Patent Application
20090238321
The present invention relates generally to nuclear power plants, and more specifically to a nuclear power plant capable of processing actinides produced as byproducts from uranium based nuclear reactors.
U.S. Publication No. 2004/0022342 A1 discloses a method of incineration of minor actinides in nuclear reactors. The minor actinides to be incinerated, are embedded in at least one finite region of a core of a thermal reactor, wherein the finite region is isolated from the rest of the core by means of a barrier layer that absorbs thermal neutrons but is transparent to fast neutrons. This publication requires a common barrier layer of fissible material that isolates the finite “fast island” from the rest of a core. The publication discloses that the thermal reactor may be a pressurized water reactor, or a high-temperature-gas-cooled reactor. U.S. Publication No. 2004/0022342 A1 is hereby incorporated by reference herein.
“A Liquid-Metal Reactor for Burning Minor Actinides of Spent Light Water Reactor Fuel—I: Neutronics Design Study” by Hangbok Choi and Thomas J. Downar discloses a decoupled core with two zones: a minor actinide zone and a plutonium-enriched zone. The minor actinide zone was used to burn the minor actinides effectively using a hard spectrum, while the plutonium zone was introduced to compensate for the deteriorating safety performance due to heavy minor actinide loading.
Advanced gas-cooled reactors (hereinafter AGRs) typically use uranium as the fuel, graphite as the neutron moderator and carbon dioxide as coolant. Several Generation III reactors are also in development, including sodium-cooled fast reactors, gas-cooled fast reactors, lead-cooled fast reactors and molten salt reactors. These are typically envisioned as fast reactors.
Byproduct actinides are transuranic byproducts created from the use of the U235 thermal spectrum fuel cycle in currently deployed light water reactors (LWRs), and include americium 241 and neptunium 237.
In accordance with an embodiment of the present invention, a thermal nuclear reactor includes a coolant and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes a plurality of second fuel structures containing thermal spectrum driver fuel, the second fuel structures being less than one thermal neutron mean free path apart.
By having the plurality of second fuel structures at less than one thermal neutron mean free path apart, the second fuel structures can shield the inner region from thermal neutrons and thereby suppress the thermal neutron flux density in the inner region. The resultant high fast neutron density in the inner region can aid in burning the byproduct actinides in the first fuel structure, while the plurality of second fuel structures permits easy provision of the thermal driver fuel, for example in the form of fuel rods containing uranium.
In accordance with a second embodiment of the present invention, a thermal nuclear reactor includes a coolant including a molten salt or metal, and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes at least one second fuel structure containing thermal spectrum driver fuel shielding the first fuel structure from thermal neutrons.
By using a molten salt or metal, which have high thermal performance, as the coolant, the second thermal spectrum driver fuel can provide good thermal neutron shielding. For example, the at least one second fuel structure may include a plurality of fuel structures less than one thermal neutron mean free path apart, or be fashioned as a single plate or cylinder surrounding the inner region. By using the molten salt or metal, the core can operate at a high power density with a plurality of the fuel assemblies while retaining the advantageous safety properties of a thermal reactor.
According to a third embodiment of the present invention, a thermal nuclear reactor includes a coolant and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material, and separated solely from each other by the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes at least one second fuel structure containing thermal spectrum driver fuel shielding the first fuel structure from thermal neutrons.
By having the plurality of fuel assemblies located next to each other and separated solely by the moderator material, a new type of reactor can be created which permits substantial burning of byproduct actinides in the inner regions while still operating with the safety characteristics of a thermal nuclear reactor. Preferably, at least nine of the fuel assemblies are provided.
A nuclear power plant having a reactor of the types described above is also provided, and may include a heat exchange system removing heat from the coolant to power a generator.
The present invention also provides a fuel assembly including an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structures containing at least one byproduct actinide, and the outer region includes a plurality of second fuel structures containing thermal spectrum driver fuel, the second fuel structures being less than one thermal neutron mean free path apart.
Preferred embodiments of the present invention will be described with respect to the drawings in which:
Nuclear reactor 10 includes a containment wall 12, a core 20, circulators 14, 16, supports 22, 24 and coolant 18. Coolant 18 preferably is a high thermal performance coolant such as a liquid salt or metal. Heat is transferred from coolant 18 to heat exchange system 106 at a heat area 107 to turn water in cool leg 110 into steam. Circulators 14, 16 move coolant 18 through core 20.
Core 20 includes a moderator 30 and a plurality of fuel assemblies 40 located in a plurality of coolant channels 32 in moderator 30. Each fuel assembly 40 is placed inside a coolant channel 32 in moderator 30. Moderator 30 may be, for example, graphite, for example, in single block form or several pieces fitted together, for example, hollow hexagonals of graphite material. Coolant 18 is circulated through core 20 and moves through coolant channels 32 past fuel assemblies 40.
In
Fuel structures 50 in thermal driver fuel region 130 are in very close proximity to each other. The pitch ps of fuel structures 50 is shown in
The behavior of core 20 in reactor 10 can be driven by thermal driver fuel region 130, thus providing the safer control and transient behavior characteristics of a thermal spectrum critical reactor system, as opposed to those of third generation fast reactors. The fuel assembly pitch p and the moderator thickness t can be selected so the temperature coefficient remains safely negative, even though a plurality of fuel assemblies can be located next to each other and separated solely by the moderator material. At the same time, byproduct actinides can be burned. The arrangement of fuel structures 50 into thermal driver fuel region 130 and fast target fuel region 140 allows for optimal transuranic burning capabilities of minor actinides in region 140 while maintaining the safety and controllability characteristics associated with thermal neutrons in region 130.
Because the outer annuli of fuel structures in the
Neither coolant channel 32 nor annuli 60, 62, 64, 66, 68 need be cylindrical, for example, prismatic or other geometric shapes may be used, as long as the fuel is zoned radially from the center of fuel assembly 40 towards an outer peripheral region where moderator 30 is located.
In addition, having the outer region of the fuel assembly be separate structures less than one thermal neutron mean free path apart is advantageous as it permits easier manufacture of the fuel assembly and safer characteristics, it is also possible to fashion the outer region as a single plate or cylinder of thermal spectrum driver fuel surrounding the inner region.
In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.
