The present patent application relates to nuclear fission reactors and fuel assemblies, particularly for fast reactors, such as a traveling wave reactor. Fast reactors include a reactor vessel containing a reactor core. The reactor core includes a plurality of fuel assemblies. Liquid coolant passes through the reactor core, absorbing thermal energy from the nuclear fission reactions that take place in the reactor core. The coolant then passes to a heat exchanger and a steam generator, transferring the thermal energy to steam in order to drive a turbine that generates electricity.
Fast reactors are designed to increase the utilization efficiency of uranium in fission reactions. Fast reactors can capture significantly more of the energy potentially available in natural uranium than typical light-water reactors. Production of energy in the fast reactor core is intense because of the high-energy neutrons that are employed. However, the high burnup and energy intensity in fast reactors also stresses the structural materials in the fuel assembly to a greater degree relative to light-water reactors.
Fuel assemblies in fast nuclear fission reactor cores traditionally include a simple solid hexagonal tube surrounding a plurality of fuel elements, such as fuel pins. The tube directs coolant past the fuel pins, which are organized into a fuel bundle. The tube allows individual assembly orificing, provides structural support for the fuel bundle, and transmits handling loads from the handling socket to the inlet nozzle. Fuel pins are composed of nuclear fuel surrounded by cladding, which prevents radioactive material from entering the coolant stream. The coolant stream may be a liquid metal, such as liquid sodium.
The hexagonal tubes degrade and deform from exposure to high temperatures (e.g., 300° C. to 700° C.), intensive radiation damage, and corrosion and other chemical interactions with the liquid metal coolant. Several phenomena, including irradiation creep, void swelling, bowing, and dilation, cause tubes to deform. The interstitial gap between adjacent tube walls closes during fuel assembly service life. For high burnup assemblies, the lifetime of the assembly is limited by mismatch between fuel pin swelling and dilation, which either allows coolant bypass around the periphery of the pin bundle, or reduction of coolant channels within the assembly due to compression of the pin bundle by the tube wall.
Irradiation creep occurs as high-energy neutrons impinge on the tube and displace tube particles. Irradiation creep, duct dilation due to coolant pressure, and void swelling increase the diameter of the tube (i.e. cause expansion). Similarly, tubes may bow due to gradients in temperature, pressure, and radiation dose. Such gradients cause an imbalance in the macroscopic forces along the tube face. These problems, which warp and embrittle the tube structure, also increase the force necessary to withdraw fuel assemblies from the reactor, thus limiting the fuel assembly service life. Despite these deficiencies, hexagonal tubes continue to be used in fast reactors.
Under service conditions in high-burnup fast reactors, such as breed-and-burn reactors (of which one type is a traveling wave reactor (“TWR”)), a simple hexagonal duct may not be able to withstand duct wall pressure differential, void swelling, and/or subsequent irradiation induced creep. This could result in unacceptable duct face dilation and duct bowing, thereby resulting in a fuel assembly design life which might not be able to support the high burnup required to achieve an equilibrium breed-and-burn cycle with depleted uranium feed assemblies. The typical approach in a core restraint system is to manage local duct dilation by adding interstitial space between ducts to allow room for duct face dilation to occur. Additionally, to manage duct bowing caused by void swelling, the core restraint system utilizes three load planes—the inlet nozzle, the above-core load pad, and the top load pad—to permit irradiation creep to offset the effects of swelling induced fuel assembly duct bowing, yet provide space for the duct faces to dilate outward toward the adjacent duct faces.