Patent Application
20160099080
1. Technical Field
This invention pertains generally to a nuclear reactor core component and, more particularly, to components such as fuel rods and control rods that employ an active ingredient within a cladding that is held in position by a plenum spring.
2. Related Art
The primary side of nuclear power generating systems which are cooled with water under pressure comprise a closed circuit which is isolated and in heat exchange relationship with a secondary side for the production of useful energy. The primary side includes the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. The fission reactions within the fuel assemblies within the core of the reactor vessel are the source of heat which are transferred to the secondary side through the steam generators for the production of useful work.
A typical fuel assembly for a pressurized water reactor is shown in
The fuel assembly 10 further includes a plurality of transverse grids 20 axially spaced along and mounted to the guide thimbles 18 (also referred to as guide tubes) and an organized, array of elongated fuel rods 22 transversely spaced and supported by the grids 20. Although it cannot be seen in
As mentioned above, the fuel rods 22 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 20 spaced along the fuel assembly length. Each fuel rod 22 includes a plurality of nuclear fuel pellets 24 and is closed at its opposite ends by upper and lower end plugs 26 and 28. The fuel pellets 24, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent fission by-products from entering the coolant and further contaminating the reactor system.
To control the fission process, a number of control rods 30 are reciprocally movable in the guide thimbles 18 located at predetermined positions in the fuel assembly 10. Specifically, a rod cluster control mechanism 32, positioned above the top nozzle 16 supports the control rods 30. The control mechanism 32 has an internally threaded cylindrical hub member 34 with a plurality of radially extending flukes or arms 36. Each arm 36 is interconnected to at least one of the control rods 18 such that the control rod mechanism 32 is operable to move the control rods vertically in the guide thimbles 18 to thereby control the fission process in the fuel assembly 10, under the motive power of a control rod drive shaft (not shown) which is coupled to the control rod hub 34, all in a well-known manner.
The fuel assemblies 10 are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the fuel assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes on the upper surfaces of the straps of many grids, which promote the transfer of heat from the fuel rod cladding to the coolant. The substantial flow forces and turbulence can result in vibration of the fuel rod cladding which can damage the fuel pellets 24 if they are not restrained. To prevent any damage to the fuel pellets during shipping, operation in the reactor and handling during loading, repositioning and removal of the nuclear fuel assembly, a holddown device 38 is inserted into the fuel rod 22 to provide a minimum preload of four times of the pellets' stack weight. Typically, a coil spring with a uniform pitch 40, as shown in
An alternative holddown device that has been employed in one type of wet annular burnable absorber rodlets is the spring clip design shown in
Another proposed alternative to the spiral spring is disclosed in U.S. Pat. No. 3,679,545 which describes a holddown device that is helically corrugated over its entire length to provide improved radial support of the cladding than is provided by a typical helical coil spring. This device occupies a larger volume of the plenum than the helical coil spring would occupy and its helical geometry can result in excessive twisting or bowing of the holddown device.
An additional alternative is disclosed in U.S. Pat. No. 4,684,504 which describes a sealed expanded bellows where the internal pressurization generates the holddown force on the pellet stack as well as radial support of the cladding. However, this device may not provide the plenum volume required to accommodate the fission gas release from the fuel pellets.
Accordingly, an improved means of holding down the fuel pellets within a fuel element cladding is desired that will provide uniform pressure on the upper surface of the top pellet in the pellet stack.
Additionally, such an improved design is desired that will facilitate installation, limit consequences of unlikely installation mistakes and minimize potential performance issues.
Furthermore, a new holddown device is desired that will increase the plenum volume efficiently.
These and other objects are achieved by an improved elongated reactive member, such as a fuel element or control rod, for use in a nuclear core. The reactive member is formed from a tubular cladding substantially extending the elongated length of the reactive member with a top end plug sealing off a top end of a central hollow cavity of the tubular cladding and a bottom end plug sealing off a bottom end of the central hollow cavity of the tubular cladding. A lower end plug sealably closes off a lower end of the tubular cladding and a column of reactive material occupies a lower portion of the interior of the tubular cladding above the lower end plug. An upper end plug sealably closes off an upper end of the tubular cladding defining a gas plenum substantially occupying the internal volume of the tubular cladding above the column of reactive material and below the upper end of the tubular cladding. A restraining device is supported above a top of the column of reactive material for pressuring the column of reactive material towards the lower end plug to restrain the reactive material from movement. The restraining device comprises a bellows-like, resilient tubular member having a hollow interior volume and an exterior sheath formed from a plurality of alternating ridges and troughs stacked in tandem. Preferably, the bellows-like resilient tubular member has a Fillet Radius approximately between 0.002-0.020 inches (0.005-0.051 cm.); a Major Radius approximately between 0.010-0.100 inches (0.025-0.254 cm.); a Minor Radius approximately between 0.005-0.080 inches (0.013-0.203 cm.); an Inner Diameter approximately between 0.100-0.350 inches (0.254-0.889 cm.); and Wall Thickness approximately between 0.002-0.010 inches (0.005-0.025 cm.). Desirably, the bellows-like resilient tubular member has a total number of ridges approximately within the range of 5-100.
In one embodiment, the elongated reactive member has a tubular cladding formed from silicon carbide and the bellows-like resilient member is constructed from one or more materials selected from a group of materials consisting of Molybdenum, Tungsten, a Nickel Iron alloy, Zirconium and Hafnium. In another embodiment, the bellows-like resilient tubular member has a thermal barrier coating extending over at least part of an outer surface and preferably, the thermal barrier coating is a low conductivity oxide or a pyrochlore compound.
The invention also contemplates a nuclear fuel assembly including a plurality of fuel rods comprising the elongated reactive member. In still another embodiment, the invention contemplates a control rod cluster assembly in which the control rods comprise the elongated reactive member.
A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
Nuclear power electrical generating stations are a very efficient and cost-effective source of electricity as long as they are running Unforced outages, such as for refueling, considerably raise the cost of power, because they require expensive replacement power to be purchased over the length of the outage. Accordingly, increasing the time between outages is a desired objective. One way to increase core residence time of a fuel assembly is to load more uranium in the fuel rods. There are several possible ways to accomplish that objective, such as employing longer pellet stacks, enlarged pellet diameters, or higher density fuel. However, these modifications require more plenum volume to accommodate the volume changes of the pellets and accommodate the increase fission gas release. Within the limited plenum volume, it is not easy for a coil spring to achieve the 4 g holddown force required. When the coil spring is compressed, the shear stress, dynamic expansion and solid height requirements have to be satisfied. In fact, the current variable pitch plenum spring shown in
Another possible holddown device currently in use in wet annular burnable absorber rodlets is a spring clip design 46 shown in
Another possible alternative to the spiral spring is disclosed in U.S. Pat. No. 3,679,545 which describes a helically corrugated tubular holddown device that provides better radial support of the cladding than a typical helical coil spring. However, it occupies a larger volume than a coil spring and its helical geometry can result in excessive twisting or bowing of the spring.
Another alternative is disclosed in U.S. Pat. No. 4,684,504 which describes an expanded sealed bellows where the internal pressure within the bellows generates the holddown force on the pellet stack as well as radial support for the cladding. This design, however, occupies more plenum volume than the helically corrugated design of U.S. Pat. No. 3,679,545 and will likely not accommodate the fission gases released from the fuel pellets if it was employed within a fuel rod plenum.
This invention employs a holddown or restraining device that can better withstand the effects of irradiation and high temperatures to maintain an adequate force to holddown the fuel pellets over their operating life and occupies less plenum volume than a spiral spring. One embodiment of the restraining device 38 is shown in
Finite element analysis confirmed that most compressions occur by elastic and plastic deformations of the corrugated region without diametric expansion.
The bellows plenum spring 48 may be used with a silicon carbide cladding provided the bellows is made of a low thermal expansion co-efficient material with a high melting point, such as Molybdenum or Tungsten or INBAR-36® (36% Nickel and 64% Iron). A thermal barrier coating can be deposited on the parts of the device experiencing the highest temperatures, such as the portions nearest the fuel and nearest the upper end cap, to prevent the device from overheating. The thermal barrier coating materials can be a variety of low thermal conductivity oxides such as ZrO2 or Pyrochlore compounds, i.e., Nd2CR2O7. The thermal barrier coating can be applied only to the heat effective zone, mainly the ends of the device which are in contact with the fuel pellets and the upper end plug. The thermal barrier coating can be applied using plasma spray, chemical vapor deposition, physical vapor deposition, cold spray or thermal spray. Accordingly, as compared to a typical plenum coil spring, this invention will provide added plenum volume to allow more uranium to be loaded into the fuel rods. In addition, it will provide sufficient holddown force on the irradiated pellet stacks to prevent pellet damage during shipping and handling. This device also provides better radial support of the cladding than the spiral coil spring. Using refractory materials this invention can withstand the high temperature environment anticipated for a silicon carbide clad fuel rod.
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
