, independently controlled control rods 1100 can also improve safety of the reactor system 102, as failure of one control rod 1100 does not preclude shutdown of the reactor core 112. [0105] FIGS. 15A is a side-cross-sectional view of another embodiment of a reactor 1500 suitable for use with the control rods 1100. A corresponding top-down view is illustrated in FIG. 15B. As shown, the reactor 1500 can include multiple control rods 1100 (e.g., four) that are configured to translate axially through penetrations 1502 in the reactor 1500 by the lifting mechanism 405. An upper plenum of the reactor 1500 (not shown) can be dimensioned to receive the control rods 1100 when withdrawn from the reactor core 112. The placement of the control rods 1100 can be selected to avoid interference with flow of the molten fuel salt composition 114 within the riser 305. In other embodiments, the riser 305 can be omitted from the reactor 110.

[0106] In certain embodiments, criticality of the reactor 1500 can be controlled solely by volume displacement using the control rods 1100. In other embodiments, the control rods 1100 can be formed from a neutron poisoning material and criticality of the reactor 1500 can be controlled by a combination of volume displacement and neutron poisoning.

[0107] Embodiments of the control rods 1100 can be configured as discussed above to allow for thermal expansion and/or provide longer lifetime in a fast neutron flux region. The shape of the control rods 1100 can also adopt any geometry that provides ease of insertion resistance (e.g., tapered) and/or minimizes the pressure shock of rapid insertion.

[0108] FIGS. 16A-16B illustrate an embodiment of control objectsl600 suitable for use in the reactor 1500. The control objects 1600 can be hollow or solid and they can be configured to remain permanently within the reactor core 112 and control criticality of the reactor 1500 by translating into and out of the center of the reactor core 112. While the reactor 1500 of FIGS. 16A-16B is illustrated as including the riser 305, in certain embodiments, it can be omitted.

[0109] In certain embodiments, criticality of the reactor 1500 can be controlled solely by volume displacement using the control objects 1600. In other embodiments, the control objects 1600 can be formed from a neutron poisoning material and criticality of the reactor 1500 can be controlled by a combination of volume displacement and neutron poisoning.

[0110] As shown, the control objects 1600 are coupled to shafts 1604 extending through the penetrations 1502 to independently raise or lower each of the control objects 1600. The lifting mechanism 405 can raise the control objects 1600 away from the reactor core 112 (FIG. 16A) to add positive reactivity or lower the control objects 1600 towards the reactor core 112 (FIG. 16B) to remove positive reactivity. In an embodiment, a shape of the control objects 1600 can maximize the displacement at about the center of the reactor core 112 during reactor shutdown. In further embodiments, the placement of the control objects 1600 within the reactor core 112 can be selected to avoid interference with flow of the molten fuel salt composition 114 in the riser 305.

[0111] Embodiments of the control objects 1600 can be configured as discussed above to allow for thermal expansion and/or provide longer lifetime in a fast neutron flux region. The shape of the control objects 1600 can also adopt any geometry that provides ease of insertion resistance (e.g., tapered) and/or minimizes the pressure shock of rapid insertion.

[0112] FIG. 10 is a flow diagram illustrating an exemplary embodiment of a method 1001 for controlling criticality of a molten salt reactor, such as reactor system 100.

Embodiments of the method 1001 can be employed with the system 100 and any embodiment of the control rods discussed herein (e.g., 301, 501, 601, 901, 1100, 1600). It may be understood that embodiments of the method can include greater or fewer operations than illustrated in FIG. 10 and the operations can be performed in any order.

[0113] In an embodiment, the method can include operations 1013-1039. As shown, the method 1001 can include lowering 1013 a control rod into the reactor (e.g., reactor 110), through the use of a lifting mechanism (e.g., 405). The method can also include displacing 1019 a portion of the molten salt from a reactor core of the reactor vessel using the control rod, thereby placing 1025 the reactor in a sub-critical state. The control rod places the reactor in a sub-critical state by decreasing a volume of the molten salt within the reactor core. The method can also include raising 1027 the control rod to make the reactor go critical 1039.

[0114] In an embodiment of the method 1001, the control rod can be formed from a neutron poisoning material to facilitate control of criticality in combination with volumetric displacement of the molten fuel salt composition. In alternative embodiments, the control rod can be formed from a material that is not neutron poisoning. In certain embodiments, the control rod can be formed from one or more of steel, lead, tungsten, and indium. [0115] The molten salt reactor of the method can include a reactor as described above, a molten salt within the reactor vessel, and a lifting mechanism operably coupled to the reactor vessel. The control rod is operably coupled to the lifting mechanism. When the lifting mechanism raises the control rod out of the reactor vessel, the molten salt flows into the reactor core and places the reactor in a critical state. Preferably the reactor vessel defines a riser above the reactor core, where when the lifting mechanism raises 1027 the control rod out of the reactor vessel, the control rod enters the riser. The method may be performed through the use of an operator station that includes a controller operably connected to the lifting mechanism. In some embodiments, an operator (e.g., a human or robot) may use the controller to cause the lifting mechanism to raise or lower the control road. In certain embodiments, the operator station comprises one or more computer systems (e.g., local client, server, or both) with software instructions that cause the lifting mechanism to perform the described steps.

[0116] In certain embodiments of the method 1001, the control rod can be hollow. When the control rod is lowered into the reactor vessel, the molten salt composition can fill the control rod and the molten fuel salt composition overflow a top of the control rod.

[0117] The reactor can also include a heat exchanger disposed above the reactor core. When the lifting mechanism has raised the control rod out of the reactor vessel, the molten fuel salt composition can flow out of the hollow control rod and into the heat exchanger. The hollow control rod can include one or more channels through its wall, such that the molten salt flows through the channels. This way, when the control rod is lifted into the riser, the reactor can operate routinely by having hot molten salt rise through the hollow control rod, flow through the channels and into the heat exchanger (where it is cooled by transferring heat to a secondary fluid), and flow as cool molten salt back to a base of the reactor vessel via a downcomer region defined substantially by a peripheral volume of the vessel.

Example - Parametric Study

[0118] The following example presents a parametric study for evaluation of reactivity worth associated to displacement of a fuel salt composition within the reactor core. [0119] The study geometry is illustrated in FIG. 17. As shown, a cylindrical geometry is employed. A 2 cm thick reactor vessel has been considered, with a 5 cm downcomer between the reactor vessel and a 25 cm reflector. A 30 cm outer reflector (external to the primary reactor vessel) is also included. The fuel salt composition is initially composed of 35 mol % UC1 ₂ , 55 mol % NaCl, 10 mol % CaCl ₂ , with a 90 % enriched Cl-27 and an average temperature of 575°C. Structures and reflectors are assumed to be formed of stainless steel with a density of 7850 kg/m ³ . Uranium is enriched at 14.8%.

[0120] The goal of the parametric study is to determine dimensions required to reach a slighdy over-breeding performance at equilibrium, with natural uranium feed. An equilibrium k _eff of 1.05 was selected to provide a conservative margin. The capability to maintain the criticality in the first years of operation with less than 20% enriched uranium was verified.

[0121] To evaluate the reactivity worth associated to salt displacement, insertion of a 1 m ³ cylinder into three different positions of the core is simulated. As shown in FIG. 18, these position are: 1) at the core center; 2) at the core top, in a central position; and 3) at the top corner of the core. Five different materials have been inserted, namely: void (e.g., hollow), steel, lead, tungsten and indium. This study adopted a H/D = 1. Results are shown in Table 1. In all cases, the statistical uncertainty is approximately 20 pcm.

Table 1 - Reactivity variation (pcm) due to insertion of a 1 m ³ cylinder of different materials

[0122] For the case of tungsten, two additional cases have been investigated to understand the effect of heterogeneity. In particular, the 1 m ³ cylinder has been substituted with four 0.25 m ³ cylinders, with the configurations shown in Fig. 19. The case with 4 central-top cylinders results in a reactivity reduction of 2783 pcm, 200 pcm more compared to the case with one single cylinder at the top. The effect of a reduced spatial self- shielding becomes notably more visible in case of 4 corner cylinders, which result in a 2187 pcm reactivity reduction, twice as much as the case of one single cylinder in the corner position. [0123] It will be understood that embodiments of the devices and methods described herein all can make use of a hollow control rod to control criticality of molten salt in a molten salt reactor via volumetric displacement (when lowered, or lack thereof, when raised) of molten salt from a core of the reactor. In the certain embodiments, the fact that the control rod is hollow can offer a utilitarian benefit to the operation of the molten salt reactor. Whether the hollow control rod is raised, lowered, or at any position between the extremes, molten salt can flow upwards from the core of the reactor through the control rod. This allows for ongoing operation of the reactor in a critical state while the control rod is raised. For example, molten salt can flow up from the core, into the control rod and, either through channels or over the top of the control rod, into a heat exchanger.

Interestingly, lowering the control rod does not prohibit the flow of molten salt through the primary fluid loop. Instead, lowering the control rod changes (decreases) a volume of molten salt within a core of the reactor, causing the reactor to go sub-critical.

[0124] All references cited throughout this application, for example patent documents including issued or granted patents or equivalents, patent application publications, and non-patent literature documents or other source material, are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application. For example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference.

[0125] One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of embodiments of the disclosure without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in the disclosed embodiments.

[0126] When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. [0127] When a Markush group, or other grouping is used herein, all individual members of the group and all combinations and sub-combinations possible of the group are intended to be individually included in the disclosure.

[0128] When a compound is described herein such that a particular isomer, enantiomer, or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

[0129] As used herein, and in the appended claims, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to "a cell" includes a plurality of such cells and equivalents thereof known to those skilled in the art, and so forth. Additionally, the terms "a" (or "an"), "one or more" and "at least one" can be used interchangeably herein.

[0130] As used herein, the term "comprising" is synonymous with "including," "having," "containing," and "characterized by" and each can be used interchangeably. Each of these terms is further inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0131] As used herein, the term "consisting of excludes any element, step, or ingredient not specified in the claim element.

[0132] As used herein, the term "consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms "comprising", "consisting essentially of," and "consisting of may be replaced with either of the other two terms. [0133] The embodiments illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0134] The expression "of any of claims XX- YY" (where XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form and in some embodiments can be interchangeable with the expression "as in any one of claims XX- YY."

[0135] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which the disclosed embodiments belong.

[0136] Whenever a range is given in the specification, for example, a temperature range, a time range, a composition range, or a concentration range, all intermediate ranges and subranges, as well, as all individual values included in the ranges given, are intended to be included in the disclosure. As used herein, ranges specifically include the values provided as endpoint values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or sub-range that are included in the description herein can be excluded from the claims herein.

[0137] In the descriptions above and in the claims, phrases such as "at least one of or "one or more of may occur followed by a conjunctive list of elements or features. The term "and/or" may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it is used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases "at least one of A and Β;" "one or more of A and Β;" and "A and/or B" are each intended to mean "A alone, B alone, or A and B together." A similar interpretation is also intended for lists including three or more items. For example, the phrases "at least one of A, B, and C;" "one or more of A, B, and C;" and "A, B, and/or C" are each intended to mean "A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together." In addition, use of the term "based on," above and in the claims is intended to mean, "based at least in part on," such that an unrecited feature or element is also permissible.

[0138] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed embodiments. Thus, it should be understood that although the present application may include discussion of preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art. Such modifications and variations are considered to be within the scope of the disclosed embodiments, as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present disclosure and it will be apparent to one skilled in the art that they may be carried out using a large number of variations of the devices, device components, and methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional compositions and processing elements and steps.