Patent Application
20230411024
Various exemplary embodiments disclosed herein relate generally to preparation of nuclear fuel elements particles of a nuclear material evenly distributed in a matrix.
Generally, nuclear fuel elements consist of particles of a nuclear material distributed in a matrix. The nuclear fuel particles may be uranium, plutonium, or thorium compounds. In various embodiments, the nuclear fuel particles kernels may contain ceramic kernels of nuclear metals. In the case of uranium, such ceramic kernels may include uranium oxide (UO2), uranium oxycarbide (UCO), uranium carbide (UC2 or UC), or uranium nitride (UN). The nuclear fuel particles may contain bare kernels, or kernels which are coated with protective carbon or ceramic layers. In various embodiments, the nuclear fuel particles may contain kernels of a uranium ceramic compound, coated with protective ceramic or carbon layers.
The nuclear fuel elements may be tri-structural isotropic (TRISO) fuel particles. Such TRISO particles include multiple layers of various thicknesses and of different chemistries (carbon, SiC or ZrC). To produce TRISO particles, ceramic nuclear fuel kernels are sequentially coated with:
Nuclear fuel elements consist of coated or uncoated particles of a nuclear fuel kernel evenly distributed in a matrix. The matrix that surrounds the fuel may be graphite, a ceramic, such as SiC or ZrC, or a resin, such as a phenolic resin. Fuel elements may be shaped as spheres, cuboids, or cylinders. The fuel elements may be:
The fuel elements may include burnable poisons in the matrix material, or as distinct particles embedded within the matrix material. Such burnable poisons prevent criticality from excess nuclear fuel early in the life of the fuel element, while being consumed by neutron absorption as the nuclear fuel is consumed.
Nuclear fuel elements containing TRISO particles may be used to generate a nuclear chain reaction, where one single nuclear reaction causes an average of one or more subsequent nuclear reactions. A neutron multiplication factor, k, represents the average number of neutrons from one fission reaction that cause another fission, and is defined as:
In general, the value of k determines how the nuclear reaction proceeds. Specifically,
Since the total amount of fissionable material that is present within the matrix material of a nuclear fuel element affects the value of the multiplication factor k, a system for producing a fuel element with a predictable mass loading is desirable. The fuel kernels should be homogeneously distributed in radioactive fuel elements for uniformity.
In light of the present need for improved methods of providing nuclear fuel elements with controlled distribution of nuclear kernels, a brief summary of various embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various embodiments, but not to limit the scope of the invention.
Various embodiments disclosed herein relate to a method for producing a nuclear fuel element having a known volume of homogeneously distributed nuclear material, including steps of:
According to various embodiments disclosed herein, the mold is filled with a volume of particulate matrix material sufficient to fill any void space remaining after feeding the target number of nuclear fuel particles into the mold, so that a ratio of the volume of nuclear material in the nuclear fuel particles to the volume of solid matrix material in the fuel element is known.
In various embodiments, driving the conveyer may involve driving a vibratory conveyer having a conveyer surface running along the length of the channel to the exit; and a motor configured to vibrate the conveyer surface.
In various embodiments, the channel may have a tubular or semicylindrical conveyer surface running along the length of the channel to the exit, with a threaded auger therein. Driving the conveyer may involve rotating the threaded auger to drive particles within a helical thread of the auger along the conveyer surface to the exit. A motor is configured to rotate the threaded auger.
In various embodiments, the channel may have a sloped metal conveyer surface running along the length of the channel to the exit. The conveyer may be driven by gravity feed, a motor configured to vibrate the conveyer surface, or a combination thereof. If the conveyer is driven by gravity feed, stopping the conveyer may involve closing a gate at the channel exit. If the conveyer is driven by vibration, stopping the conveyer may involve stopping the motor.
In various embodiments, driving the conveyer may involve driving a conveyer having at least two rollers and an endless belt carried by the at least two rollers, the endless belt running along the length of the channel to the exit; and a motor configured to rotate the at least two rollers.
In the disclosed method, the optical counter may include:
In the disclosed method, the optical counter may include:
In the disclosed method, the optical counter may include:
In various embodiments disclosed herein, the step of converting the particulate matrix material into the solid matrix material may be performed by subjecting the nuclear fuel particles and the particulate matrix material within the mold to hot isostatic pressing, cold isostatic pressing, spark plasma sintering, or uniaxial pressing. The particulate matrix material within the mold may be graphite, phenolic resin, or a metal carbide, e.g., SiC or ZrC. The particulate matrix material within the mold may also include a polymeric binder and/or a burnable poison. Suitable burnable poisons include gadolinium, boron, hafnium, and/or compounds thereof.
Various embodiments disclosed herein relate to a system for producing a nuclear fuel element having a known amount of homogeneously distributed nuclear material, including:
In various embodiments of the system, the conveyer is a vibratory conveyer having a conveyer surface running along the length of the channel to the exit; and a motor configured to vibrate the conveyer surface. In some embodiments of the system, the conveyer has at least two rollers and an endless belt carried by the at least two rollers, the endless belt running along the length of the channel to the exit; and a motor configured to rotate the at least two rollers. In some embodiments of the system, the conveyer may include a threaded auger or a sloped metal conveyer.
The optical counter in the disclosed system may include:
The optical counter in the disclosed system may include:
Various embodiments disclosed herein relate to a method for producing a nuclear fuel element having a predictable multiplication factor k. The method involves feeding nuclear fuel particles having a known particle size along a channel having a conveyer configured to transmit the nuclear fuel particles to an exit. The conveyer is driven until a target number of nuclear fuel particles exits the channel through the exit. An optical counter is used to count the number of nuclear fuel particles which pass through the exit of the channel. The conveyer is stopped after the target number of nuclear fuel particles exits the channel, and a mold is filled with the target number of nuclear fuel particles and a particulate matrix material. During the filling step, the mold is vibrated so as to homogeneously distribute the nuclear fuel particles within the particulate matrix material. The particulate matrix material is converted into a solid matrix material. Since the nuclear kernels are evenly distributed within the matrix material, and have a similar particle size, the resulting fuel element has a minimized value of multiplication factor k.
In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein:
Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments.
As used herein, the term “about” encompasses the stated value “plus or minus 10%.” “Substantially,” as applied to a value, may allow a variance of up to 15%.
As used herein, the term “homogeneous distribution,” as it relates to particles in a matrix, means that the particles are evenly distributed through the matrix material, so that any two sections of equal volume through the matrix material have a substantially similar number of particles.
When discussing nuclear fuel particles, the term “kernel” relates to a radioactive ceramic particle. The term “particle” may relate to a kernel, or to a particle produced by coating a kernel with a carbon layer, a ceramic layer, or a combination thereof. The term “TRISO particle” relates to a particular class of coated kernels, which are sequentially coated with a porous carbon layer; an inner pyrolytic carbon layer; a ceramic layer, e.g., a metal carbide, oxide, or nitride layer; and an outer pyrolytic carbon layer.
In various embodiments, the present disclosure describes systems and methods for producing multiple nuclear fuel elements with an accurately determined amount of uranium in each fuel element. Each fuel element contains substantially the same number of nuclear fuel kernels as any other fuel element. Each nuclear fuel kernel has about the same volume and about the same mass as any other nuclear fuel kernel. Each nuclear fuel kernel has a mass within ±10% of a target mass M. The number of nuclear fuel kernel included in each fuel element is counted with an optical counter. Counting nuclear fuel kernels, where each kernel has a mass M±10%, until a target number of kernels N is reached, and then preparing fuel elements containing N kernels, results in fuel elements containing a predictable amount of nuclear material. The fuel elements are manufactured so that the kernels are homogeneously distributed within each fuel element. Each fuel element contains the same number of substantially identical kernels homogeneously distributed within a matrix. While the kernels may contain ceramic and/or carbon coatings, the mass of each kernel is substantially identical.
Conventional processes, in contrast, use nuclear fuel particles having a defined total mass, rather than a defined number of particles or a defined particle volume. In such a system, the number of particles cannot be readily predicted, as the particles may contain a mixture of small and large particles. Further, particles are frequently coated kernels, where the kernels are coated with a carbon layer, a ceramic layer, or a combination of carbon and ceramic layers. The mass of each particle contains a contribution from the kernel and from the coating layers, each with its own uncertainty. As a result, the error in determining the amount of nuclear material from measuring mass is higher than the error from counting a predetermined number of particles.
As discussed above, measuring an amount of nuclear material based only on mass may produce a fuel element with kernels having variable sizes. If the kernels have different surface area, i.e., if small kernels and large kernels are mixed, the total kernel surface area is reduced. Under these conditions, k-infinity is increased. The present disclosure describes a system for counting nuclear fuel kernels with substantially identical size and mass to produce fuel elements with a uniform kernel size and a reduced k-infinity. Various counting processes disclosed herein may be carried out very quickly, so measuring an amount of nuclear material based on particle count rather than particle mass improves accuracy without significantly sacrificing productivity.
In various embodiments, nuclear fuel pellets or particles 2 have a defined mean particle size and/or a defined particle size range. Based on such information regarding the size of particles 2, knowledge of the number of nuclear fuel pellets or particles 2 allows a good estimate of the amount of nuclear material present in the nuclear fuel element prepared in mold 7. Additionally, use of a known number of nuclear fuel pellets or particles 2 having a controlled mean particle size or particle size range provides a nuclear fuel element containing nuclear fuel pellets 2 with a controlled surface area. If all particles have a similar surface area, the total kernel surface area is maximized. Under these conditions, k-infinity, or a ratio of neutrons resulting from fission in a current generation to neutrons absorbed in a preceding generation in a system of infinite size, is a minimum.
To solve this, the system of
The term “control circuit,” as used herein, represents any type of information processing unit. The control circuit may be a central processing unit (CPU), external to the optical sensor, where which may communicate with the optical sensor through a wired or wireless communication network. The control circuit may be a microprocessor included within the optical sensor, specifically within sensor 6. The control circuit may be a logic circuit or logic gate included on an integrated circuit within sensor 6. The control circuit may be a combination of a logic gate included on an integrated circuit within sensor 6 and a CPU or microprocessor.
Each time a particle 2 passes through the beam from laser 5, the beam intensity is reduced and sensor 6 sends a signal to the control circuit 6a. Control circuit 6a records the number of signals received from sensor 6 as a count of the number of particles passing sensor 6. Once a target number of particles passes by sensor 6, the control circuit 6a sends a signal to motor 16, switching off the motor 16 and stopping the vibratory conveyer 4. This allows a precise number of nuclear fuel pellets or particles 2 to enter mold 7. The control circuit may be implemented using a logic gate implemented in an integrated circuit, or by using a CPU or microprocessor.
The control circuit 6a may be a sequential logic circuit implemented on an integrated circuit, which counts particles leaving channel 3 until the target number is reached, and then sends a signal shutting down motor 16. The logic circuit may be designed to reset the particle number to 0 upon sending the signal to motor 16.
The control circuit 6a may include a logic circuit implemented on an integrated circuit, and a CPU or microprocessor. The logic circuit sends a signal to the CPU or microprocessor each time a particle leaves channel 3, and the CPU or microprocessor counts the number of particles until the target number is reached, and then sends a signal shutting down motor 16. In various embodiments, the logic circuit may be a NOT gate which sends a signal each time a particle 2 passes through the beam from laser 5. In various embodiments, the logic circuit may be a two-input logic gate. For example, sensor 6 may be configured to detect both a reduction in laser intensity as a particle 2 passes through a beam from laser 5, and a time T until the laser reaches its original strength. This may be used to screen out phantom signals from momentary fluctuations in laser intensity. The two-input logic gate may, for example, be an AND gate, configured to sends a signal each time a particle 2 passes through the beam from laser 5, as determined when:
The control circuit 6a may include a CPU or microprocessor configured to record an output from sensor 6, count particles leaving channel 3 based on this output, and then send a signal shutting down motor 16 once the target number is reached.
In various embodiments, the optical sensor comprises a light source 5 and a sensor 6, where sensor 6 is a camera positioned at the exit from the conveyer. The camera is configured to transmit a first signal each time one of the nuclear fuel particles exits the channel. The optical sensor also includes a control circuit, configured to receive the first signal from the camera each time one of the nuclear fuel particles exits the channel, and calculate a number of nuclear fuel particles which exit the channel. The control circuit transmits a second signal to a motor driving the conveyer when the target number of nuclear fuel particles exits the channel, wherein the second signal stops the motor.
In various embodiments, the optical sensor comprises an LED as light source 5, and a camera as sensor 6. The camera is positioned at the exit from the conveyer, and is configured to record a sequence of images of a stream of particles exiting the conveyer. The camera is configured to sequentially transmit each image in the sequence of images to a control circuit. The control circuit is configured to sequentially analyze each image for dark spots, i.e., spots where brightness of the image falls below a threshold value. Each dark spot corresponds to a particle. The control circuit counts a number of particles in each image, and calculates a total number of nuclear fuel particles which exit the channel in the sequence of images. Once the total number of nuclear fuel particles exiting the channel reaches a target value, the control circuit transmits a signal to a motor driving the conveyer, wherein the motor stops the conveyer upon receipt of the signal from the control circuit.
In various embodiments, the control circuit is configured to analyze a diameter or area of each dark spot, generally corresponding to particle size. The control circuit may send an alert signal if a threshold number or percentage of particles falls outside a target size range.
Since the number of nuclear fuel pellets or particles 2 which enter mold 7 are known, and the mean particle size or the particle size range of nuclear fuel pellets or particles 2 is known, both the number of pellets 2 and a good estimate of the total volume of nuclear fuel material may be determined.
Finally, referring back to
In various embodiments, the mold contains a core element which is free of nuclear kernels prior to filling with nuclear fuel particles and matrix material. The nuclear fuel particles and matrix material are added to the mold so as to surround the core element, so that the final fuel element contains a zone which is free of nuclear fuel particles, and a zone containing a homogeneous distribution of nuclear particles within a matrix material. The core element may be cylindrical, spherical, or cuboid. The core element may have a hollow bore therethrough, so that a first zone containing a homogeneous distribution of nuclear particles exists within the hollow bore of the core element, and a second zone containing a homogeneous distribution of nuclear particles surrounds an outer surface of the core element.
Efficient distribution of fuel particles within the matrix is important, as k-infinity increases with:
Prior art procedures fill a mold for a nuclear fuel element with a known mass of uranium particles, rather than a known number of particles having a known particle size. Such systems may include a small number of large particles, which contribute disproportionately to the total mass, and a large number of small particles. Even when vibrating the mold during filling, such systems may produce a non-homogeneous distribution of uranium particles or pellets within the matrix material, due to the non-homogeneous particle size distribution.
Additionally, the method disclosed herein counts nuclear fuel kernels or coated nuclear fuel particles having a narrow kernel size distribution. Counting such particles produces an accurate total mass of fissionable material. Merely weighing particles, as in the prior art, is less accurate than counting particles because the particle size distribution may not be sufficiently controlled. Additionally, in the case of coated particles, weighing the mass of the coated particles to determine a target amount of fissionable material is inaccurate because of the uncertainty in kernel mass and the uncertainty in coating mass in each particle. TRISO particles and other coated particle fuels have considerable non-fissionable mass. Even if the total mass of a particle is known, the combined uncertainty in kernel mass and coating mass may leave substantial uncertainty in the kernel mass for each coated fuel particle.
In various embodiments disclosed herein, the average fissionable mass per kernel and the particle size per kernel are each known before any non-fissionable mass, e.g., coating layers, is added. The distribution of kernel size and/or kernel mass is very narrow, e.g., ±10%, so that the average value of kernel mass is very representative. In various embodiments, the number of fuel particles in each fuel element is determined by dividing a target fissionable mass per fuel element by the known fissionable mass per particle. This produces a total result which is more accurate than simply measuring the total mass of fuel particles. Since the mass of fissionable material per kernel is precisely known, a wider range of coated fuel particle sizes can be accepted while maintaining a high accuracy and precision on the fissionable mass loading of the fuel element.
When measuring coated particles by total mass, the number of kernels and the mass of fissionable material per kernel are not precisely known, and a narrower range of particle sizes can be accepted. For example, larger particles may be rejected on the grounds that they may have oversize kernels; some of these particles may simply have thicker coatings.
The particulate matrix material 9 is fed into mold 7 from hopper 10 until the mold is filled. Since the total volume of nuclear fuel material in the mold is known, the volume of matrix material in mold 7 is also known. This allows a determination of a ratio of the volume of nuclear fuel material to the volume of matrix material.
In various embodiments, nuclear fuel pellets or particles 2 should have a substantially uniform size. In the case of a TRISO particle, the mean kernel size is about 200 to 800 microns, 300 to 700 microns, or 350 to 500 microns in diameter, and the mean multilayer coated TRISO particle size is about 500 to 1500 microns, 600 to 1200 microns, or 800 to 1000 microns in diameter. If a set of particles is determined to have an unacceptably wide particle size range, e.g., unacceptably large particles 2a or unacceptably small particles 2b as shown in
In various embodiments, the particle size range may be narrowed with a roller sorter, as shown in
In various embodiments, the nuclear fuel kernels are sorted by roller sorting or screen sorting to produce kernels having a narrow size distribution. As a result, the mass of nuclear material in each particle is known. The kernels may then be coated with ceramic layers, carbon layers, or a mixture thereof. While coating may introduce some variation in total particle size, each particle has substantially the same kernel size. If desired, after coating the kernels, the coated particles may be sorted by screen sorting or roller sorting to produce particles having a narrow size distribution, providing nuclear fuel particles with:
If a mean diameter of a spherical uranium oxide kernel is known to be ˜500 microns, then the volume of uranium oxide in each kernel is ˜0.52 mm3. Based on an accurate count of the number of nuclear fuel pellets or particles 2, one knows the volume of uranium oxide in a mold 7 for a nuclear fuel element. Additionally, if the volume of uranium oxide, or of the volume of coated uranium oxide particles, e.g., the volume of TRISO particles, in the mold is known, then the amount of particulate matrix material 9 added to the mold 7 can be determined, allowing an accurate determination of the ratio of nuclear material to matrix material. Specifically, the volume of particulate matrix material added to mold 7 may be equal to the amount of void volume remaining in the mold after addition of the nuclear fuel pellets or particles 2 to the mold.
In various embodiments, the system of
In various embodiments, the system of
Once the mold 7 is filled with nuclear fuel pellets or particles 2 and a defined amount of a particulate matrix material 9, the contents are subjected to heat and or pressure to convert the particulate matrix material 9 into a solid matrix material 13, as shown in
After mold 7 is positioned in vessel 27, vessel 27 is filled with a liquid material 26, e.g., water, under high pressure through pipe 28. When the desired pressure is reached, valve 29 in pipe 28 is closed, and pressure from the high-pressure liquid 26 is applied to the contents of elastomeric mold 7 until particulate matrix material 9 sinters into a solid mass of matrix material 13. In some embodiments, the liquid material 26 may be heated, to allow application of both heat and pressure to mold 7. Use of a pressurized heated liquid may enhance the sintering process. Dry bag isostatic pressing procedures are known in the art, and may be used to form a nuclear fuel element.
After mold 7 is positioned in vessel 27, vessel 27 is filled with a gas 30, e.g., air, nitrogen, argon, or any other nonreactive gas, under high pressure through pipe 28. When the desired pressure is reached, valve 29 in pipe 28 is closed, and pressure from the pressurized gas 39 is applied to the contents of elastomeric mold 7. Also, gas 30 is heated to a sintering temperature. The hot pressurized gas 7 applies both heat and pressure to mold 7 until particulate matrix material 9 sinters into a solid mass of matrix material 13.
In the processes of forming a nuclear fuel element by hot or cold isostatic pressing, spark plasma sintering, and uniaxial pressing, the mold may be any desired shape, including spherical, cylindrical, or cuboid.
Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.
