Patent Application
20090022259
The invention relates generally to fuel elements for nuclear reactors, and more particularly to braces, spacers, or support lattices which are usually placed at predetermined distance(s) along the length of the fuel elements in order to provide lateral bracing and spacing, and to maintain the fuel elements in fixed positions.
Boiling water nuclear reactors operate for many years. Commencing with their initial construction and through their service lives, these reactors may accumulate debris in their closed circulation moderator systems. This debris can become an operating hazard if the debris is allowed to enter into the fuel bundle containing core region having the heat generating fuel rods. In order to understand this problem, a summary of reactor construction as it relates to the accumulation of debris in the core needs first to be given. Thereafter, fuel bundle construction will be set forth. Emphasis will be given to the need to preserve substantially unchanged the regions of pressure drop within the fuel bundles. Thereafter, the effects caused by debris entering into the fuel rod region of the fuel bundles will be summarized.
Boiling water nuclear reactor construction can be simply summarized for purposes of understanding the debris entrainment problem. Such nuclear reactors are provided with a large, central core. Liquid water coolant/moderator flow enters the core from the bottom and exits the core as a water steam mixture from the top.
The core includes many side-by-side fuel bundles. Water is introduced into each fuel bundle through a fuel bundle support casting from a high pressure plenum which is situated below the core. Water passes in a distributed flow through the individual fuel bundles, is heated to generate steam, and exits the upper portion of the core as a two phase water steam mixture from which the steam is extracted for the generation of energy.
The core support castings and fuel bundles are a source of pressure loss in the circulation of water through the core. This pressure loss assures the substantially even distribution of flow across the individual fuel bundles of the reactor core. When it is remembered that there are as many as 750 individual fuel bundles in a reactor core, it can be appreciated that assurance of the uniformity of flow distribution is important. To interfere with the pressure drop within the fuel bundles could affect the overall distribution of coolant/moderator within the fuel bundles of the reactor core.
Having set forth the construction of the boiling water nuclear reactor in so far as is appropriate; attention can now be directed to the construction of the fuel bundles themselves.
The fuel bundles for a boiling water nuclear reactor include a fuel rod supporting lower tie plate assembly, in which the lower tie plate is a cast structure. The lower tie plate assembly includes at its lowest point a downward protruding bail covering an inlet nozzle. This inlet nozzle includes entry to an enlarged flow volume within the lower tie plate. At the upper end of the flow volume, there is located a rod supporting grid. Between the supporting grid and the nozzle there is defined a flow volume.
The rod supporting grid has two purposes. First, the rod supporting grid provides the mechanical support connection for the weight of the individual fuel rods to be transmitted through the entire lower tie plate to the fuel support casting. Secondly, the rod supporting grid provides a flow path for liquid water moderator into the fuel bundle for passage in between the side-by-side supported fuel rods.
Above the lower tie plate, each fuel bundle includes a matrix of upstanding fuel rods, which are sealed tubes each containing fissionable material which when undergoing nuclear reaction produce the power generating steam. At the upper end of the matrix of upstanding fuel rods is located a so-called upper tie plate. This upper tie plate holds at least some of the fuel rods in vertical side-by-side alignment. Some of the fuel rods are attached to both the upper and lower tie plates. Between the upper and lower tie plates, there are usually included water rods or equivalent devices for improvement of the water moderator to fuel ratio, particularly in the upper, highest void fraction region of the fuel bundle.
Fuel bundles also include about seven fuel rod spacers at varying elevations along the length of the fuel bundle. These spacers are required because the fuel rods are long (about 160 inches) and slender (about 0.4 to 0.5 inches in diameter), and would come into contact under the dynamics of fluid flow and nuclear power generation within the fuel bundles. The spacers provide appropriate restraints for each fuel rod at their respective elevations and thus prevent abrading contact between the fuel rods and maintain the fuel rods at uniform spacing relative to one another along the length of the fuel bundle for optimum performance. As will hereafter be developed, these spacers are sites where debris can be trapped and damage the fuel rods.
Each fuel bundle is surrounded by a channel. This channel causes water flowing between the tie plates to be restricted to only one bundle in an isolated flow path between the tie plates. The channel also serves to separate the steam generating flow path through the fuel bundles from the surrounding core bypass region, this region being utilized for the penetration of the control rods. The water in the bypass region also provides neutron moderation.
In the operation of a boiling water nuclear reactor, it is important to understand that the maintenance of the originally designed flow distribution is important. Specifically, from the lower (high pressure) plenum inlet to the core to the outlet from the core of the steam and water mixture through the upper tie plates of the fuel bundles, about 20 pounds per square inch (psi) of pressure drop is encountered at typical 100% power/100% flow operating conditions. About 7 to 8 psi of this pressure drop occurs through the fuel support casting. This pressure drop is mainly to assure the uniform distribution of coolant/moderator flow through the many fuel bundles making up the core of the reactor and is related to the prevention of operating instabilities within the reactor at certain power rates of the reactor. At the lower tie plate of each fuel bundle, from the inlet nozzle into the flow volume and through the fuel rod supporting grid, about 1 to about 1½ psi pressure drop occurs which contributes to establishing flow distribution between the individual fuel rods of each fuel bundle. Finally, through the fuel bundle itself—from the lower supporting grid to the exit at the upper tie plate—about 11 psi of pressure drop usually occurs.
Having summarized the construction and operation of a boiling water nuclear reactor, the problem of debris resident within the closed circulation moderator system of the reactor can now be understood. Typically debris within boiling water nuclear reactors can include extraneous materials left over from reactor construction. Further, corrosion during the reactor lifetime also liberates debris. Finally, further debris can be introduced during the numerous outages and repairs of the nuclear reactor. It will therefore be understood that nuclear reactors constitute closed circulation systems that essentially accumulate debris with increasing age.
It has been discovered that a particularly vexing and usual place for the accumulation of debris is in the fuel bundles between the fuel rods particularly in the vicinity of the fuel rod spacers. It will be remembered that each fuel rod is surrounded by the spacer at the particular elevation of the spacer. Debris particles tend to lodge between the spacer structure and the fuel rods and often dynamically vibrate with the coolant/moderator flow in abrading contact to the sealed cladding of the fuel rods. Such flow induced vibration within the reactor, can cause fretting and eventually damage and/or rupture of the cladding of the fuel rods. The rupture of the cladding may lead to the undesirable release of fission gas accumulated and sealed within the fuel rod prior to the rupture. If sufficient fission gas has been released due to cladding ruptures, plant shutdown could be necessary.
It is to be understood that modern nuclear plants have both redundancy and many safety systems designed to counter act anticipated operating casualties, such as fuel rods becoming punctured by debris. Such failures are not of safety concern. However, in almost all cases they result in the plant operating at less than optimum efficiency. Thus, it is highly desirable to reduce the incidence of debris damage to fuel rods.
Briefly, one aspect of the invention resides in a fuel rod comprising a cladding tube and a hard metallic material bonded to an outer surface of the cladding tube, wherein the hard metallic material is selected from the group consisting of NiCrAlY, NiCr, FeCrAlY, FeCr, or combinations thereof.
Another aspect of the invention resides in a fuel rod comprising a cladding tube and a composite of a metal matrix and a ceramic material or a metal oxide hard phase material, wherein the metal matrix acts as a binding agent for bonding the ceramic material or the metal oxide hard phase material to the cladding tube.
In another aspect of the invention, a method for enhancing the fretting wear characteristics of a fuel rod comprises the step of thermal spraying a hard metallic powder material selected from the group consisting of NiCrAlY, NiCr, FeCrAlY, FeCr, or combinations thereof, onto an outside surface of a cladding tube to form a wear-inhibiting coating having a thickness of between about 25 microns and about 175 microns.
In yet another aspect of the invention a method for enhancing the fretting wear characteristics of a fuel rod, comprising the steps of:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring to
Coolant supplied from below the lower tie plate 14 may carry debris of the type noted above. As the coolant (water) flows upwardly, some debris will be intercepted and may drop down below the lower tie plate 14. Some debris, however, can impact upon the exterior surface of the fuel rods 16, the spacer grids 20 and, in the case of BWR assemblies, enclosing channel structure 22, particularly at the lower ends thereof. In the case where a fuel assembly does not include a debris-catching device or screen, an even greater amount of debris may be expected to impact upon the exterior surface of the fuel rods 16, the spacer grids 20 and other components in the fuel assembly 10. The debris particles are often hardened by irradiation and are known to rapidly fret the exterior of the fuel rods 16 as the water flows in the direction indicated by the arrow.
Referring now to
In accordance with an embodiment, a wear-inhibiting coating 26 is applied to an exterior surface 32 of the cladding tube 22 in one or more areas or zones where fretting is expected to occur. One expected area of fretting is in the vicinity of the spacer grid or spacer 20. The relative dimension of the coating 26 is exaggerated in
Referring now to
In an alternate embodiment of the wear-inhibiting coating 26 shown in
In general, the wear-inhibiting coating 26 can be applied to the cladding tube using many different types of thermal spray processes, depending on the materials applied to the cladding tube.
For example, in the case where the coating comprises a single hard metallic layer shown in
In the HVOF (High Velocity Oxygen Fuel) Thermal Spray Process there are a number of HVOF guns that use different methods to achieve high velocity spraying. One method is basically a high pressure water cooled HVOF combustion chamber and long nozzle. Fuel (kerosene, acetylene, propylene and hydrogen) and oxygen are fed into the chamber. Combustion produces a hot high pressure flame which is forced down a nozzle increasing its velocity. Powder may be fed axially into the HVOF combustion chamber under high pressure or fed through the side of laval type nozzle where the pressure is lower. Another method uses a simpler system of a high pressure combustion nozzle and air cap. Fuel gas (propane, propylene or hydrogen) and oxygen are supplied at high pressure, combustion occurs outside the nozzle but within an air cap supplied with compressed air. The compressed air pinches and accelerates the flame and acts as a coolant for the HVOF gun. Powder is fed at high pressure axially from the centre of the nozzle.
In the alternate embodiment of the wear-inhibiting coating 26 shown in
For example, the composite coating 26 can be created by simultaneously using a separate thermal spray process for the metal powder material and a different type of thermal spray process for the ceramic powder material and/or metal oxide hard phase powder material. The two different processes would be aimed at the same location on the cladding surface so that the melted metal and the ceramic or oxide metal would mix when impacting the cladding surface. In this way, each thermal spray process can be optimized for the metal powder material, ceramic powder material and/or metal oxide hard phase powder material to create the desired coating structure, such as the type of coating structure shown in
Thermal spray processes that can be used to apply the metal powder material are HVOF, HVAF, and the like, that would minimize oxidation of the particles while in the hot flame, but have enough energy to splat out on to the surface of the cladding to form a dense coating. Thermal spray processes, such as air plasma spray, and the like, with higher temperatures are needed to be used to melt the metal oxides. If cermet powder is used with carbides, it is preferred not to melt or over heat the carbides. Over heating the carbides would cause an undesirable decomposition of the carbides. Therefore, a HVOF process, a HVAF process, and the like, is preferred to minimize decomposition of the carbides and oxidation of the metal material within the cermet powder while they are entrained in the hot flame.
The plasma spray process used for the metal oxide hard phase powder material is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. For example, the flame temperature of a plasma torch may be about 15000 K. The hot material impacts on the substrate surface and rapidly cools forming a coating. This plasma spray process carried out correctly can be called a “cold process” (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.
The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled, plasma gases (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localized ionization and a conductive path for a DC arc to form between cathode and anode. The resistance heating from the arc can cause the gas to reach an extreme temperature, dissociate, and ionize to form plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) which is quite different to the Plasma Transferred Arc coating process where the arc extends to the surface to be coated. When the plasma is stabilized ready for spraying the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of about 25 mm to about 150 mm (about 0.984 inches to about 5.906 inches).
As shown in
Alternatively, the metal powder material, a ceramic powder material and/or metal oxide hard phase powder material can be premixed in a ratio so that there will be sufficient metal matrix 30 to bind the melted ceramic material or metal oxide hard phase material 32 to the cladding tube 22. The size and percent of the ceramic powder material or metal oxide hard phase powder material that would be added to the composite coating 26 is a function of the expected size of the debris causing the fretting and the trade offs between fretting wear resistance and impact resistance. Adding too much ceramic powder material or metal oxide hard phase powder material for a specific application may cause the composite coating 26 to fail from dynamic impact and not from fretting wear.
In one form of the wear-inhibiting coating 26 similar to the type shown in
In another form of the wear-inhibiting coating 26 similar to the type shown in
The cladding tubes with the NiCrAlY coating and the NiCrAlY/TiO2 coating were placed in a test rig that simulated the water temperature, water pressure, water chemistry, debris hardness and debris pressure commonly found during operation of the nuclear reactor.
Table I shows data from a rig wear test that was conducted on a new Zircaloy tube without the wear-inhibiting coating, a new Zircaloy tube with the wear-inhibiting coating comprising NiCrAlY that was deposited using the HVOF process, and a new Zircaloy tube with the wear-inhibiting coating comprising NiCrAlY/TiO2 that was deposited using the HVOF and air plasma spray processes, respectively.
As shown in Table I and
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
