CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Prov. App. No. 62/445,537, filed January 12, 2017, and U.S. Prov. App. No. 62/488,315, filed April 21, 2017, with the entire contents of each of these applications hereby incorporated herein by reference.

BACKGROUND

[0002] Nuclear reactors produce energy based on fission of fuel through interaction between neutrons and the fuel. The fission of fuel produces more neutrons, which leads to further fission of fuel in a neutron chain reaction. Given that the reactivity feedback coefficient of some reactor designs is positive, the rate of the neutron chain reaction can increase as the temperature of the fuel, coolant, or other reactor components increases. In nuclear reactors with such reactor designs, a coolant is used to conduct heat away from the fuel. As the temperature of the coolant increases in positive reactivity reactors through heat transfer from the fuel, the density of the coolant decreases. With such a decrease in density, neutron flux in the positive reactivity reactor increases, producing a corresponding increase in the rate of the neutron chain reaction. Accordingly, the use of a coolant to conduct heat away from the fuel can present challenges for controlling certain nuclear reactors to avoid or reduce the likelihood of a runaway neutron chain reaction, particularly during power excursions.

SUMMARY

[0003] Devices, systems, and methods of the present disclosure are generally directed to controlling temperature of a nuclear reactor (e.g., a fast neutron reactor) that is at least partially cooled by a molten salt. The molten salt can cooperate with an absorbent material in at least one control rod in a core of the reactor to control reactivity in the nuclear reactor. Such control can be useful for reducing the likelihood of a runaway nuclear chain reaction during power excursions of the nuclear reactor.

[0004] According to one aspect, a method of controlling a nuclear reactor includes producing a neutron chain reaction, passing heat and neutrons from the neutron chain reaction through a molten salt, a moderator material and into a volume defined by at least one control rod, and absorbing at least a portion of the neutrons in a vapor phase of an absorbent material in the volume of the at least one control rod. The moderator material decreases energy of the neutrons passing though the moderator material, and the heat passing into the volume defined by the at least one control rod increases pressure of the vapor phase of the absorbent material in the volume.

[0005] In some implementations, the molten salt can have a macroscopic elemental absorption cross-section of greater than about 0.003 [1/cm] at about 0.025 eV. Additionally, or alternatively, the molten salt can include one or more of a nitrate salt, a fluoride salt, and a chloride salt. For example, the molten salt can include a nitrogen-enriched nitrate salt.

[0006] In certain implementations, a combined temperature coefficient of the molten salt and the at least one control rod can be negative.

[0007] In some implementations, a rate of the neutron chain reaction can decrease as a rate of absorption of the neutrons in the volume defined by the at least one control rod increases, and the rate of absorption of the neutrons in the volume can increase with an increase in the pressure of the vapor phase of the absorbent material in the volume.

[0008] In certain implementations, the at least one control rod can be thermally conductively insulated from the neutron chain reaction. For example, the at least one control rod can be thermally conductively insulated from the neutron chain reaction by one or more of a vacuum sleeve, a gas sleeve, and a low thermal conductivity solid sleeve. Additionally, or alternatively, passing heat and neutrons from the chain reaction through the molten salt and the moderator material and into the volume defined by the at least one control rod can include radiative transfer of heat into the volume through absorption of photons (e.g., gamma rays) in the absorbent material in the volume. Further, or instead, a rate of radiative heat transfer into the volume can be greater than a rate of conductive heat transfer into the volume.

[0009] In some implementations, the moderator material can include a metal hydride (e.g., zirconium hydride). Additionally, or alternatively, the moderator material includes one or more of hydrogen, beryllium, lithium, and carbon.

[0010] In certain implementations, the neutron chain reaction can include fission of a fuel (e.g., uranium). [0011] In some implementations, absorbing at least a portion of the neutrons in the vapor phase of the absorbent material transmutes nuclei of the absorbent material. For example, transmuting nuclei of the absorbent material forms a stable isotope of the absorbent material.

[0012] In certain implementations, the absorbent material can include mercury, and absorbing at least a portion of the neutrons in the vapor phase of the absorbent material forms a mercury isotope. Additionally, or alternatively, the absorbent material can include boron halide (e.g., boron iodide), and absorbing at least a portion of the neutrons in the absorbent material forms lithium.

[0013] In some implementations, pressure of the vapor phase of the absorbent material in the volume can increase non-linearly with an increase in temperature of the volume resulting from the heat passed into the volume of the at least one control rod. For example, pressure of the vapor phase of the absorbent material in the volume can double with about a 50 °C increase in temperature of the volume.

[0014] In certain implementations, upon passing the heat and the neutrons into the volume, the absorbent material in the volume can be above a boiling temperature of the absorbent material at atmospheric pressure. For example, the absorbent material in the volume can have a boiling temperature of greater than about 0 °C and less than about 600 °C at atmospheric pressure.

[0015] In some implementations, the volume can contain a second material (e.g., one or more of rubidium, sodium, and potassium) having an elemental cross-section less than an elemental cross-section of the absorbent material such that the second material is relatively inert, as compared to the absorbent material, as the neutrons are received in the volume. For example, the absorbent material and the second material can have respective vapor pressures. The vapor pressure of the absorbent material can vary with temperature according to a first rate, and the vapor pressure of the second material can vary with temperature according to a second rate, the first rate differing from the second rate over at least a portion of a temperature range from about 0 °C to about 1000 °C.

[0016] In certain implementations, as the neutrons pass into the volume, a first portion of the absorbent material can be in a liquid phase and a second portion of the absorbent material can be in a vapor phase. For example, the absorbent material in the liquid phase can occupy greater than about 5 percent and less than about 15 percent of the volume. [0017] In some implementations, the at least one control rod includes a substantially cylindrical housing, the substantially cylindrical housing defining the volume.

[0018] In certain implementations, the volume can be a constant volume up to at least about 100 atmospheres.

[0019] In some implementations, producing the neutron chain reaction includes producing more than about 50 percent of power of the nuclear reactor by neutrons with an energy of about 1 keV or greater.

[0020] In certain implementations, producing the neutron chain reaction includes producing less than about 50 percent of power of the nuclear reactor by neutrons with an energy of about 1 keV or less.

[0021] According to another aspect, a nuclear reactor can include a core, a plurality of fuel rods disposed in the core, a molten salt movable through the core to conduct thermal energy away from the plurality of fuel rods, a moderator material, the molten salt movable in the core between the moderator material and the plurality of fuel rods,, energy of neutrons from a neutron chain reaction of the plurality of fuel rods reduceable by the moderator material, at least one control rod defining a volume, heat and the neutrons from the neutron chain reaction passable into the volume via at least the molten salt, and an absorbent material in the volume, pressure of a vapor phase of the absorbent material increasable as heat passes into the volume, and at least a portion of the neutrons in the volume absorbable by the absorbent material in the vapor phase.

[0022] In certain implementations, the moderator material can be disposed about at least a portion of the at least one control rod, and heat and neutrons from the neutron chain reaction are passable into the volume via the molten salt and the moderator material.

[0023] In some implementations, the moderator material can be spaced apart from the at least one control rod, and the molten salt is movable in the core between the moderator material and the at least one control rod.

[0024] In certain implementations, the molten salt can have a macroscopic elemental absorption cross-section of greater than about 0.003 [1/cm] at about 0.025 eV.

[0025] In some implementations, the molten salt includes a nitrate salt. For example, the nitrate salt can be enriched with N-15 nitrogen.

[0026] In certain implementations, a combined temperature coefficient of the molten salt and the absorbent material is negative. [0027] In some implementations, the nuclear reactor can further include an insulator disposed about the at least one control rod, the insulator thermally conductively insulating the at least one control rod from the molten salt. For example, the insulator can include a sleeve disposed about the at least one control rod. Additionally, or alternatively, the insulator can include a vacuum sleeve, a gas sleeve, a solid sleeve, or a combination thereof. In certain instances, radiative heat can be passable into the volume at a rate greater than conductive heat is passable into the volume.

[0028] In certain implementations, the moderator material includes a metal hydride (e.g., zirconium hydride). Additionally, or alternatively, the moderator material can include one or more of hydrogen, beryllium, lithium, and carbon.

[0029] In some implementations, the plurality of fuel rods includes uranium.

[0030] In certain implementations, nuclei of the absorbent material are transmutable by absorption of the neutrons to form a stable isotope of the absorbent material. For example, the absorbent material can include mercury. Additionally, or alternatively, the absorbent material can include boron halide (e.g., boron iodide).

[0031] In some implementations, pressure of the vapor phase of the absorbent material in the volume is non-linearly increasable with an increase in temperature of the volume as heat passes into the volume. For example, pressure of the vapor phase of the absorbent material in the volume can double with about a 50 °C increase in temperature of the volume. In addition, or instead, the absorbent material in the volume can have a boiling temperature of greater than about 0 °C and less than about 600 °C at atmospheric pressure.

[0032] In certain implementations, the system can further include a second material (e.g., one or more of rubidium, sodium, and potassium) in the volume. The second material can have, for example, an elemental cross-section less than an elemental cross-section of the absorbent material such that the second material is relatively inert, as compared to the absorbent material, with respect to neutrons passing into the volume. Further, or instead, the absorbent material and the second material can have respective vapor pressures, the vapor pressure of the absorbent material varying with temperature according to a first rate, the vapor pressure of the second material varying with temperature according to a second rate, the first rate differing from the second rate over at least a portion of a temperature range from about 0 °C to about 1000 °C.

[0033] In some implementations, the volume can be substantially cylindrical. [0034] In certain implementations, the volume can be a constant volume up to at least about 100 atmospheres.

[0035] Implementations can include one or more of the following advantages.

[0036] In certain implementations, the nuclear reactor includes a molten salt. In general, the molten salt can be a useful alternative to lead-based or sodium-based coolants used in certain types of nuclear reactors. For example, as compared to lead-based or sodium-based coolants, the molten salt can offer advantages with respect to one or more of cost, corrosion, chemical reactivity, melting point, and operational challenges in certain types of nuclear reactor designs. Additionally, or alternatively, as compared to the use of a coolant (e.g., water) that has significant neutron moderation properties, the molten salt can be used as a coolant in a fast neutron reactor design, in which propagation of a neutron chain reaction is dependent upon fast neutrons.

[0037] In some implementations, the nuclear reactor includes an absorbent material in a volume defined by at least one control rod. The absorbent material can have a vapor phase having a pressure that increases with an increase of absorbed heat. Because the vapor phase of the absorbent material absorbs at least a portion of neutrons passing through the vapor phase and the pressure of the vapor phase increases with absorbed heat, the absorbent material can have a negative temperature coefficient. In certain instances, the negative temperature coefficient of the absorbent material can be useful for facilitating the use of a molten salt as a coolant in a nuclear reactor design in which a positive temperature coefficient of the molten salt is otherwise prohibitive. Thus, for example, the absorbent material can be placed in a position and in an amount suitable for achieving a reactivity temperature coefficient that is negative, neutral, or otherwise achieves a target value in a nuclear reactor cooled by a molten salt. Additionally, or alternatively, the absorbent material can facilitate the use of a molten salt as a coolant in a wider range of reactor designs to realize one or more of the advantages associated with the use of the molten salt as an alternative to other types of coolants such as, for example, lead-based or sodium-based coolants.

[0038] In certain implementations, the nuclear reactor includes an absorbent material in a volume defined by at least one control rod, with a temperature coefficient of the absorbent material in the volume being substantially self-regulating. As compared to temperature control based on active control of one or more actuators, the absorbent material in the volume defined by the at least one control rod can facilitate, for example, rapid and robust control of a nuclear reactor over power excursions.

[0039] Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE FIGURES

[0040] FIG. 1 is a schematic representation of a nuclear reactor.

[0041] FIG. 2 is a cross-section of the core of the nuclear reactor of FIG. 1, the cross- section taken along line A-A in FIG. 1.

[0042] FIG. 3 is a two-dimensional schematic representation of a fuel assembly of the core of FIG. 2.

[0043] FIG. 4 is a cross-section of a control rod of the fuel assembly of FIG. 3.

[0044] FIG. 5 is a graph of simulated normalized fission rates as a function of axial height of the nuclear reactor of FIG. 1 with and without the at least one control rod of FIG. 4.

[0045] FIG. 6 is a flow chart of a method of controlling a nuclear reactor.

[0046] FIG. 7 is a cross-section of a control rod of a fuel assembly with an insulator disposed about the control rod.

[0047] FIG. 8 is a graph of a simulation of macroscopic absorption cross-section as a function of energy for a nitrate salt and for a nitrate salt enriched with N-15 nitrogen.

[0048] FIG. 9 is a schematic representation of a vapor phase of an absorbent material and a second material mixed with one another in a volume defined by the at least one control rod of FIG. 4.

[0049] FIG. 10 is a graph of a simulated combined temperature coefficient of a molten salt and different compositions of a material contained in the control rod of FIG. 4, the simulated combined temperature coefficient shown as a function of temperature.

[0050] FIG. 11 is a two-dimensional schematic representation of a fuel assembly of a core.

[0051] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION [0052] The embodiments will now be described more fully hereinafter with reference to the accompanying figures, in which preferred embodiments are shown. The foregoing may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein.

[0053] All documents mentioned herein are hereby incorporated by reference in their entirety. References to items in the singular should be understood to include items in the plural, and vice versa, unless explicitly stated otherwise or clear from the text. Grammatical

conjunctions are intended to express any and all disjunctive and conjunctive combinations of conjoined clauses, sentences, words, and the like, unless otherwise stated or clear from the context. Thus, the term "or" should generally be understood to mean "and/or," and the term "and" should generally be understood to mean "and/or."

[0054] Recitation of ranges of values herein are not intended to be limiting, referring instead individually to any and all values falling within the range, unless otherwise indicated herein, and each separate value within such a range is incorporated into the specification as if it were individually recited herein. The words "about," "approximately," or the like, when accompanying a numerical value, are to be construed as including any deviation as would be appreciated by one of ordinary skill in the art to operate satisfactorily for an intended purpose. Ranges of values and/or numeric values are provided herein as examples only, and do not constitute a limitation on the scope of the described embodiments. The use of any and all examples or exemplary language ("e.g.," "such as," or the like) is intended merely to better illuminate the embodiments and does not pose a limitation on the scope of those embodiments. No language in the specification should be construed as indicating any unclaimed element as essential to the practice of the disclosed embodiments.

[0055] In the following description, terms such as "first," "second," "top," "bottom," "above," "below," and the like, are words of convenience and are not to be construed as limiting unless expressly stated otherwise.

[0056] The use of the term "temperature coefficient" should be understood in the context of temperature feedback in a control loop including a neutron chain reaction. Thus, for example, a negative temperature coefficient of an element or elements of a reactor should be understood to refer to a decrease in power of the reactor as the local temperature of the element or elements increases. Similarly, a positive temperature coefficient of an element or elements of a reactor should be understood to refer to an increase in power of the reactor as the local temperature of the element or elements increases. If left uncontrolled, such a positive temperature coefficient in an element or elements of a reactor can increase the likelihood of a runaway condition of the neutron chain reaction (e.g., with an increase in temperature of the element increasing the rate of the neutron chain reaction which, in turn, increases the temperature of the element, etc.).

[0057] The term "reactivity temperature coefficient," as used herein, is an overall temperature coefficient of the reactor. Thus, it should be understood that the temperature coefficient of each element can contribute to the reactivity temperature coefficient. For example, as described in greater detail below, an element with a positive temperature coefficient can be controlled through an arrangement of one or more elements with a negative temperature coefficient such that the reactivity temperature coefficient of the reactor is negative, neutral, or otherwise tuned to a desired reactivity temperature coefficient.

[0058] Referring now to FIGS. 1-4, a nuclear reactor 10 can include a core 12 and a cooling system 14 in fluid communication with the core 12. The core 12 can include a pressure vessel 16 in which one or more fuel assemblies 18 is disposed. Each fuel assembly 18 can include, for example, a plurality of fuel rods 20, at least one control rod 22, and a moderator material 24 disposed about at least a portion of the at least one control rod 22. The nuclear reactor 10 can be, for example, a fast neutron reactor in which a neutron chain reaction is propagated by fast neutrons moving through each fuel assembly 18. As used herein, a fast neutron reactor should be understood to include a nuclear reactor in which a majority (e.g., greater than about 50 percent) of power is produced by neutrons with an energy of about 1 keV or greater.

[0059] As described in greater detail below, the at least one control rod 22 can have a negative temperature coefficient that is self-regulating through power excursions of the nuclear reactor 10. As also described in greater detail below, the negative temperature coefficient of the at least one control rod 22 can be used in combination with a molten salt 26 (which can have a macroscopic elemental absorption cross-section of greater than about 0.003 [1/cm] at 0.025 eV and a positive temperature coefficient) to achieve an overall negative, neutral, or otherwise tune reactivity temperature coefficient of the nuclear reactor 10. More generally, the devices, systems, and methods of the present disclosure should be understood to facilitate the use of the molten salt 26 in fast neutron reactors, in which the positive temperature coefficient of the molten salt 26 would otherwise be considered unsuitable. Thus, the devices, systems, and methods of the present disclosure can facilitate using the molten salt 26 as an alternative to other types of coolants used in fast neutron reactors, such as lead-based and sodium-based coolants, which can offer significant advantages with respect to, for example, one or more of cost, corrosion, chemical reactivity, melting point, and operational challenges.

[0060] The core 12 can include, for example, an extrusion defining a plurality of substantially parallel orifices in which one or more of the plurality of fuel rods 20, the at least one control rod 22, and the moderator material 24 can be positioned. In certain instances, the core 12 can include an active fuel section of about 2 meters and a radial dimension of about 2 meters and, more generally, can be of a size sufficient to facilitate retrofitting existing reactor designs to replace light water reactors (-33 percent efficient) with more efficient fast neutron reactors (e.g., greater than about 40 percent efficient) cooled by a proven coolant such as the molten salt 26. Additionally, or alternatively, an outer portion of the core 12 can include a material that reflects neutrons such that the neutrons produced in the neutron chain reaction of the material of the plurality of fuel rods 20 remains substantially within the core 12.

[0061] The plurality of fuel rods 20 can include an actinide (e.g., uranium, plutonium, or a combination thereof). In certain implementations, the plurality of fuel rods 20 can include fuel in a pellet oxide form. Additionally, or alternatively, the plurality of fuel rods 20 can extend through at least a portion of each fuel assembly 18 such that the molten salt 26 flows through the core 12 in a direction substantially parallel to an axial direction of the plurality of fuel rods 20 (e.g., in a direction from the bottom of the core 12 to the top of the core 12).

[0062] In some implementations, the molten salt 26 can be solid at standard temperature and pressure and a thermally stable liquid at an operating temperature of the nuclear reactor 10. For example, the molten salt 26 can be in a thermally stable liquid phase at a temperature of greater than about 150 °C and less than about 525 °C. Additionally, or alternatively, the molten salt 26 can include one or more of a nitrate salt, a fluoride salt, and a chloride salt.

[0063] In use, the cooling system 14 can move the thermally stable liquid phase of the molten salt 26 into the core 12. For example, the cooling system 14 can pump the molten salt 26 through the core 12 at a rate based on one or more of an operating temperature of the core 12 and power output of the nuclear reactor 10. In certain implementations, the cooling system 14 can be a closed system such that the molten salt 26 is recirculated through the cooling system 14 during operation of the nuclear reactor 10. Further, or instead, the cooling system 14 can move the molten salt 26 through a heat exchanger external to the core 12. More generally, unless otherwise specified or made clear from the context, it should be understood that the cooling system 14 can be any one or more of various different cooling systems known in the art for moving a coolant through a core of a nuclear reactor.

[0064] In the core 12, the molten salt 26 can be in thermal communication with the plurality of fuel rods 20. For example, the molten salt 26 can be in thermally conductive communication with the plurality of fuel rods 20 through one or more thermally conductive layers between the molten salt 26 and the plurality of fuel rods 20. As a more specific example, each of the fuel rods 20 can be disposed in a sleeve (e.g., a steel sleeve) and the molten salt 26 can be in contact with the respective sleeve of each fuel rod of the plurality of fuel rods 20 to conduct heat away from the plurality of fuel rods 20. It should be appreciated that such a sleeve can be useful for protecting the material of the plurality of fuel rods 20 from direct contact with the molten salt 26.

[0065] The molten salt 26 can flow in each fuel assembly 18 to conduct thermal energy away from the plurality of fuel rods 20 in the respective fuel assembly 18. As the molten salt 26 conducts thermal energy away from the plurality of fuel rods 20, at least a portion of the neutrons generated by the nuclear chain reaction of the material of the plurality of fuel rods 20 can pass through the molten salt 26. Thus, more generally, at least a portion of the thermal energy and neutrons generated by the nuclear chain reaction of the material of the plurality of fuel rods 20 can move through the molten salt 26 and into the at least one control rod 22 via the moderator material 24.

[0066] In certain implementations, the density of the molten salt 26 can decrease as the temperature of the molten salt 26 increases. As the density of the molten salt 26 decreases, more neutrons can pass through the molten salt 26 which, in turn, can increase the rate of propagation of the nuclear chain reaction of the plurality of fuel rods and, thus, further increase temperature of the molten salt 26. Thus, in such implementations, the molten salt 26 should be understood to have a positive temperature coefficient. As described in greater detail below, the increase in neutrons passing through the molten salt 26 as temperature increases can be counteracted by a combination of the moderator material 24 and the at least one control rod 22, as described in greater detail below. [0067] The moderator material 24 can act as a flux trap for at least a portion of the neutrons generated in a neutron chain reaction of the plurality of fuel rods 20. That is, the moderator material 24 can absorb at least a portion of the energy of some of the neutrons to slow down the neutrons (e.g., through collisions with atoms of the moderator material 24). As described in greater detail below, in the context of a fast neutron reactor, slowing down at least a portion of the neutrons through the moderator material 24 can be useful for controlling an operating temperature of the nuclear reactor 10 through the use of the at least one control rod 22.

[0068] The moderator material 24 can be substantially stationary with respect to the plurality of fuel rods 20 and the at least one control rod 22. For example, the molten salt 26 can flow between the moderator material 24 and the plurality of fuel rods 20 as the material of the plurality of fuel rods 20 undergoes a neutron chain reaction. In certain implementations, the moderator material 24 can be disposed in a sleeve such that the molten salt 26 does not come into direct contact with the moderator material 24. Such isolation of the moderator material 24 from the molten salt 26 can be useful, for example, for prolonging the life of the moderator material 24 by reducing the likelihood of unintended reactions between the moderator material 24 and the molten salt 26. In such instances, it should be appreciated that the sleeve can be formed of one or more materials having a high thermal conductivity (e.g., steel) such that heat in the molten salt 26 is readily conducted into the moderator material 24 through the sleeve.

[0069] The moderator material 24 can be, for example, disposed among the plurality of fuel rods 20 in each fuel assembly 18. As a more specific example, the moderator material 24 can be positioned as pins spaced at regular intervals with respect to the plurality of fuel rods 20. More generally, the amount and position of the moderator material 24 in each fuel assembly 18 can be based on a variety of factors related, for example, to absorbing neutrons in the at least one control rod 22 to achieve a target reactivity temperature coefficient of the nuclear reactor 10.

[0070] The moderator material 24 can be any one or more of a solid, a liquid, or a gas confined to a respective pin of the moderator material 24 within the core 12. For example, the moderator material 24 can include any one or more of zirconium hydride, hydrogen, beryllium, lithium, and carbon. Additionally, or alternatively, the moderator material 24 can include a metal hydride. More generally, the moderator material 24 can be any one or more material suitable for absorbing energy from neutrons through collisions between atoms of the moderator material 24 and the neutrons. In certain implementations, the moderator material 24 can be formed of layers of different types of material.

[0071] In some implementations, the moderator material 24 can define an orifice to accommodate positioning the at least one control rod 22. Thus, for example, the at least one control rod 22 can be slid into the moderator material 24 as the core 12 is assembled. In general, for reasons associated with one or more of overall cost and system complexity, it can be advantageous to use the fewest number of the at least one control rod 22 to achieve a target reactivity temperature coefficient of the nuclear reactor 10. Thus, it should be appreciated that the at least one control rod 22 can be in less than all of the pins of the moderator material 24.

[0072] The at least one control rod 22 can define a volume 28. In general, an axial dimension of the volume 28 can be larger than a radial dimension of the volume 28. As an example, the volume 28 can be substantially cylindrical, which can, among other advantages, facilitate withstanding high pressures of gasses in the volume 28. In certain implementations, the volume 28 can be a constant volume up to at least about 100 atmospheres. Additionally, or alternatively, the axial dimension of the volume 28 can be substantially parallel to an axial dimension of the plurality of fuel rods 20 and an axial dimension of the moderator material 24. Such parallel positioning of the axial dimension of the volume 28 relative to the respective axial dimensions of the plurality of fuel rods 20 and the moderator material 24 can be useful for presenting a maximum area of the volume 28 as a target for neutrons moving from the plurality of fuel rods 20 through the moderator material 24.

[0073] In general, the at least one control rod 22 can be penetrable by conductive heat, radiative heat (e.g., photons in the form of gamma rays), and neutrons passing through the moderator material 24 such that the heat and neutrons can pass from the moderator material 24 into the volume 28. For example, the at least one control rod 22 can be formed of steel.

Additionally, or alternatively, the at least one control rod 22 can be formed of a material having a thickness greater than about 200 microns and less than about 1500 microns. Such a range of material thickness can, for example, provide sufficient structural strength of the at least one control rod 22 (e.g., to resist deformation under pressure of contents of the volume 28) while facilitating suitable passage of heat and neutrons into the volume 28 to achieve temperature control, as described in greater detail below. [0074] An absorbent material 30 can be contained by the volume 28 defined by the at least one control rod 22. Pressure of a vapor phase 30v of the absorbent material 30 can increase as heat passes into the volume 28, and at least a portion of the neutrons in the volume 28 are absorbable by the absorbent material 30 in the vapor phase 30v. Thus, an increase in

temperature in the volume 28 can lead to increased absorption of neutrons and, thus, a reduction in the rate of propagation of the nuclear chain reaction of the material of the plurality of fuel rods 20. Similarly, a decrease in temperature in the volume 28 can lead to decreased absorption of neutrons and, thus, an increase in the rate of propagation of the nuclear chain reaction of the material of the plurality of fuel rods 20. More generally, therefore, the absorbent material 30 should be understood to be a passive control mechanism with a negative temperature coefficient. As compared to actively controlled mechanisms (e.g., actuators, valves, etc.), the passive control achievable through changes in the vapor phase 30v of the absorbent material 30 in the volume 28 can, for example, provide advantages with respect to safe operation of the nuclear reactor 10 through excursions in power.

[0075] In general, the response of the vapor phase 30v of the absorbent material 30 to heat and neutrons in the volume 28 can be useful for overcoming certain constraints associated with the positive temperature coefficient of the molten salt 26. For example, the combined temperature coefficient of the molten salt 26 and the absorbent material 30 can be negative. In instances in which the temperature coefficient of the molten salt 26 is ordinarily positive, however, it should be appreciated that achieving the combined negative temperature coefficient of the molten salt 26 and the absorbent material 30 is based on balancing design tradeoffs associated with the nuclear reactor 10. For example, while the absorbent material 30 has a negative temperature coefficient, the vapor phase 30v of the absorbent material 30 absorbs neutrons that would otherwise be used to produce power and, thus, the absorbent material 30 can be associated with a slight power offset as compared to a reactor without the absorbent material 30. Thus, while the negative temperature coefficient of the molten salt 26 and the absorbent material 30 can be achieved through the use of a large volume of the absorbent material 30 relative to the molten salt 26, it is generally desirable to use as little of the absorbent material 30 as required to achieve the desired combined negative temperature coefficient of the molten salt 26 and the absorbent material 30. [0076] FIG. 5 is a graph of simulated normalized fission rates 51 and 52 as a function of axial height of the nuclear reactor 10 (FIG. 1). Referring to FIGS. 1-5, the simulated normalized fission rate 51 corresponds to a condition in which the at least one control rod 22 is inserted in the core 12, and the simulated normalized fission rate 52 corresponds to a condition without the at least one control rod 22 inserted in the core 12. The simulated normalized fission rates 51 and 52 were calculated with a three-dimensional fuel assembly model in a Monte Carlo nuclear reactor analysis code. The simulated normalized fission rates 51 and 52 correspond to fission rates of fuel rods of the plurality of fuel rods 20 next to the position of the at least one control rod 22 at a steady state operating condition of the nuclear reactor 10. As shown by a comparison of the simulated normalized fission rate 51 to the simulated normalized fission rate 52 in FIG. 5, the absorbent material 30 can be selectively positioned in the nuclear reactor 10 in a quantity that has little to no impact on the normalized fission rates of the plurality of fuel rods 20 under steady state operation of the nuclear reactor 10.

[0077] Referring again to FIGS. 1-4, the absorbent material 30 can have a boiling point in a range suitable for achieving an increase in vapor pressure of the vapor phase 30v of the absorbent material 30 at typical operating temperature of the nuclear reactor 10. Thus, for example, the absorbent material 30 can have a boiling temperature of greater than about 0 °C and less than about 600 °C at atmospheric pressure. The portion of the absorbent material 30 that is not in the vapor phase 30v can be, for example, in a liquid phase, a solid phase, or a combination thereof. Thus, for example, the pressure of the vapor phase 30v of the absorbent material 30 can increase through sublimation of a solid to a vapor. Additionally, or alternatively, the pressure of the vapor phase 30v of the absorbent material 30 can increase through evaporation of a liquid to a vapor.

[0078] In some implementations, the vapor phase 30v of the absorbent material 30 is within a pressure range of about 1 atmosphere to about 100 atmospheres in the volume 28. For example, the vapor phase 30v of the absorbent material 30 can occupy less than about 95 percent and greater than about 85 percent of the volume 28 over a pressure range of about one atmosphere to about 100 atmospheres. The presence of at least a portion of the absorbent material 30 in a non-vapor phase in the volume 28 can be useful, for example, for temperature control that can accommodate brief variations in conditions beyond intended design points. [0079] In certain implementations, nuclei of the absorbent material 30 can be transmutable by absorption of the neutrons to form a stable isotope of the absorbent material. The formation of such a stable isotope can, for example, facilitate accurate control of the temperature of the nuclear reactor 10. In some implementations, the absorbent material 30 can include mercury. In such instances, neutrons penetrating the at least one control rod 22 and entering the volume 28 can be absorbable by the absorbent material 30 to form a mercury isotope. Additionally, or alternatively, the absorbent material 30 can include a boron halide (e.g., boron iodide). In such instances, neutrons penetrating the at least one control rod 22 and entering the volume 28 can be absorbable by the absorbent material 30 to form lithium. Because the absorbent material 30 is transformed into a stable isotope over time, it should be appreciated that the at least one control rod 22 may need to be replaced periodically over the life of the nuclear reactor 10 to ensure that a sufficient amount of the absorbent material 30 is present in the volume 28 to achieve suitable temperature control of the nuclear reactor 10.

[0080] In certain implementations, pressure of the vapor phase 30v of the absorbent material 30 in the volume 28 can be non-linearly increasable with an increase in temperature of the volume 28 as heat (through conductive heat transfer, radiative heat transfer, or both) passes into the volume 28. Such a non-linear increase in pressure of the vapor phase 30v as a function of temperature can be useful for providing rapid control in a given temperature range. As an example, the pressure of the vapor phase 30v of the absorbent material 30 in the volume 28 can double with about a 50 °C increase in temperature of the volume 28.

[0081] FIG. 6 is a flow chart of an exemplary method 60 of controlling a nuclear reactor. Unless otherwise specified or made clear from the context, the exemplary method 60 can be carried out using any one or more of the corresponding devices and systems described herein. Thus, for example, any one or more of the various different implementations of the absorbent material 30 (FIG.4) can be used to carry out the exemplary method 60 unless otherwise indicated or made clear from the context.

[0082] The exemplary method 60 can include producing 62 a neutron chain reaction, passing 64 heat and neutrons from the neutron chain reaction through a molten salt, a moderator material, and into a volume defined by at least one control rod, and absorbing 66 at least a portion of the neutrons in a vapor phase of the absorbent material in the volume of the at least one control rod. The moderator material can be, for example, any of the various different moderator materials described herein and, in general, can decrease energy of the neutrons passing 64 through the moderator material to facilitate absorbing 66 at least a portion of the neutrons in the vapor phase of the absorbent material. Additionally, or alternatively, heat passing into the volume can increase pressure of the vapor phase of the absorbent material in the volume to increase the number of neutrons absorbed 66 in the vapor phase of the absorbent material. Because the absorption of the neutrons can reduce the rate of the production 62 in the neutron chain reaction, it should be appreciated that the increased absorption 66 of neutrons resulting from increased heat passed into the volume of the at least one control rod results in a negative temperature coefficient useful for controlling temperature of a nuclear reactor during, for example, power excursions.

[0083] In general, producing 62 the neutron chain reaction can include reacting fuel formed as a plurality of fuel rods. For example, a nuclide of the fuel can undergo a nuclear chain reaction in which a neutron interacts with the nuclide to form a compound nucleus (e.g., U-236), which can fission into two fission products to emit neutrons, photons (e.g., high energy photons such as gamma rays), and heat. The interaction of the neutrons with additional nuclides of the fuel can propagate a nuclear chain reaction, with the rate of propagation being a function of temperature.

[0084] Passing 64 heat and neutrons from the neutron chain reaction through a molten salt, a moderator material, and into the volume can be based on relative positioning of the molten salt and the moderator material relative to the plurality of fuel rods such that the passing 64 heat and neutrons from the neutron chain reaction occurs passively during operation of the nuclear reactor. In general, the amount of heat and neutrons passed 64 into the volume is a function of the temperature of the nuclear reactor. Because the temperature is a function of the rate of propagation of the nuclear chain reaction, the amount of heat and neutrons passed 64 into the volume should be understood to be a function also of the rate of propagation of the nuclear chain reaction. Thus, as the temperature of the nuclear reactor and, thus, the rate of propagation of the nuclear chain reaction increases for a given set of conditions, the amount of heat and neutrons passing into the volume increases.

[0085] As the amount of heat passing 64 into the volume increases, the vapor pressure of the vapor phase of the absorbent material in the volume increases. With this increase in vapor pressure, it is more likely that the neutrons passing 64 into the volume will be absorbed by molecules of the absorbent material in the vapor phase. The absorption of neutrons in the volume, in turn, can reduce the rate of propagation of the nuclear chain reaction, which has an associated reduction in temperature of the reactor. Thus, the absorbent material in the volume should be understood to have a negative temperature coefficient useful for controlling the temperature of the nuclear reactor, particularly given that the molten salt has a positive temperature coefficient. Further, or instead, because the negative temperature coefficient of the absorbent material is based on properties of the absorbent material itself, the negative temperature coefficient of the absorbent material should be understood to be rapid and self- regulating. Thus, as compared to controlling temperature coefficient of a nuclear reactor through the use of an actuator or other similar actively controlled component, controlling temperature of the nuclear reactor through the use of the absorbent material can offer significant advantages with respect to safe operation of the nuclear reactor.

[0086] While certain implementations have been described, other implementations are additionally or alternatively possible.

[0087] For example, while heat transfer into the volume of the at least one control rod has been described as occurring through conductive heat transfer and through radiative heat transfer, other configurations are additionally or alternatively possible. For example, referring to FIG. 7, an insulator 70 can be disposed about the at least one control rod 22. The insulator 70 can be, for example, a sleeve disposed between the at least one control rod 22 and the moderator material 24. In general, the insulator 70 can insulate the at least one control rod from conductive heat transfer from the molten salt (e.g., conductive heat transfer from the molten salt 26 in FIGS. 1 and 3). Additionally, or alternatively, high energy photons (e.g., gamma rays) produced by the neutron chain reaction of the material of fuel rods (e.g., the plurality of fuel rods 20) can penetrate the insulator 70 so that the absorbent material 30 in the volume 28 undergoes heating through radiative heat transfer. For example, the insulator 70 can facilitate heating the absorbent material 30 in the volume primarily through radiative heat transfer. Because conductive heat transfer from the fuel rods into the volume 28 can be significantly slower than radiative heat transfer from the fuel rods into the volume 28, conductive heat transfer into the volume 28 can, in some implementations, interfere with the control of temperature using the absorbent material 30 in the volume. Accordingly, the insulator 70 can facilitate rapid control of temperature of the nuclear reactor 10 (FIG. 1) as the nuclear reactor 10 undergoes rapid changes in operating conditions.

[0088] In general, the insulator 70 can have a thermal conductivity less than a thermal conductivity of one or both of the at least one control rod 22 and the moderator material 24. Thus, for example, the insulator 70 can include a vacuum sleeve. Additionally, or alternatively, the insulator 70 can include a gas sleeve. Further, or instead, the insulator can include a solid sleeve.

[0089] As another example, while the insulator has been described as being a discrete material, separate from the at least one control rod and the moderator material, it should be appreciated that other configurations are additionally or alternatively possible. For example, one or both of the at least one control rod and the moderator material can be formed of a low thermal conductivity material to insulate the volume 28 from thermally conductive heat transfer.

[0090] As yet another example, while the molten salt has been described as including, in certain implementations, one or more of a nitrate salt, a fluoride salt, and a chloride salt, other components can be additionally or alternatively added to the molten salt to achieve target performance parameters in a given reactor design. For example, FIG. 8 is a graph of

macroscopic absorption cross-sections as a function of energy. The macroscopic absorption cross-section as a function of energy is shown for a nitrate salt 81 and for a nitrate salt enriched with N-15 nitrogen 82. The macroscopic absorption cross-section values were calculated from the E DF/B-VII.1 ACE data, the Brookhaven National Laboratory National Nuclear Data Center, available at http://www.nndc.bnl.gOv/endf/b7. l/acefiles.html.

[0091] Nitrogen is naturally about 99.6% N-14 and about 0.4% N-15. The absorption cross-section of N-14 is much larger than the absorption cross-section of N-15. Accordingly, as shown in FIG. 8, enriching the nitrate salt with N-15 can substantially reduce the absorption cross-section of nitrate salt. This can be useful, for example, for reducing the number of control rods required to achieve suitable temperature control in a given reactor design. That is, the nitrogen-enriched nitrate salt can have a lower temperature coefficient than the unenriched nitrate salt and, thus, for a given reactor design, fewer control rods can be used to achieve a target reactivity temperature coefficient.

[0092] As still another example, while heat and neutrons have been described as interacting with the absorbent material in the volume, it should be appreciated that heat and neutrons from the neutron chain reaction can interact with additional materials in the volume. For example, referring to FIG. 9, the vapor phase 30v of the absorbent material 30 can be mixed with a second material 90 in a volume (e.g., the volume 28 of the at least one control rod 22 in FIG. 4). The second material can facilitate, for example, achieving a target reactivity

temperature coefficient of a nuclear reactor (such as the nuclear reactor 10 in FIG. 1). For example, the second material 90 can have an elemental cross-section that is less than an elemental cross-section of the vapor phase 30v absorbent material 30 such that the second material 90 is relatively inert with respect to neutrons passing into the volume 28 (FIG. 4).

Additionally, or alternatively, the absorbent material 30 and the second material 90 can have respective vapor pressures. As an example, the vapor pressure of the absorbent material 30 can vary with temperature according to a first rate, and the vapor pressure of the second material 90 can vary with temperature according to a second rate differing from the first rate over a temperature range from about 0 °C to about 1000 °C. Additionally, or alternatively, the second material 90 can include one or more of rubidium, sodium, and potassium.

[0093] FIG. 10 is a graph of the simulated temperature coefficient expressed as a function of temperature of various compositions of material contained in a control rod (e.g., the at least one control rod 22 in FIGS. 3 and 4). In some simulations, the control rod was placed in a reactor cooled by a molten salt (e.g., the molten salt 26 in FIGS. 1 and 3) and the simulated combined temperature coefficient was computed. The results shown in FIG. 10 are based on a simulation of the nuclear reactor 10 using a Monte Carlo reactor analysis tool. Multiple Monte Carlo reactor simulations were performed at varying power levels (i.e. varying temperatures and densities), and the calculated neutron multiplication factors from the simulations were used to determine the combined temperature coefficient of the molten salt and the control rod. In particular, FIG. 10 shows the influence of changing the nuclide composition of the absorbing material in the control rod. More specifically, FIG. 10 shows temperature coefficient as a function of temperature for a nitrate salt along curve 102, for a combination of Hg and nitrate salt along curve 104, for Hg along curve 106, and for ¹⁹⁹ Hg along curve 108.

[0094] As shown in the curve 108 corresponding to ¹⁹⁹ Hg in FIG. 10, the use of ¹⁹⁹ Hg in the at least one control rod can facilitate achieving a strongly negative combined temperature coefficient that decreases as the temperature increases. Given the variation of the combined temperature coefficient shown in FIG. 10, it should be appreciated that the combined temperature coefficient, and thus the reactivity temperature coefficient, can be tuned by adjusting the composition of the contents in the material in the at least one control rod. As a specific example, the combined temperature coefficient of the molten salt and the contents of the at least one control rod can be increased slightly through the addition of sodium.

[0095] As yet another example, while the nuclear reactors described herein have been described as fast neutron reactors for the sake of clarity of explanation, it should be understood that any one or more of the nuclear reactors described herein can be any of various different types of nuclear reactors, unless otherwise specified or made clear from the context. Thus, for example, referring again to FIGS. 1-4, the nuclear reactor 10 can be a thermal reactor or an epi- thermal reactor (e.g., a reactor in which a greater than about 50 percent of the powder is produced by neutrons with an energy of less than about 1 keV). More generally, one or more of the size, type, and position of the moderator material 24, and the at least one control rod 22, or a combination thereof can be varied to achieve neutron energy levels suitable for configuring the nuclear reactor 10 as a given type of reactor. That is, a thermal reactor configuration of the nuclear reactor 10 can generally include more of the moderator material 24 than a fast neutron reactor configuration of the nuclear reactor 10.

[0096] As a specific example, in implementations in which the nuclear reactor 10 is a thermal reactor, the molten salt 26 can be used as a coolant instead of water (which is commonly used as a moderator material in thermal reactors). Because water reduces neutron energy and, thus, acts as a moderator in a thermal reactor, it should be appreciated that replacing water with the molten salt 26 can require the use of a separate moderator material. For example, the degree of neutron moderation required to configured the nuclear reactor 10 as a thermal reactor can be achieved by using a larger amount of the moderator material 24, the at least one control rod 22, or a combination thereof, as compared to analogous amounts used in a fast reactor configuration.

[0097] As compared to water, the molten salt 26 has a higher boiling point. Accordingly, as compared to the use of water as a coolant in a thermal reactor, the use of the molten salt 26 in the nuclear reactor 10 configured as a thermal reactor can facilitate the use of a higher outlet temperature (resulting in higher efficiency) at lower pressures. By facilitating achievement of higher efficiency at lower pressures as compared to water, the use of the molten salt 26 in a configuration in which the nuclear reactor 10 is configured as a thermal reactor can have significant advantages with respect to safety, as compared to water. Given such advantages, it should be appreciated that it can be desirable to form the nuclear reactor 10, configured as a thermal reactor, through retrofitting a water-cooled thermal reactor.

[0098] As still another example, while fuel assemblies have been described as including a moderator material disposed about at least a portion of at least one control rod, other orientations of the moderator material relative to the at least one control rod are additionally, or alternatively, possible. For example, referring now to FIG. 11, a fuel assembly 18' can include the plurality of fuel rods 20, at least one control rod 22', and a moderator material 24'. The at least one control rod 22' can be separate from the moderator material 24' such that, for example, the molten salt 26 can move between the at least one control rod 22' and the moderator material 24' . Unless otherwise specified or made clear from the context, elements with primed numbers in FIG. 11 should be understood to be the same as corresponding unprimed element numbers described above. Thus, as a specific example, the at least one control rod 22' should be understood to be the same as the at least one control rod 22 described above with respect to FIG. 3, except that the at least one control rod 22' is spaced apart from the at moderator material 24' . Thus, more generally, it should be understood that fuel assembly 18' can be used

interchangeably with the fuel assembly 18 described above with respect to FIG. 3, unless otherwise specified or made clear from the context.

[0099] The above systems, devices, methods, processes, and the like may be realized in hardware, software, or any combination of these suitable for the control, data acquisition, and data processing described herein. This includes realization in one or more microprocessors, microcontrollers, embedded microcontrollers, programmable digital signal processors or other programmable devices or processing circuitry, along with internal and/or external memory. This may also, or instead, include one or more application specific integrated circuits, programmable gate arrays, programmable array logic components, or any other device or devices that may be configured to process electronic signals. It will further be appreciated that a realization of the processes or devices described above may include computer-executable code created using a structured programming language such as C, an object oriented programming language such as C++, or any other high-level or low-level programming language (including assembly languages, hardware description languages, and database programming languages and technologies) that may be stored, compiled or interpreted to run on one of the above devices, as well as

heterogeneous combinations of processors, processor architectures, or combinations of different hardware and software. At the same time, processing may be distributed across devices such as the various systems described above, or all of the functionality may be integrated into a dedicated, standalone device. All such permutations and combinations are intended to fall within the scope of the present disclosure.

[0100] Embodiments disclosed herein may include computer program products comprising computer-executable code or computer-usable code that, when executing on one or more computing devices, performs any and/or all of the steps of the control systems described above. The code may be stored in a non-transitory fashion in a computer memory, which may be a memory from which the program executes (such as random access memory associated with a processor), or a storage device such as a disk drive, flash memory or any other optical, electromagnetic, magnetic, infrared or other device or combination of devices. In another aspect, any of the control systems described above may be embodied in any suitable transmission or propagation medium carrying computer-executable code and/or any inputs or outputs from same.

[0101] The method steps of the implementations described herein are intended to include any suitable method of causing such method steps to be performed, consistent with the patentability of the following claims, unless a different meaning is expressly provided or otherwise clear from the context. So, for example, performing the step of X includes any suitable method for causing another party such as a remote user, a remote processing resource (e.g., a server or cloud computer) or a machine to perform the step of X. Similarly, performing steps X, Y and Z may include any method of directing or controlling any combination of such other individuals or resources to perform steps X, Y and Z to obtain the benefit of such steps. Thus, method steps of the implementations described herein are intended to include any suitable method of causing one or more other parties or entities to perform the steps, consistent with the patentability of the following claims, unless a different meaning is expressly provided or otherwise clear from the context. Such parties or entities need not be under the direction or control of any other party or entity, and need not be located within a particular jurisdiction.

[0102] It will be appreciated that the methods and systems described above are set forth by way of example and not of limitation. Numerous variations, additions, omissions, and other modifications will be apparent to one of ordinary skill in the art. In addition, the order or presentation of method steps in the description and drawings above is not intended to require this order of performing the recited steps unless a particular order is expressly required or otherwise clear from the context. Thus, while particular embodiments have been shown and described, it will be apparent to those skilled in the art that various changes and modifications in form and details may be made therein without departing from the spirit and scope of this disclosure and are intended to form a part of the invention.