This disclosure is directed to nuclear reactor control systems, including, but not limited to, small scale reactor units for space-based and terrestrial applications. In particular, this disclosure is directed to reflector-based nuclear reactor control systems, with improved neutron reflection and parasitic absorption properties.
One of the current challenges facing small-scale power applications is the need for an energy source capable of providing useful energy for the entire mission duration or product lifetime. Historically, radioisotope batteries have been used to provide load power in spacecraft, underwater systems, and remote scientific stations, but these systems are not capable of the load flexibility and higher power requirements that more advanced fission energy systems provide. To remedy this, many forays into nuclear powered spacecraft have been investigated, but no completely suitable, robust system for long-term high density power generation has been found.
Nuclear reactors rely on the process of nuclear fission to generate power. Protons and neutrons make up the nucleus, and are the building blocks for all nuclear reactions including not only radioactive decay and fission, but also fusion processes. More specifically, the number of protons in the nucleus determines the atomic number (that is, which element is present), while the number of neutrons determines the atomic mass of each specific isotope. While the physical and chemical properties of different isotopes may be similar, as determined by the atomic number of the element, the nuclear properties can be vastly different. Neutrons are also important to the fission process through their interaction properties, including elastic and inelastic scattering, and neutron absorption.
Neutron absorption causes the nucleus to become more energetic. The excited nucleus can achieve de-excitation by emission of a gamma ray (γ), which does not change the radioactive isotope, or through alpha (α) or beta (β) emission. In alpha emission, an alpha particle (a helium nucleus consisting of two protons and two neutrons) is ejected, lowering the nuclear energy state and reducing both the number of protons and the number of neutrons by two. In beta emission, a neutron (n) can be converted into a proton (p) by emission of positron (e+) and an electron neutrino (ve), or a neutron can be converted into a proton by emission of an electron (e−) and an antineutrino.
In special cases, unstable nuclei can reach a lower total energy state by splitting into two or more pieces or fission products. Fission processes can also emit additional neutrons, so there may be more free neutrons present at the end of a fission reaction than there were at the beginning. It follows that if a fission process occurs in a group of fissile atoms (say, in the middle of a fuel element), the emitted neutrons could repeat the process with other nearby atoms, starting a nuclear chain reaction.
In reactor theory, a fission system that creates exactly as many neutrons as it consumes is said to be exactly critical, and has a multiplication factor or “k value” of 1.0. If the reactor is consuming more neutrons than is creates, it is said to be subcritical and has k<1.0. Conversely, a supercritical reactor has a k>1.0, and creates more neutrons than it consumes.
Subcritical reactors do not sustain chain reactions, while supercritical reactors are difficult if not impossible to control. Thus, power reactor designs typically operate with a goal of approximately k=1.0.
Because not all neutron absorptions lead to fission, it is often beneficial to examine the number of neutrons emitted per absorption in the fuel or other reactor mass. This is commonly denoted as 11, and can be defined by the following equation, where the numerator and the denominator are the microscopic fission and macroscopic absorption cross sections, respectively:
The total absorption cross section σa for a particular fuel includes any reaction that leads to neutron absorption, regardless of outcome, including alpha, beta and gamma decays as well as the nuclear fission cross section σf. In order to maintain criticality η must be large enough to account for leakage and parasitic absorption, and materials with η≈1.5 or higher may be considered suitable for use as nuclear fuels.
Neutron scattering and absorption cross sections are highly energy dependent, and the cross section for fission may increase substantially at energies on the order of about 1 keV or less. This is well below the typical neutron emission energy of a few MeV or more, so moderators such as graphite or beryllium may be used to slow or thermalize the neutron flux to the energy range that would increase the fission process probability. This tends to increase the fission cross section, as compared to fast neutron reactor designs, but there also materials (e.g., U-238) which are only fissile at high neutron energy.
Neutron “poisons” with high absorption cross section may also be produced as fission products, or used to control neutron populations in the reactor core as (e.g., using a metalloid such as boron or gadolinium absorbers). Reactor control is thus a highly complex and challenging area of nuclear systems design, in which there is a constant need for more robust and responsive control systems. The need extends to small-scale, remote, and space-based applications, where maintenance and service requirements are major design considerations.
This application is directed to reactor control systems and methods. The reactor may include a reactor core with a fuel assembly disposed inside a core barrel. The reactor core generates a neutron flux, based on a fission reaction rate in the fuel assembly. A reactor vessel can be disposed about the reactor core, with a neutron reflector disposed between the outer surface of the active core (where the fission process occurs) and the inner surface of the reactor vessel, or between the outer surface of the core barrel and the inner surface of the reactor vessel.
At least one rotary control element can be disposed within the reactor vessel, for example a number of control drums disposed about the circumference of the reactor core, or a ring or annular reflector disposed coaxially about the reactor core. The control element typically includes a “primary” reflector material configured to reflect the neutron flux back toward the reactor core, and a “lesser reflector” disposed on a selected surface of the primary. The lesser reflector can be formed of a metal such as aluminum or steel, and may be borated so as to have a boron-10 concentration that decreases the absorption rate as a function of exposure to the neutron flux during the reactor lifetime.
The control system can be configured to regulate the neutron population inside the active core region and fission process by controlling reflected neutron flux back to the core based on rotation of the control drums or control ring, and thus to regulate the fission reaction rate within the reactor core. In particular, the reflected neutron flux depends upon the rotational or angular position of the control drums or control ring. In multiple drum embodiments, for example, the reflected neutron flux can be substantially greater with the primary reflectors disposed toward the reactor core, as compared to an angular position with the lesser reflector disposed toward the reactor core.
The reflected neutron flux also depends on the Boron-10 concentration, which decreases with exposure to the neutron flux. Thus, the reflected flux and reactor rate can also be controlled based on the burn rate of the borated lesser reflector, in order to maintain more efficient operation over the useful service life of the reactor system.
There are two approaches to regulating the neutron flux inside the reactor via leaking of neutrons from the active core region to the reflector. One approach is based on letting the neutrons stream through the reflector region to the outside of reactor region via void holes in the reflector, provided in different configurations and forms, which may create some radiation shielding problems. The second approach is directed to controlling the backscattering neutron population via absorption and/or by reducing the backscattering efficiency, similar to that of albedo boundary conditions, where less neutron current is reflected back to the active core region.
This disclosure encompasses the first option and the second option, which has advantages through the large attainable reactivity worth (and reactor safety), and better reactivity control during the core lifetime (including burnup of neutron absorbing material), and improving radiation protection for terrestrial applications. A combination of both approaches, is also a viable option in these designs, e.g., in rotating drum or ring configurations, by creating void channels through the borated aluminum (Al+B4C) with or without other reflector material (e.g., Pb-208+Al+B4C), all the way through the drum or other reflector element, providing a neutron path from the active reactor core to the outside of the reactor.
Reactor core 12 includes a number of fuel assemblies or block assemblies 18 formed of fuel blocks, fuel rods, fuel cells or other fuel elements, as described below, surrounded by a structural wall such as a core barrel 19 or thermal shield (or both). Reactor core 12 may also include additional neutron reflectors, moderators, absorbers and structural materials, and reactor system 10 may include a combination of active and/or passive fluid (liquid or gas) flow systems for heat extraction and cooling of reactor core 12. The heat can be utilized to generate power for external use, for example in a turbine-type generator operating on a Brayton or Stirling cycle, or via another thermodynamic or thermoelectric power generating process.
Reactor vessel 14 is typically formed of a structural metal such as stainless steel or aluminum. Depending on reactor size and environment, additional containment and shielding structures may also be provided, for example a single or double-walled pressure vessel, a reinforced concrete containment shell or other containment structure, or a combined pressure vessel and containment system.
The size scale of reactor system 10 and core 12 is determined based on fissile fuel type (e.g., enriched uranium, slightly enriched uranium, reprocessed uranium, highly enriched uranium, plutonium, or other actinide metal), fertile materials (e.g., thorium), and other design factors including active core radius, fuel spacing parameters such as fuel block flat-to-flat distance, and reflector thickness. Each of these parameters can be varied using a full model of the reactor core to determine power demands, service lifetime, and other operational considerations, until a suitable configuration is defined. The configuration of reactor system 10 may also depend upon a working model of the individual fuel elements or blocks, which is utilized to determine reaction products and reactivity trends that can affect reactor operation over the lifetime of core 12.
Suitable representative reactor power and energy control systems include, but are not limited to, those described in references 1-4, below, each of which is incorporated by reference herein. In particular, HTGR technology may be utilized with or without a thorium fuel cycle to design lightweight nuclear power sources capable of continuous electric power output of wide range from couple of kWe up 10 MWe or more, with long term operational periods of up to 15 years or more. In one such embodiment, the energy system may utilize a combination of fissile fuel (e.g., low and highly enriched uranium dioxide) and a fertile material (e.g., thorium carbide or natural UC), for example in a Tri-Structural Isotropic or TRISO fuel particle medium can be embedded in a graphite or beryllium oxide matrix (e.g., cylindrical pelts that forms hexagonal matrix block). As the primary fissile material is consumed in such a reactor system, the fertile material breeds new fissile fuel, resulting in a more steady fuel loading over the lifetime of the core.
Representative reactor designs may be selected based on reactor core and fuel block modeling, as described above, for example with a packing fraction of about 10-45% by volume fissile material 22 and about 10-50% by volume fertile material 28, or with a packing fraction of about 25±5% fissile uranium oxide and about 40±5% fertile thorium carbide. In particular examples, flat-to-flat distances of about 4 cm or below and up to about 10 cm or more may also be selected, in order to maintain a negative reactivity temperature component so that the fission reaction rate decreases with increasing core temperature for a given active core radius, e.g., about 50 cm or less, for example about 30 cm or less. Similarly, the active core height may be limited to about two meter or less, for example about 150 cm or less, or about 140 cm or less. At about 10 cm flat-to-flat distances, the reactivity temperature coefficient may become positive in some of designs, for example in a graphite moderated reactor core, which can be undesirable unless other methods are used to reduce the thermal neutron flux and resulting fission rate.
References. The following references are incorporated by reference herein, in their entirety and for all purposes:
Fuel 22 may be formed uranium, plutonium or mixed uranium/plutonium oxide, or another suitable fissile nuclear fuel such as a uranium-zirconium hydride (UZrH) material. Fuel 22 may be provided in pelletized, particle or microparticle form and stacked or poured along the central axis or centerline CL of fuel block 21, within a suitable high temperature cladding material 23 such as a nickel-based superalloy or zirconium alloy. Depending reactor design, additional fuel 22 or fertile material 28 may also be included in different locations within fuel block 21, for example a fertile thorium, uranium or plutonium isotope that is converted into a fissile material by neutron interactions.
Block or matrix material 24 can be selected for a combination of high temperature structural characteristics in combination with selected nuclear reflection, moderation, and absorption properties, for example graphite (C) or beryllium oxide (BeO). In the particular example of
Internal channels 26 are provided for cooling fluid flow, in order to achieve heat transfer from fuel block 21. Suitable cooling fluids include liquids and gases, for example an inert gas such as helium or argon, or a molten material such as a liquid fluoride salt. Depending on reactor design, other coolants such as water can also be used, and such materials may be selected for both cooling and neutron moderation properties. A neutron reflector or neutron absorber material 29 can be inserted into one or more internal channels 26, for example a pre-criticality safety rod made of boron carbide (B4C).
As shown in
In some reactor designs, a tristructural isotropic (TRISO) or other microparticle fuel may be utilized, for example in a high temperature gas cooled reactor (HGTR) or very high temperature reactor (VHTR) system, or an advanced heavy water reactor (AHWR) design. Depending on application, such microparticle fuels can be dispersed in a moderating matrix (e.g. matrix material 24), with a homogenized fissile and fertile material composition. Alternatively, the fissile and fertile materials can be provided in separate sub-elements, for example with “pins” or cylinders of fertile material 28 arranged about the central fissile cylinder 22 or “pin” as shown in
Fertile material cylinders 28 can also be either larger or smaller than the corresponding “pins” or cylinders of fissile material 22. Alternate configurations are also known, for example a simple stacked fuel rod assembly.
Individual fuel pellets 30 range in size, for example from about 300 μm or less to about 500 μm or more, or from about 500 μm up to about 1 mm or more. The particular size and layer configuration also depends on the selected fuel in kernel 32, and the corresponding fuel cycle and other operational criteria of the reactor energy system.
Suitable fuel kernels 32 include both fissile and fertile materials, for example fissile isotopes in the form of uranium oxide (UO2), uranium nitride (UN), or mixed uranium and plutonium oxide (MOX), or a fissile zirconium actinide alloy, as described above. Suitable fertile isotopes include uranium-238 and thorium-232, for example in the form of a uranium oxide, uranium carbide (UCx), thorium carbide (ThC) or thorium dioxide (ThO2).
Buffer layer 34 may comprise a relatively low density pyrolytic carbon or pyrocarbon buffer material, selected to provide thermal and mechanical stress relief when during production of fission gases and other fission processes in fuel kernel 32. Outer layer 35 provides structural integrity and containment, for example utilizing a silicon carbide (SiC) diffusion barrier layer 37, with inner and outer pyrolytic carbon layers 36 and 38, as shown in
Control system 40 includes a number of control drums 20, each having a major or primary reflector portion 42 made of a neutron reflector material and a control feature 44 made of a parasitic absorber or lesser reflector material to control the backscattering neutrons, or a combination thereof. In the low reaction rate or OFF position of
Depending on configuration, fuel assembly 18 may be formed of a substantially uniform array of individual fuel blocks 21, for example in a close-packed hexagonal configuration as described above. A number of peripheral blocks 46 may also be provided along the outer circumference of fuel assembly 18, for example using a moderator or other matrix material 24, or additional reflector material 16. Peripheral elements 46 can also include additional internal channels 26 for cooling fluid or heat exchange, or for the introduction of control rods or other absorbing or moderating components.
When lesser reflector 44 is rotated near or toward (proximate) reactor core 12, relatively more neutrons from fuel assembly 18 are absorbed in control drum 20 and relatively fewer neutrons are reflected back from control drum 20 toward fuel assembly 18. This reduces or limits the number of available fission neutrons within reactor core 12, decreasing the fission rate within fuel assembly 18 and reducing the power output of reactor system 10.
In some such configurations, neutrons from fuel assembly 18 are absorbed within (or not reflected back from) lesser reflector 44 of control drum 20, sufficient to reduce the fission reaction rate below the level of a sustained nuclear chain reaction within reactor core 12. This may be referred to as a subcritical configuration of reactor system 10, in which control system 40 is operated to substantially shut off or shut down reactor core 12.
In some configurations, sufficient neutrons are reflected back from control drums 20 toward reactor core 12 to allow a controlled chain reaction to proceed within fuel assembly 18. This may be referred to as a critical (or substantially critical) configuration, in which control system 40 is operated to start or turn on reactor core 12 in order to generate energy or extract power from reactor system 10. Control system 40 can also be operated to regulate the fission reaction rate within fuel assembly 18, in order to increase or decrease the power output from reactor system 10 while reactor core 12 remains in a substantially critical configuration.
The number and configuration of individual control drums 20 varies from design to design, along with the material composition, thickness, and other dimensions of neutron reflector 42. Suitable materials for neutron reflector 42 include, but are not limited to, beryllium oxide (BeO) and other neutron reflector materials such as beryllium, tungsten, tungsten carbide, lead, steel, and alloys thereof. Alternatively, materials with both neutron reflecting and neutron moderating properties may be utilized, for example graphite, or a combination of neutron reflecting and neutron moderating materials.
Lesser reflectors 44 are provided at one or more selected locations on or within outer surface 48 of control drum 20. Lesser reflectors 44 can be provided in discrete form, for example as chord-like shim 44A embedded within primary reflector 42, or in the form of a discrete layer 44B plated onto or embedded within outer surface 48. Alternatively, the material of lesser reflector 44 can be mixed homogenously into the material of reflector 42, at selected locations along outer surface 48.
Axially, lesser reflector 44 may extend substantially along the length or height of control drum 20, or along the corresponding axial length or height of reactor core 12. The angular extent of lesser reflector 44 along outer surface 48 of control drum 20 varies as a fraction of the total circumference, for example about ten to thirty degrees, about fifteen to twenty degrees, or about twenty degrees. In one particular example, lesser reflector 44 extends for about 18.75° along outer surface 48.
For reactor system 10 in the OFF (or reduced power) state, as shown in
The lesser reflector control option relies on a reduction in neutron reflection, or an increase in the average neutron mean free path in the reflecting region, in order to moderate the neutron economy. For this option, part of the primary reflector material 42 (e.g., BeO) in each control drum 20 is replaced with a control material 44 having lesser reflecting capabilities (e.g., aluminum, silicon, or steel). Suitable materials for lesser reflector 44 also include parasitic absorbers such as boron (B-10), boron carbide (B4C), gadolinium (Gd), and hafnium (Hf), and lesser reflective materials such as aluminum (Al), silicon (Si), as well as steel and stainless steel. A composition of neutron absorbing and a lesser neutron reflecting material may also be used, for example borated silicon, borated aluminum, or borated steel.
In borated materials, the boron component provides a “burnable” nuclear poison, in which the neutron absorbing isotope boron-10 is consumed over time by one of two different neutron capture reactions. These are alpha decay to lithium-7, or (less commonly) gamma decay to stable boron-11:
10B+n→
11B*→α+7Li. [2]
10B+n→11B*+γ. [3]
The lesser reflector material (e.g., aluminum, silicon, steel, or stainless steel) has a reduced cross section for elastic neutron scattering, as compared to lesser reflector 44 (e.g., boron, boron carbonate, tungsten, or tungsten carbonate). Thus, the effective neutron capture and neutron reflection cross sections of lesser reflector 44 vary over time, based on the integrated neutron flux, allowing control system 40 to provide improved power regulation and energy capability over the extended service lifetime of reactor system 10.
Based on
Aluminum (line 53) offers the largest reduction in cross section, as compared to beryllium (line 57). Both aluminum (line 53) and silicon (line 54), however, maintain a similar spectral shape, as compared to beryllium (line 57), and both aluminum (line 53) and silicon (line 54) offer almost an order of magnitude reduction in scattering cross section, over a wide range of incident neutron energies.
To determine suitable quantities of lesser reflector materials 44 in each control drum 20, reactor control system 40 was modeled in ON and OFF positions until the relative reaction rate or effective “multiplication factor” (keff) began to drop from an initial preselected threshold, for example about 1% excess radioactivity. The threshold was selected to determine a nominal proportion of lesser reflective material 44 that could be introduced into each control drum 20, without substantially changing the neutron properties of reactor core 12 and reactor system 10. In the particular configuration of
Generally, less aluminum (line 71) than silicon carbide (line 72) may be needed to provide a given control factor, because aluminum has a lower scattering cross-section. Thus, replacing the same proportion of beryllium (or BeO) scattering material with aluminum (Al) lesser reflector offers a greater reduction in neutron reflection, than may be achieved by replacing the same proportion with silicon (or silicon carbide).
The aluminum option can also provide for a greater control range and maximum power level, as shown in
To further increase the effective control range, a borated lesser reflector material may be used, as described below. For example, borated aluminum or silicon carbide may be used, with a trace amount of naturally occurring boron, for example less than about 5% or less than about 2% by weight. Borated steel and stainless steel may also be used, with similar trace boron content.
As shown in
For more general configurations,
Note that the numbers in
The maximum suitable ranges of boron content also vary, for example about 2% or less natural boron content by weight, or about 1% or less boron-10 atomic concentration. Alternatively, the upper bound is larger, for example about 1% or above about 2% for either or both quantities.
As shown in
In control systems utilizing boron absorbers, the amount of boron-10 available for absorption will decrease with core age, and this becomes an important operating consideration over the operating lifetime of the reactor. With typical core lifetimes of 10 to about 20 years, moreover, for example about 15 to 16 years or more, the remaining boron may become insufficient to adequately control the reactor, without taking boron depletion (or “burning”) into account.
The amount (N) of original boron-10 present at any given time (t) can be estimated from the original amount (No). Using the simple exponential depletion (or decay) equation:
N=N0 e−kt. [5]
The decay constant (k) depends on the total cross section for absorption (σa) and the neutron flux (φ). That is:
k=σaφ. [6]
Modeling the flux (φ) and total cross-section (σa) in each of the control designs, a maximum reactivity change or swing can be calculated as the difference in reactivity from the ON position to the OFF position, over the lifetime of the reactor power system. The reactivity worth can be determined from the reactivity swing, allowing different designs to be compared.
An additional option or alternative to borated Aluminum is to consider using a mixture of two or more of the following materials: lead (e.g., Pb-208) and/or lead borate, aluminum (Al) and/or borated aluminum, and boron carbide (B4C), in drums, reflectors or lesser reflectors with a different weight percentage of each material, for example from at least about 1-10% or more of each different material. As the boron-10 in the boron carbide or borated metal is depleted as a function of burnup or operational time (e.g., in effective full power days or EFPD), the Pb-208, Al and/or C components are left. The result is a significant improvement in backscattering of neutrons into the active core over the reactor lifetime, where the increased backscattered neutron flux provides for core life extension and more efficient fuel utilization. This design may also have a significant impact on the economics of the nuclear energy system, based on using Pb-208 in combination of borated Al as given below.
The effectiveness of using lead in the lesser reflector can be illustrated through examination of moderation, the neutron slowing process. Hydrogen has the highest moderating ability, with average logarithmic energy decrement ξ=1, where ξ=ln(Ei/Ef) and Ei/Ef is defined as the ratio of average initial neutron energy Ei to average final neutron energy Ef. This value is approximately six times higher than that of carbon and beryllium, and the number of collisions required to slow down neutrons from an average initial energy Ei of 1 MeV to an average final energy Ef of 0.5 eV is about N=14 in hydrogen, as compared to about N=92 in carbon. The carbon absorption cross-section, however, is about 82 times less than that of hydrogen. Therefore, the moderation ratio (MR=ξΣs/Σa) of hydrogen is comparable to that of carbon, where Σs is the macroscopic cross section for scattering, Σa is the macroscopic cross section for absorption, and MSDP=ξ×Σs is the macroscopic slowing down power.
1H
9Be
12C
208Pb
The neutronic characteristics of some light elements are given in Table 1 for comparison. Table 1 shows that the elastic cross section of Pb-208 is higher than for Be-9 and C-12, but about N=1514 collisions are required to slow neutrons with an initial average energy Ei of about 1 MeV down to a final average energy Ef of about 0.5 eV, due to the high atomic mass. This material is still more effective as a moderator than other solid materials, however, due to the very low absorption cross section of Pb-208 (about 0.23 mbarns).
As a result, Pb-208 (or naturally occurring lead containing Pb-208) can be a significantly better neutron reflector than graphite (C) or Be. Therefore, the overall lesser reflector effectiveness may be higher using either Pb-208 alone, or a combination of Pb-208 mixed with borated Al and/or other reflector materials such as boron carbide.
Reactivity worth plot 80 includes a boron carbide-only baseline option (no control drums, line 81), a void channel design (without lesser reflector, line 82), an aluminum lesser reflector (unborated, line 83), and a borated aluminum lesser reflector design (line 84). A silicon carbide lesser reflector is provided for comparison (line 85), along with an axially rotating reflector (without lesser reflector or boron absorber plugs, line 86 (see
Based on
The borated aluminum design (line 84) also loses some initial advantage in reactivity worth over time, but once the boron is substantially depleted (e.g., after about 3,000 to about 4,000 EFPD), the borated aluminum design and the “virgin” aluminum lesser reflector design (line 83) are substantially the same. Thus, at some point in time (e.g., after about half the expected reactor lifetime), the borated aluminum (line 84) and unborated aluminum (line 83) reactivity worth predications merge. The void channel and axially rotating reflector designs nominally have somewhat lower reactivity performance, but this may be improved with the use of additional lesser reflector materials, and other controller geometries.
From a thermal perspective, the goal of the reactor control system is to maintain system power over the greatest possible useful reactor lifetime, conserving the available fuel for the most efficient, long-term neutron production profile while controlling the reaction rate to prevent thermal damage to the reactor core and excess radioactivity exposure. In particular, reactor control parameters (e.g., drum or reflector position) must also take into account changes in the neutron spectrum of the reactor core over time, not only as a result of fuel depletion but also due to the production of neutron poisons and other fission products that absorb neutrons.
While these effects may require some reduction in the reflected neutron flux, particularly at beginning of lifetime (BOL) and in the first few years of reactor operations, there is a competing design constraint to avoid “wasting” neutrons, which could be used to generate power by inducing additional fission reactions. Control system operation thus also depends on the depletion of boron-10 and other absorbers in the control system, because the same control elements may provide more neutron absorption and less neutron reflectivity at beginning of lifetime (BOL), and less neutron absorption and more neutron reflectivity at end of lifetime (EOL). Thus, the rotational positions of the drums and other control elements will depend not only upon the evolving neutron production and fission reaction properties of the reactor core itself, but also rate of depletion or “burnup” of boron-10 in the borated lesser reflector, and in other boron components of the reactor control system.
When control drum reflectors 20 are rotated to the off position, as shown in
The geometries of individual channels 92 vary based on control system design. In one example, each channel 92 is formed as a void in primary reflector 42, for example with a substantially square, rectangular or circular cross section. A number of individual channels 92 can also be formed in each primary reflector or control drum 20, oriented in a parallel configuration and arranged in series along the rotational axis. In one particular application, a set of parallel circular channels 92 is formed in each control drum 20, for example with a diameter of about 2 cm, or about 10-20% of the diameter of control drum 20, and with a void fraction of about 10-20% or 15-20% of the drum volume, for example about 17%.
One or both of annular drum reflectors 94 and 95 are rotatable about the axis of reactor core 12. For example, inner annular drum or ring 94 may be considered a primary reflector, rotating coaxially within outer ring or secondary reflector 95. Alternatively, outer (secondary) reflector 95 may rotate coaxially about inner (primary) reflector 94, or primary reflector 94 may be disposed about secondary reflector 95.
In the OFF position of
In some embodiments, lesser reflector material 44 may also be provided, for example in selected regions of the inner circumference of outer annular reflector 95. This allows relative rotation of annular drum reflectors 94 and 95 to introduce either primary reflector 42 or lesser (secondary) reflector 44 into the neutron path, in order to further modulate the reflected neutron flux for improved reactor rate control. Alternatively, the relative leakage rate and reflected neutron flux can be regulated by partially aligning complementary inner about outer control channels 96 and 97, varying the neutron-induced fission rate in reactor core 12 according to the open cross-sectional area long each escape path S.
The relative size and configuration of annular drums 94 and 95 vary, along with the corresponding dimensions of radial channels 96 and 97. In particular, inner drum or ring reflector 94 may have a radial thickness of about 30-50% of the total thickness of neutron reflector 16, producing a radial split in the range of about 70/30 to 60/40 or 50/50, as defined by the ratio of outer and inner ring thickness, as a fraction of the whole thickness. Alternatively, inner ring reflector 94 may have a substantially smaller thickness than outer ring reflector 95, for example about 10-30% of the total thickness. Inner annular ring 94 may also have a greater thickness than outer ring reflector 95, in similar but complementary proportions.
Inner and outer channels 96 and 97 can be formed as voids within primary reflector material 42, for example with a circular, square, rectangular, hexagonal or other cross sectional geometry, as described above. In addition, a number of generally parallel radial channels 96 and 97 can be distributed along the axial length or height of each annular reflector 94 and 95, in either a staggered or aligned (“stacked”) configuration.
Inner control channels 96 can also be somewhat larger or smaller in cross sectional area than outer channels 97, in order to make angular alignment relatively easier, or to provide for more precision in relative angular positioning. The diameters of individual channels 96 and 97 also vary, for example from about 35-50% or less of the radial thickness of the corresponding annular reflectors 94 and 95, to about 70-150% or more. In an aligned configuration, the neutron leakage or “escape” paths formed along control channels 96 and 97 may approach up to 10-20% or more of the total reflector volume, or of the total surface area of reactor core 12.
As shown in
Neutron absorber plugs 98 can be positioned in the “safe shutdown” configuration of
Depending on void proportion and control channel configuration, neutron absorber plugs 98 can also be positioned to reduce the likelihood accidental water entry into inner control channels 97, which could result in a critical event due to the reduced neutron leakage rate and water's neutron moderating properties. Neutron absorber plugs 98 can thus be provided to reduce the likelihood of reactor accident during a forced water re-entry, or accidental immersion of reactor system 10.
The operating configuration of
In the low power or OFF configuration of
Note that the rotational positions of individual control drums or reflectors 20 can be the same or individual controlled, for example in the event reactor core 12 has non-uniform thermodynamic performance, or when another asymmetric control configuration is required. Control system 40 can also be configured to rotate individual drums 20 to accommodate a “stuck rod” (or stuck drum) event, in which one or more control drums 20 cannot be rotated.
In this configuration, the other (functional) control drums 20 can be individually positioned to increase or decrease the fission rate accordingly, for example to reduce the fission rate in sections of reactor core 12 adjacent to a control drum stuck in the ON position, in order to avoid thermal damage to fuel assembly 18. Alternatively, the other (functional) control drums 20 can be individually rotated to increase the neutron flux in sections of reactor core 12 adjacent to a control drum stuck in the OFF position. The materials of primary reflector 42 and lessor or secondary reflector 44 can also be selected for substantially uniform density, in order to provide rotational balance and reduce vibrations and asymmetric torque during rotation of control drums 20 (or annular drums 94 and 95, see
Reactor control is thus achieved through an innovative approach to the conventional boron carbide neutron absorber, for example by utilizing sections of borated aluminum placed in rotating control drums within the reactor. Borated aluminum allows for smaller boron concentrations, reducing or substantially eliminating the potential for 10B(n,α)6Li reactions and other heating issues, which are common in other (e.g., boron carbide) systems. A wide range of other reactivity control systems are also encompassed, such as a radially-split rotating reactor and the other reactor configurations described herein.
Extensions to both space-based and terrestrial energy systems are also encompassed, for example with uranium enrichment dropped by up to 20% or more in order to meet regulatory and/or design requirements. A solid uranium-zirconium hydride fissile driver may also be used in place of the uranium dioxide or TRISO fuel particles, and a graphite moderating material can also be employed, as an alternative to beryllium oxide. The core size may also be increased, while maintaining or increasing long-term power generation potential. Small amounts of erbium can also be added to the hydride matrix, in order to further extend core lifetime.
While this invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes can be made and different equivalents may be substituted for particular elements thereof, without departing from the spirit /and scope of the invention. The invention is thus not limited to the particular examples that are disclosed, but can also be adapted to different problems and situations and applied to different materials and techniques, without departing from the essential scope of embodiments encompassed by the appended claims.
This application claims the benefit of priority pursuant to 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/977,375, filed Apr. 9, 2014, which is hereby incorporated by referenced in its entirety.
Number | Date | Country | |
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61977375 | Apr 2014 | US |