Patent Number: 056278667
Section: summary

TECHNICAL FIELD This invention relates to the structure of a fuel assembly in a boiling water nuclear reactor vessel and, more particularly, to a fuel assembly structure that utilizes the channel to support its load, thereby eliminating the need for fuel tie rods. BACKGROUND A conventional fuel assembly in a boiling water nuclear reactor vessel includes a lower tie plate, an upper tie plate and a matrix of the sealed fuel rods supported between the upper and lower tie plates. The fuel rods contain nuclear fuel pellets in sealed containment for supporting a required critical reaction for the generation of steam. One or more coolant rods is included in the matrix of the fuel rods and is also supported between the upper and lower tie plates. A channel surrounds the tie plates, fuel rods and coolant rod. This channel is commonly square in cross-section and made of metal (preferably an alloy called Zircaloy). Water passes from the bottom of the fuel assembly to the top of the fuel assembly. Water enters through the lower tie plate within the channel and passes between the upstanding fuel rods. Water and generated steam exit from within the channel between the fuel rods and out through the upper tie plate. The channel confines the required moderator coolant flow to a flow path that is restricted between the tie plates. The lower tie plate and the upper tie plate serve to support the sealed fuel rods in the vertical and upstanding matrix. Typically, the upper tie plate forms an overlying matrix of fuel rod support points. Into about eight of these support points are conventionally placed correspondingly male threaded tie rods and fittings. The tie rods, which contain fuel similar to the fuel rods, are threaded at their lower ends for corresponding attachment to the lower tie plate. The lower tie plate similarly forms an underlying matrix of fuel rod support points. These underlying support points correspond for the most part to the overlying support points of the upper tie plate. Conventionally, about eight of these support points are threaded with female apertures, which correspond to the overlying apertures in the upper tie plates. Into these threaded support points in the lower tie plates are placed the lower threaded ends of the fuel tie rods. Thus, conventionally, the two tie plates are tied together with the fuel tie rods. The tie plates also define a matrix of apertures for permitting fluid flow into and out of the fuel assembly. Specifically, the lower tie plate defines a first matrix of apertures for permitting the in flow of water coolant. This coolant functions in the capacity of moderating or slowing down reaction produced fast neutrons to produce reaction continuing slow or thermal neutrons. At the same time, as the coolant passes upwardly through the fuel assembly within the channel, a portion of the coolant is turned to steam. This steam and the coolant that is not turned into steam and remains in the liquid phase must pass out through the upper tie plate. Consequently, the upper tie plate forms its own matrix of apertures in between its matrix of fuel rod support points. The upper tie plate matrix of apertures permits the out flow of the two phase steam/water mixture from the fuel assembly. The fuel bundle must be periodically replaced and/or inspected during so-called "outages" of a reactor. These outages occur when the central steam generating core of a nuclear reactor has its overlying component removed to provide access through shielding water to the core. During such "outages," sections of the reactor vessel core are removed, inspected and/or replaced. The core, submerged in a radiation quenching bath of water, has the fuel bundles to be replaced for inspection removed by remotely grasping the fuel assembly at a handle. The handle must define, at the top of the fuel assembly, a support point for the entire weight of the fuel assembly in a depending relationship when the assembly is removed from the vessel. Once the fuel assembly is supported at the handle, the entire weight of the fuel assembly is carried through the handle. This weight includes the weight of the fuel and coolant rods, the weight of the upper tie plate, the weight of the lower tie plate and the weight of the surrounding channel (upwards of 600 pounds). Once the fuel assembly is removed from the vessel, the tie plates, fuel rods and coolant rods can be separated from the channel. After separation from the channel, the fuel rods can easily be inspected and/or replaced. Conventionally, however, the threaded end plugs of the fuel tie rods tend to seize in their threaded connections, thus making replacement of the fuel tie rods difficult and time consuming. Moreover, as fuel assembly design lifetimes are extended, corrosion effects weaken the fuel tie rods. This weakening occurs due to corrosion thinning of the material and by a reduction in ductility due to the formation of hydrogen and its absorption. Thus, there is a need to provide a fuel assembly structure that does not include fuel tie rods threadedly connected between the upper and lower tie plates. Moreover, there is a need to utilize a structural load path for the fuel assembly that is less affected by corrosion effects. In general, since corrosion is a surface phenomena, a structure with a high volume to surface area provides more margin in this regard. Without adding additional structure to the general design of boiling water reactor fuel assemblies, the component with the highest volume to surface area is the channel. DISCLOSURE OF THE INVENTION It is therefore an object of the invention to provide a fuel assembly structure that utilizes the channel as the structural member that limits the axial extension between the upper and lower tie plates. Another object of the invention is to provide a fuel assembly structure that provides redundancy in the attaching features of the upper tie plate to the channel as well as between the transition member and the channel and allows lifting of the assembly through a load path that does not utilize fuel tie rods that are threaded into the tie plates. It is yet another object of the invention to provide a method of removing the fuel bundle from the fuel assembly. These and other objects and advantages of the invention are achieved by providing a fuel assembly for a nuclear reactor vessel that includes a plurality of fuel rods; a coolant rod; a channel surrounding the plurality of fuel rods and the coolant rod; and a lower tie plate supporting the plurality of fuel rods and the coolant rod, the lower tie plate being supported by the channel such that the channel carries a load of the fuel assembly. The fuel assembly may further include an upper tie plate disposed inside the channel that laterally supports the plurality of fuel rods and the coolant rod; and connecting structure releasably connecting the upper tie plate and the channel. The upper tie plate may be provided with a bolt aperture adjacent the coolant rod that opens to a spring channel in the upper tie plate. In this regard, the fuel assembly further includes a coolant rod main spring disposed surrounding the coolant rod; a coolant rod main spring support supporting the coolant rod main spring; a bolt inserted in the bolt aperture and extending into the spring channel; a substantially cylindrical member fixed to an end of the bolt and delimiting the spring channel; and a spring disposed surrounding the bolt in the channel between the coolant rod main spring support and an end of the spring channel. The connecting structure can include two opposed extendible and retractable latch pins, wherein the channel has a corresponding two opposed apertures for receiving the latch pins. The upper tie plate preferably has two boss members, which may be integral with the upper tie plate, each having a channel therein, wherein each of the latch pins is movably disposed in the each of the channels, respectively. Two springs may be provided, one each disposed in each of the channels surrounding a respective one of the latch pins. The springs urge the latch pins toward an extended position, the latch pins having a first outer periphery configured to be received in the apertures, respectively, and a second outer periphery, larger than the first outer periphery, configured to sit against the channel in the extended position. In this regard, each of the latch pins may include a spring engaging surface delimiting the channels, respectively, wherein the springs are disposed in the channels between an end surface of the channels and the spring engaging surfaces, respectively. The fuel assembly may further include a transition member partially disposed in the channel and adjacent the lower tie plate. The transition member is attached to the channel, and the lower tie plate rests on the transition member in this regard, a bolt may be provided threadedly secured through the channel and the transition member. Still further, a channel clip may be provided fixed to the channel and inserted into a slot in the transition, wherein the channel clip is partially disposed axially interior of the channel, thereby restricting axial displacement of the transition member relative to the channel. In a preferred embodiment, four channel clips are welded to the channel. In accordance with another aspect of the invention, the connecting structure includes a channel guide fixed to the upper tie plate and disposed outside of the channel. The channel guide includes an ear, and the channel includes a corresponding ear aperture configured to receive the ear. The channel guide may be provided with two arms disposed at substantially 90.degree. and two ears, each fixed to a respective one of the two arms. The channel guide may include two legs extending from ends of the two arms, respectively, wherein the two ears are each fixed to one of the two legs, respectively. In accordance with yet another aspect of the invention, there is provided a fuel assembly for a nuclear reactor vessel that includes a plurality of fuel rods; a coolant rod; a channel surrounding the plurality of fuel rods and the coolant rod; a lower tie plate supporting the plurality of fuel rods and the coolant rod, the lower tie plate being supported by the channel such that the channel carries a load of the fuel assembly; an upper tie plate disposed inside the channel and laterally supporting the plurality of fuel rods and the coolant rod; a transition member partially disposed in the channel and adjacent the lower tie plate, the transition member being attached to the; channel, wherein the lower tie plate rests on the transition member; and connecting structure releasably connecting the upper tie plate and the channel, wherein the connecting structure includes a channel guide fixed to the upper tie plate and disposed outside of the channel, the channel guide including an ear, and the channel including a corresponding ear aperture configured to receive the ear. The connecting structure may further include an extendible and retractable latch pin, and the channel may include a corresponding aperture for receiving the; latch pin. Finally, in accordance with a further aspect of the invention, there is provided a method of removing a fuel bundle from a fuel assembly, the fuel bundle including a plurality of fuel rods, a coolant rod, and a lower tie plate supporting the plurality of fuel rods and the coolant rod, the coolant rod being fixed to the lower tie plate. The fuel assembly includes a channel surrounding the fuel bundle, and the lower tie plate is supported by the channel such that the channel carries a load of the fuel assembly. The method includes detaching the lower tic; plate from being supported by the channel; and removing the fuel bundle by lifting the coolant rod with a lifting tool. The fuel assembly may further include an upper tie plate disposed inside of the channel and laterally supporting the plurality of fuel rods and the coolant rod, and connecting structure releasably connecting the upper tie plate and the channel. In this regard, the detaching step may include removing the connecting structure to detach the upper tie plate and the channel.