The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear fuel arts, and related arts.
In a typical nuclear reactor, for example a pressurized water type reactor (PWR), a nuclear reactor core is disposed in a pressure vessel containing primary coolant (usually water). The reactor core generally includes a large number of fuel assemblies each of which includes top and bottom end fittings or nozzles with a plurality of elongated transversely spaced guide tubes extending longitudinally between the end fittings, and a plurality of transverse support grids (also called spacer grids) axially spaced along and attached to the guide tubes. Each fuel assembly includes a plurality of elongated fuel elements, also called fuel rods, transversely spaced apart from one another and from the guide tubes and supported by the transverse spacer grids between the top and bottom end fittings. The fuel rods each contain fissile material, and an array of such fuel assemblies are arranged to provide a radioactive nuclear reactor core with a designed volume of fissile material. The primary coolant flows upwardly through the core in order to provide heat sinking, and in so doing the primary coolant extracts heat generated in the core which can be used for the production of power. Various arrangements can be used to extract useful power from the heated primary coolant. For example, in a boiling water reactor (BWR) the primary coolant is allowed to boil and the primary coolant steam is piped out of the pressure vessel to drive a turbine. In PWR designs, the primary coolant remains in a subcooled liquid state and is piped out of the pressure vessel to boil secondary coolant in external steam generators, or alternatively a steam generator is disposed in the pressure vessel (i.e., an integral PWR) and the secondary coolant is piped into the internal steam generators.
In general, each fuel rod includes multiple nuclear fuel pellets containing fissile material loaded into a cladding tube, with end plugs secured to opposite (e.g., bottom and top) ends of the tube. It is possible for a nuclear fuel rod to generate temperatures higher than would be safe for the zirconium alloy lower end plug, potentially causing failure of the lower end plug and breach of the fuel rod. Traditional boiling water reactors (BWR) have kept temperatures lower at the bottom of the fuel through several techniques such as providing a 6 inch “blanket” of non-enriched fuel at the bottom of the fuel rods. Some BWRs also have control rods that enter the core from the bottom, which reduces power at the bottom of the core. Generally, such designs limit the maximum heat flux to less than 2 kw/ft, which prevents excessively high temperature at the lower end plug.
This approach is not applicable to PWR designs employing control rods entering from above the reactor core, such as a small modular reactor (SMR). Integral PWR designs are typically taller than traditional PWRs because the pressure vessel contains internal steam generators that add to the vessel height. Because of this, traditional BWR and PWR designs have a more mild axial shape at the bottom of the core than SMRs. One contemplated SMR design of the PWR variety has control rods that enter the core from the top in combination with fuel enrichments on the order of about 5% at the bottom of the fuel. It has been determined that this combination creates the potential for high heat flux at the bottom of the fuel. Analysis of anticipated rod pattern maneuvers suggests the potential for a heat flux as high as 9 kw/ft at the bottom of the fuel, resulting in temperatures in excess of 1400° F. Even during steady state operation, heat flux as low as 3 kw/ft would result in temperatures higher than the 750° F. design limit criteria.
Some PWR designs have employed a spacer between the fuel pellets and the lower end plug. These spacers are typically a solid cylinder of a ceramic material such as Al2O3, which is placed into the rod at time of fuel pellet loading. Because there are many fuel rods (e.g., more than one hundred rods per fuel assembly and 10,000 or more rods in the reactor core of some designs), there is a non-negligible likelihood that the spacer may be inadvertently omitted in one or more fuel rods, potentially resulting in fuel failure.
Disclosed herein is an approach that provides benefits such as reducing or eliminating the possibility of excess temperature on the lower end plug and reducing or eliminating the likelihood of human error in assembling the fuel rods.
In accordance with one aspect, a pedestal plug is sized to fit into a cladding of a nuclear fuel rod. A lower end plug is sized and shaped to plug the lower end of the nuclear fuel rod. One of the pedestal plug and the lower end plug includes a protrusion and the other of the pedestal plug and the lower end plug includes a hollow region into which the protrusion fits. In one embodiment the pedestal plug is a hollow cylindrical pedestal plug and the protrusion is disposed on the lower end plug. The protrusion disposed on the lower end plug suitably press fits into the hollow cylindrical pedestal plug.
In accordance with another aspect, a method of assembling a fuel rod of a nuclear reactor is disclosed. A pedestal plug and a lower end plug are connected. After the connecting, the lower end plug is welded to a cladding of the fuel rod with the pedestal plug disposed inside the cladding. In one embodiment the pedestal plug and the lower end plug are connected by press fitting a protrusion on one of the pedestal plug and the lower end plug into a hollow region of the other of the pedestal plug and the lower end plug. The method may further include loading fuel pellets comprising fissile material into the cladding of the fuel rod.
In accordance with another aspect, a lower end plug comprises a solid cylindrical element having a tapered first end and an opposite second end with a protrusion or blind hole surrounded by an annular surface of reduced diameter compared with the cylindrical portion of the lower end plug.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.
With reference to
With reference to
In the assembled lower end of the fuel rod 16 (shown in
The illustrative lower end plug 18 best seen in
The cylindrical portion of the lower end plug 18 is suitably of the same diameter as the outer diameter of the fuel rod cladding 13, so that the plugged lower end of the fuel rod (
In the illustrated embodiment of
In a suitable configuration the lower end plug 18 (
With reference to
One aspect of the disclosed lower fuel rod design that contributes to achieving this temperature reduction is the hollow center of the pedestal plug 10 (see
The disclosed configuration also has the advantage of reducing or eliminating the likelihood of human error in assembling the fuel rods. In existing designs that employ a “dummy” or low-enriched fuel pellet adjacent the lower end plug, this “spacer” is of similar size, shape, and appearance to the standard fuel pellets that are loaded into the fuel rod cladding. It is therefore possible to forget to load this dummy or low-enriched pellet, or to inadvertently load an enriched fuel pellet in place of the intended spacer. Since each fuel assembly typically includes dozens or hundreds of fuel rods, and the overall reactor core includes dozens or more fuel assemblies, the likelihood of such human error occurring is multiplied.
The disclosed approach prevents this possibility by connecting the pedestal plug 10 (
Another advantage is improved welding robustness. For good welding, it is best that the metal-metal contact of items next to the welding location be similar and consistent during the weld. While a separate spacer would be non-symmetric by having a metal-metal contact on one side due to gravity, the opposite side would have a wider gap. The disclosed configuration ensures non-contact for the full 360° rotation of the weld, resulting in improved weld consistency and predictability.
Another advantage is reduced manufacturing cost due to the geometry of the pedestal plug (standard cylinder, one centered through-hole, and chamfering). The pedestal plug 10 is expected to have a production cost well below that of a Al2O3 “dummy” spacer pellet, resulting in significant cost reduction. For a fuel assembly utilizing the pedestal plug, cost saving up to about 90% may be achieved over that of utilizing a Al2O3 “dummy” spacer pellet (estimated based on 2011 cost), thereby significantly reducing overall reload cost.
Another advantage is an increase in fuel rod plenum. During the irradiation process of a fuel rod, gases are produced within the fuel rod. These gases can limit the length of time a rod can be used. To address this problem, geometric voids in the fuel rod (sometimes known as plenum) are optionally added. Because the pedestal plug 8 is hollow (see
Another advantage is an increase in active fuel length and resulting reactor power. Because of effective temperature reduction, the pedestal plug 18 can be made shorter than a ceramic spacer pellet while still meeting thermal design criteria. In one alternative design, it is expected that an additional fuel pellet could be added to every rod in the core when the pedestal plug was about 3/16″ in length. This would result in an increase in uranium and several additional days of power on a multiple year fuel cycle.
Yet another advantage is improved material robustness and expected enhanced customer acceptance. The use of stainless steel as a reactor component has a proven track record for decades and is widely accepted as an allowed reactor component material, even in the fuel bundle.
With reference to
In both the illustrative embodiment of
The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
This application claims the benefit of U.S. Provisional Application No. 61/625,367 filed Apr. 17, 2012. U.S. Provisional Application No. 61/625,367 filed Apr. 17, 2012 is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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61625367 | Apr 2012 | US |