Patent Application
20110091004
Projections by the Energy Information Agency and current Intergovernmental Panel on Climate Change (IPCC) scenarios expect worldwide electric power demand to double from its current level of about 2 terawatts electrical power (TWe) to 4 TWe by 2030, and could reach 8-10 TWe by 2100. They also expect that for the next 30 to 50 years, the bulk of the demand of electricity production will be provided by fossil fuels, typically coal and natural gas. Coal supplies 41% of the world's electric energy today, and is expected to supply 45% by 2030. In addition, the most recent report from the IPCC has placed the likelihood that man-made sources of CO2 emissions into the atmosphere are having a significant effect on the climate of planet earth at 90%. “Business as usual” baseline scenarios show that CO2 emissions could be almost two and a half times the current level by 2050. More than ever before, new technologies and alternative sources of energy are essential to meet the increasing energy demand in both the developed and the developing worlds, while attempting to stabilize and reduce the concentration of CO2 in the atmosphere and mitigate the concomitant climate change.
Nuclear energy, a non-carbon emitting energy source, has been a key component of the world's energy production since the 1950's, and currently accounts for about 16% of the world's electricity production, a fraction that could—in principle—be increased. Several factors, however, make its long-term sustainability difficult. These concerns include the risk of proliferation of nuclear materials and technologies resulting from the nuclear fuel cycle; the generation of long-lived radioactive nuclear waste requiring burial in deep geological repositories; the current reliance on the once through, open nuclear fuel cycle; and the availability of low cost, low carbon footprint uranium ore. In the United States alone, nuclear reactors have already generated more than 55,000 metric tons (MT) of spent nuclear fuel (SNF). In the near future, we will have enough spent nuclear fuel to fill the Yucca Mountain geological waste repository to its legislated limit of 70,000 MT. The commercial spent nuclear fuel destined for the repository is approximately 90% of this value, or approximately 63,000 metric tons of heavy-metal uranium (MTHM).
Fusion is an attractive energy option for future power generation, with two main approaches to fusion power plants now being developed. In a first approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ion beams, or pulsed power to rapidly compress capsules containing a mixture of deuterium (D) and tritium (T). As the capsule radius decreases and the DT gas density and temperature increase, DT fusion reactions are initiated in a small spot in the center of the compressed capsule. These DT fusion reactions generate both alpha particles and 14.1 MeV neutrons. A fusion burn front propagates from the spot, generating significant energy gain. A second approach, Magnetic Fusion Energy (MFE) uses powerful magnetic fields to confine a DT plasma and to generate the conditions required to sustain a burning plasma and generate energy gain.
Important technology for ICF is being developed primarily at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), assignee of this invention, in Livermore, Calif. There, a laser-based inertial confinement fusion project designed to achieve thermonuclear fusion ignition and burn utilizes laser energies of 1 to 1.3 MJ. Fusion yields of the order of 10 to 20 MJ are expected. Fusion yields in excess of 200 MJ are expected to be required in central hot spot fusion geometry if fusion technology, by itself, were to be used for cost effective power generation. Thus, significant technical challenges remain to achieve an economy powered by pure inertial confinement fusion energy.
According to the present invention, techniques related to nuclear engine fuel are provided. More particularly, an embodiment of the present invention provides an enhanced fuel particle suitable for use in a laser inertial confinement fusion-fission power plant. Merely by way of example, the invention has been applied to the design and fabrication of a robust tristructural-isotropic (TRISO) particle capable of high burn-up as well as a fuel pebble including a plurality of the robust TRISO particles. The methods and systems described herein are also applicable to other nuclear power plant designs.
According to an embodiment of the present invention, a fuel particle for use in a fusion-fission nuclear engine is provided. The fuel particle includes a fuel kernel and a buffer layer surrounding the fuel kernel. The fuel particle also includes a pyrolytic carbon layer surrounding the buffer layer and a silicon carbide layer surrounding the buffer layer. The silicon carbide is characterized by a stress less than 450 MPa at 95% burn-up. In this case, the intrinsic tensile strength of the SiC capsule is assumed to be approximately 450 MPa. Of course this value may change for different types of silicon carbide. As the strength increases above this level, the required wall thickness will decrease. As the strength decreases below this level, the required wall thickness will increase. The fuel particle further includes a second pyrolytic carbon layer surrounding the silicon carbide layer.
According to another embodiment of the present invention, a method of fabricating a fuel particle for a fusion-fission nuclear engine is provided. The method includes forming a fuel kernel and forming a buffer layer surrounding the fuel kernel. The method also includes forming a first pyrolytic carbon layer surrounding the buffer layer and forming a silicon carbide layer surrounding the first pyrolytic carbon layer. The silicon carbide layer is characterized by a thickness greater than 60 μm. The method further includes forming a second pyrolytic carbon layer surrounding the silicon carbide layer.
According to an alternative embodiment of the present invention, a fuel pebble for use in a fusion-fission nuclear engine is provided. The fuel pebble includes a plurality of fuel particles disposed in a matrix material. Each of the fuel particles includes a fuel kernel, a buffer layer surrounding the fuel kernel, and a pyrolytic carbon layer surrounding the buffer layer. Each of the fuel particles also includes a silicon carbide layer surrounding the buffer layer and a second pyrolytic carbon layer surrounding the silicon carbide layer. The silicon carbide layer is characterized by a thickness greater than 60 μm. The fuel pebble also includes a cladding layer enclosing the plurality of fuel particles and the matrix material.
Numerous benefits are achieved by way of the present invention over conventional techniques. For example, the present technique provides a robust fuel for Laser Inertial-confinement Fusion-fission Energy (often referred to herein as LIFE) nuclear engines that is able to achieve high (e.g., over 95% and up to 99.9%) burn-up of the fissile material in the kernel of the fuel. Additionally, fuel particles described herein provide a mechanism for disposing of weapons grade plutonium and highly enriched uranium. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more detail throughout the present specification and more particularly below.
These and other objects and features of the present invention and the manner of obtaining them will become apparent to those skilled in the art, and the invention itself will be best understood by reference to the following detailed description read in conjunction with the accompanying drawings.
According to the present invention, techniques related to nuclear engine fuel are provided. More particularly, an embodiment of the present invention provides an enhanced fuel particle suitable for use in a laser inertial confinement fusion-fission power plant. Such an engine is described in more detail in our commonly assigned copending U.S. patent application Ser. No. ______, entitled “Control of a Laser Inertial Confinement Fusion-Fission Power Plant,” filed contemporaneously with this application, the contents of which are incorporated by reference. Merely by way of example, the invention has been applied to the design and fabrication of a robust tristructural-isotropic (TRISO) particle capable of high burn-up as well as a fuel pebble including a plurality of the robust TRISO particles. The methods and systems described herein are also applicable to other nuclear power plant designs. Additional discussion related to nuclear fusion-fission engines is provided in U.S. patent application Ser. No. ______, entitled “Control of a Laser Inertial Confinement Fusion-Fission Power Plant” (Attorney Docket No. 027512-000400) and U.S. patent application Ser. No. ______ (Attorney Docket No. 027512-000700US), entitled “Solid Hollow Core Fuel for Fusion-Fission Engine,” the disclosures of which are hereby incorporated by reference in their entirety for all purposes.
The TRISO fuel particles described herein are designed to resist cracking, which results from stresses associated with processes such as fission gas pressure. An example of a reactor design in which TRISO fuel particles are utilized is the pebble bed reactor (PBR), in which thousands of TRISO fuel particles are dispersed into graphite pebbles. The PBR is a high temperature reactor.
The porous buffer layer not only serves to provide space for entrapment of fission gases, but may include sacrificial silicon carbide that can react with palladium and other fission products, thereby preventing these deleterious elements from reacting with the silicon carbide encapsulation shell 240. The sacrificial carbon can be formed as a layer or can be distributed throughout the porous buffer layer. In some embodiments, a zirconium carbide (ZrC) diffusion barrier is positioned surrounding the fuel kernel in order to prevent direct contact of fission products from the kernel with the SiC containment shell 240. This ZrC diffusion barrier could be positioned either on the inner surface of layer 230, the inner surface of layer 240, or both. Additionally, ZrC can serve as an oxygen getter to reduce the oxygen pressure due to the generation of free oxygen from, for example, UOC.
The silicon carbide sacrificial material and/or the zirconium carbide diffusion barrier may be formed, either as sequential single layers or as a multi-layer stack in which each of the layers, which may be referred to as sub-layers, is deposited one or more times in a periodic or non-periodic manner. Thus, for example, several layers of the zirconium carbide diffusion barrier may be deposited in conjunction with the other sub-layers to form the multipurpose buffer “layer” 220.
An inner high thermal conductivity (ρ) pyrolytic carbon (PyC) layer 230, a silicon carbide layer 240, an outer, low p PyC layer 232, and a protective layer of PyC 250 complete the structure of the fuel particle 200. The inner PyC layer 230 protects the SiC layer 240 by limiting the interaction between the SiC layer and the fuel kernel. The PyC layer 230 provides structural support to the SiC layer 240 in addition to reducing or preventing reactions between the metallic fission products and the SiC shell 240. The outer PyC layer 232 and the protective PyC layer 250 protect the particle. If a particle were to crack internally, these layers will serve to prevent the molten salt coolant from leaching radioactive materials, such as UOC, from the fuel particles.
The SiC shell 240 is preferably substantially thicker than corresponding layers found in conventional TRISO fuel particles. In an embodiment, the thickness of the SiC shell 240 (sometimes referred to as a containment shell or vessel) ranges from about 60 μm to about 200 μm. In a particular embodiment, the thickness of the SiC shell 240 is 120 μm. In another embodiment, the thickness is about 70 μm, about 80 μm, about 100 μm, about 120 μm, or thicker than 120 μm. In other embodiments, the thickness varies as appropriate to the particular application.
The SiC shell 240 serves as a pressure vessel to contain the gaseous and metallic fission products. Typically, the SiC layer is formed using a chemical vapor deposition (CVD) process, although other layer formation processes are included within the scope of the present invention. In embodiments of the present invention, the thickness of the SiC shell is increased in comparison to conventional particles and is sufficient to resist stress from the fission gases as they accumulate with the burning of the fuel kernel in the particle during a high burn-up fuel cycle. Without the protection provided by the SiC shell of the present invention, it is possible that fission gases can escape from the fuel particle and then circulate in the coolant loop.
In contrast with conventional fuel particles, the enhanced TRISO particles described herein are optimized to provide mechanical strength against cracking or failure of the particle during a high burn-up cycle. As an example, the burn-up may progress to 99.9% FIMA utilizing embodiments of the present invention. Additionally, the enhanced or high burn-up TRISO particles described herein provide a high mass fraction of fissile material. Thus, the design of the various materials and layer dimensions provides for a kernel of sufficient size, a buffer layer able to absorb fission gases and other byproducts produced during the high burn-up fuel cycle, a SiC containment shell able to withstand the fission gas accumulation pressure at high burn-up percentages (e.g., in excess of 95% FIMA), and the like.
In contrast with conventional TRISO particles, in order to achieve high burn-up, the inventors have balanced the strength of the TRISO particle to withstand high burn-up and the accompanying fission gas pressures against the mass fraction of fissile material in the particle and the fuel pebble. As an example, in an embodiment, the mass fraction of fissile fuel in the enhanced TRISO particle is increased as illustrated in
The fuel pebble includes a cladding layer 270 made of a material compatible with molten salt coolants, i.e., a material that is resistant to attack from the molten salt coolant utilized to remove heat from the fuel pebble. Tritium fluoride, which behaves like hydrofluoric acid is formed in the molten salt coolant as a result of the transmutation of lithium by the neutron flux. Therefore, the cladding layer 270 is incorporated into the design to provide resistance to attack by hydrofluoric acid. Accordingly, embodiments of the present invention utilize cladding layers including refractory metals such as tungsten and vanadium, refractory metal carbides, oxide-dispersion strengthened (ODS) ferritic steels, or the like. The fuel particles 200 are supported in the cladding 270 by an inert matrix material 280 such as graphite, zirconium carbide, ODS ferritic steels, or the like.
During typical use, the fuel particles and pebble described herein are only exposed to temperatures below the melting point of the fuel particles and pebble, typically between 500° C. and 750° C.
According to an embodiment of the present invention, the fuel pebbles are marked or encoded with a unique identifier, for example, a bar code, an individual number, or the like. This unique identifier can be used to individually track the fuel pebbles for accounting of the fuel. Because the fuel pebbles can be individually marked and tracked, diversion of large numbers of fuel pebbles is difficult. It should be noted that such individual marking and tracking is not generally possible with conventional TRISO particles. In addition each fuel pebble contains enough of the TRISO fuel particles to emit enough radiation to prevent manual removal without personal harm. Thus, the pebbles are self protecting. One fuel pebble emits more radiation than a convention fuel rod, yet to accumulate enough nuclear material to be of concern, on the order of 30,000 fuel pebbles need to be acquired. Therefore, an attempt to refine the fuel kernel from the fuel particles is a difficult task at best.
The uranium ore is pelletized (312) and dissolved in a broth, typically utilizing nitric acid (HNO3), Urea, or the like. The uranium broth is flowed through a drop column (316), washed in water (318) and alcohol (320). After these processes, the uranium fuel is kiln dried (322), sintered (324), calcined (326), tabled (328), and screened (330). The screening is the final step in the fabrication of the kernel in the embodiment illustrated in
The kernel, illustrated as element 210 in
In addition to providing an expansion volume for fission gases, the buffer layer can include a sacrificial silicon carbide material. For example, the sacrificial silicon carbide can react with palladium produced as a fission byproduct to form Pd5Si. Since the LIFE engine typically operates at a temperature of less than 800° C., the Pd5Si remains in a solid form and does not melt. Thus, even if the palladium gas reaches layer 240, it can react to form a stable, solid material. The consumption of the palladium in the buffer prevents the palladium from reacting with and thereby degrading the silicon carbide layer 240. Embodiments using a ZrC diffusion barrier prevent or reduce the ability of fission products to react with the SiC containment shell. The sacrificial SiC will react with palladium to form the high-melting 1:3:3:5 Pd:U:Si:C compound.
The fuel kernel and buffer layer combination are placed in a chemical vapor deposition (CVD) reactor to deposit the inner pyrolytic carbon layer (332). The CVD reactor may utilize a reduced or atmospheric pressure, plasma enhancement, or the like. The thickness and resistivity of the inner PyC layer are predetermined depending on the particular application. Either the same or a different CVD reactor is utilized 334 to form the silicon carbide layer 240. As illustrated in
Either the same or a different CVD reactor 332 or 336 is utilized to form the outer PyC layer 232. It should be appreciated that in some embodiments, the interfaces between various layers of the structure are not exposed to an ambient environment during the CVD process, improving the fuel performance. Thus, in some embodiments, a single CVD reactor is utilized with multiple gas sources. In other embodiments, multiple CVD reactors joined by a load-lock vacuum interface can be utilized to achieve results similar to those achieved with a single reactor. Thus, embodiments of the present invention utilize the equivalent to a single CVD process to form fuel particles that avoid interfacial problems. Additional protective PyC layers (e.g., layer 250) may be formed as appropriate.
Additionally, the inventors have determined that the anisotropic swelling of the inner and outer pyrolytic graphite layers 230 and 232, as well as the graphite binder in the pebbles, may adversely affect the lifetime of high burn-up TRISO fuels. It has been determined that the swelling of the lattice normal to the hexagonal planes (along the c-axis) is substantially greater than that parallel to the planes (along the a-axis). Thus, a material that swells isotropically, and to a lesser extent, is preferable. Thus, in some embodiments, the deposition processes utilized to form the PyC layers is modified to result in smaller grain sizes in the graphite, which can result in less anisotropy. Additionally, the inventors have noted that continuous growth processes tend to produce smaller grain sizes.
Once the particles with the outer PyC layer have been fabricated, they are tabled (338), screened (340), and put through elutriation columns (342) to purify and separate the particles on the basis of particle density. After elutriation, the particles are combined with a binder in a compaction press (344) and/or the inert matrix material 280. The compacted fuel particles/matrix material forms a portion of the fuel pebble, which is about 2-4 cm in diameter. The partially formed pebble is placed in a carburization furnace (346) and heat treated (348) before formation of the cladding layer (350). The cladding layer, which is typically a refractory metal, is resistant to dissolution in the molten salt coolants utilized in some engine designs.
It should be appreciated that the specific steps illustrated in
Similarly to the fuel particles, the stress in the cladding layer of the fuel pebble illustrated in
For both the fuel particles and the fuel pebble, the wall stress in the SiC layers is less than the published strength of SiC, which is about 450 Mpa. It should be noted that after the fuel pebbles are removed from the LIFE engine, at approximately year 40, the stresses in the SiC layers drop substantially during interim storage and repository conditions. Thus, by operating the LIFE engine at relatively lower temperatures (e.g., below 800° C.), the wall stress from fission gas accumulation remains significantly below the published strength of silicon carbide. Therefore, the fuel particles and fuel pebble provided by embodiments of the present invention prevent cracking of the SiC layers and thereby release of materials from the fuel kernel into the LIFE engine.
Embodiments of the present invention maintain the fuel particles and the fuel pebbles at temperatures less than about 1000° C., preferably in a range from about 600° C. to about 1000° C. Thus, by maintaining the fuel for the LIFE engine at relatively low temperatures (e.g., on the order of 800° C.), the swelling of the SiC remains below the non-saturable void-swelling regime. As illustrated by
Since the LIFE engine is designed to operate at high burn-up levels, the inventors have appreciated that conventional TRISO particles utilizing thin layers of SiC are unsuitable for long lived operation at the high fission gas pressures associated with LIFE engines. Based on the determination that failure of the SiC layer as a result of fission gas pressure makes conventional particles unsuitable, the inventors have developed new designs that provide long life operation at high burn-up levels.
In contrast with the conventional TRISO particle, the high burn-up TRISO fuel particles provided according to embodiments of the present invention are characterized by reduced wall stress as a function of burn-up in comparison with conventional designs. As shown in
Embodiments of the present invention are utilized in LIFE engines that avoid operating above the eutectic temperature of palladium silicide and similar compounds formed during fission of heavy elements in TRISO fuels. If the engine were operated at a temperature above the eutectic temperature, there is a propensity for some of the inner containment of the TRISO particles to liquefy as particular fission products are created by the neutron fluence. If liquefication occurs, the kernel of the TRISO particle can migrate within the particle to contact the silicon carbide shell. This will likely cause damage to the particle, and possibly damage to the pebble containing the TRISO particles.
Thus, the propensity of the silicon carbide to swell when irradiated is overcome by designing the LIFE engine to operate at a low enough temperature to avoid the non-saturable void-swelling regime as discussed in relation to
The inventors have determined that the enhanced TRISO fuels described herein overcome a number of challenges, both scientific and engineering, inherent in conventional fuel particle designs. For example, in some embodiments, the reaction of carbon and oxygen released from the kernel of the TRISO particle as a result of fission processes, can form carbon monoxide. Accordingly, in these embodiments, the production of CO is minimized by the use of a zirconium carbide oxygen getter, thereby capturing liberated oxygen in place of the formation of CO. Furthermore, by substituting UOC for UO2 in the fuel kernel, fuel kernel migration up the temperature gradient is minimized. The reduction in fuel kernel migration results in less interaction between the palladium produced by the fission processes and the silicon carbide containment layer and thereby prolongs the lifetime of the fuel particle, enabling high burn-up. Additionally, since UOC has one half the oxygen content of UO2, the amount of free oxygen generated during fission processes is approximately half for UOC fuels, reducing the reactions between oxygen and elements of the fuel particle.
Some embodiments provide sacrificial silicon carbide material in the vicinity of the fuel kernel, enabling the formation of the stable compound 1:3:3:5 U:Pd:Si:C. This stable compound has a melting point of approximately 1952° C. and serves to absorb palladium that would otherwise react with the silicon carbide containment shell. Since the LIFE engine is designed to operate at a lower operating temperature in order to avoid the non-saturable void-swelling regime, irradiation swelling of SiC and PyC layers is kept at acceptable levels.
Since the wall thickness of the SiC containment layer is thicker than in conventional designs, the wall stress in the SiC containment shell due to fission gas (e.g., Kr and Xe) pressure is less than the failure stress of the containment vessel. In order to reduce the impact of fission products attacking the SiC containment shell, a sacrificial SiC material is positioned close to the fissile kernel in some embodiments to provide a reactant to form the stable compound 1:3:3:5 U:Pd:Si:C. Additionally, in some embodiments a ZrC diffusion barrier can be utilized to prevent fission products from contacting the SiC containment shell.
The design of the various elements of the particle will result in a volume ratio between the fissile kernel and the fuel pebble. The latter designs illustrated in
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
The present application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/997,780, filed on Oct. 4, 2007, entitled “Hybrid Fusion-Fission Reactor,” and U.S. Provisional Patent Application No. 61/130,200, filed on May 29, 2008, entitled “Hybrid Fusion-Fission Reactor Using Laser Inertial Confinement Fusion,” the disclosures of which are hereby incorporated by reference in their entirety for all purposes.
The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/78336 | 9/30/2008 | WO | 00 | 1/10/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 60997780 | Oct 2007 | US | |
| 61130200 | May 2008 | US |
