Patent Number: 046408134
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending U.S. application dealing with related subject matter and assigned to the assignee of the subject application: "Light Water Moderator Filled Rod For A Nuclear Reator"; by P. K. Doshi et al; assigned U.S. Ser. No. 654,709; and filed Sept. 26, 1984. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique design of a soluble burnable absorber rod for use in a nuclear reactor which achieves substantially complete absorber burnup and has reduced fabrication cost. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison at stationary positions in a nuclear reactor core are described in U.S. Pat. No 3,510,398 to Wood. Heretofore, rods containing burnable poison intended to be stationarily positioned within the reactor core have been of the "fixed" type. By a rod being of the fixed type, it is meant that the absorber content of the burnable poison at any axial elevation on the rod is fixed by the initial loading of the material during manufacture of the rod. The burnable poison rod in the Wood patent is representative of the fixed type. A major disadvantage of the fixed type absorber rod, such as the one illustrated and described in this patent, is that not all of the poison material in the rod burns up completely or depletes evenly. The shape of the axial depletion curve for the fixed type absorber rod is approximately the same as the axial neutron flux distribution curve averaged over the core life cycle. However, because of the lack of correspondence between the average axial distribution of neutron flux and some of the neutron flux peaks occurring in the reactor core over the life cycle of core operation, poison material at certain axial locations of the fixed type burnable poison rod depletes more rapidly than at other locations. This results in incomplete absorber depletion at the other locations, which means there is a substantial residual absorber penalty at the end of the cycle. Consequently, a need exists for a burnable poison rod design which will have an improved fuel cycle cost benefit over the previous fixed design, as represented by the rod design in the Wood patent, in terms of fabrication costs and increased length of core cycle. SUMMARY OF THE INVENTION The present invention provides a soluble burnable poison or neutron absorber rod designed to satisfy the aforementioned needs. Unlike the prior art rod, the absorber content of the material at any axial location of the rod of the present invention is not fixed at the time of manufacture nor at any time thereafter. Instead, the absorber material can circulate so that axial zones which are depleted faster than the average (due to neutron flux peaks) can be replenished with absorber material from other axial regions of the rod having lower depletion rates. Underlying the present invention is the recognition that by simply providing the absorber material in mobile rather than fixed form, it will be driven into circulation within the rod by thermal gradients which are normally present along the height of the rod. No external driving source is required. Thus, the absorber content tends to maintain a constant value over the full height of the rod of the invention as it is depleted rather than burning out faster at certain local elevations. Since during the manufacture of the rod the amount of absorber material required is calculated for the peak neutron flux location, the fixed absorber rod type requires more absorber material than the circulating type of the present invention. In contrast to the substantial residual absorber penalty at the end of the core cycle in the case of the fixed absorber type, the circulating type of the invention can be entirely depleted over its full length so there is no significant residual absorber penalty associated with it. In summary, therefore, as compared to the prior fixed type of absorber rod, the circulating absorber rod extends the burnup and thereby results in an increased cycle length. Also, it is felt that its fabrication costs would be less. Accordingly, the present invention sets forth in a fuel assembly for a nuclear reactor including a plurality of guide thimbles and a plurality of nuclear fuel rods spaced apart from one another and from the guide thimbles and grouped together in an array organized to generate a neutron flux in the fuel assembly, an improved burnable absorber rod for insertion into at least one of the guide thimbles for regulating the reactor neutron flux. The improved burnable absorber rod is composed of: (a) an elongated hollow tubular member having opposite ends and a hermetically sealed chamber defined therein between its ends; (b) a neutron absorber material in liquid form contained in the sealed chamber within the tubular member; (c) means providing a hydride sink disposed at one end of the tubular member and in communication with the sealed chamber; and (d) means providing a hydrogen getter disposed at the other end of the tubular member and in communication with the sealed chamber. More particularly, the tubular member is formed by a tubular body of thin wall construction and a pair of end plugs attached to the opposite ends of the body so as to hermetically seal the same. The tubular body has one or more reinforcing convolutions formed therein which enhance the structural rigidity and integrity of the rod so as to enable it to better withstand both high internal and external pressures acting thereon. The reinforcing convolutions can take on the shape of a recess or groove formed in the body so as to extend along a spiraling path between the ends of the tubular member. Or, alternatively, the convolutions can be a series of ring-like circular grooves formed in the body so as to extend circumferentially about and be spaced axially along the tubular member between the ends thereof. Still further, some combination of both can be used. The liquid neutron absorber material is preferably composed of boron dissolved in water with the boron enriched with B-10 over the proportion naturally contained therein. Also, since the rod is designed to have absorber material at full core height during operation, the column of liquid absorber material in the rod is reduced during non-operating periods. Thus, there will be some empty vapor space left within the chamber of the tubular member. Further, more specifically, the material composing the tubular body and end plugs of the tubular member is preferably Zircaloy-4. To reduce corrosion of the inside of the tubular member, the material is beta quenched. This also reduces the hydride pickup in the member due to free hydrogen from the oxidation or burn process. The solid lower end plug provides the means serving as the hydride sink and has an outer end portion of reduced diameter adapting it to fit into a dashpot in the lower end of the guide thimble. The upper end plug of the tubular member is an attachment fitting and disposed adjacent thereto is the means providing the hydrogen getter which takes the form of a Zircaloy sponge adaped to remove hydrogen from the vapor space in the chamber of the tubular member. The sponge is retained at the upper end against the fitting by a disc which has a central opening for allowing passage of the hydrogen gas to the sponge and is held against the sponge by a circumferential bulge formed in the body of the tubular member. Finally, the sealed chamber of the tubular member is prepressurized with helium gas so that at core operating temperature the internal pressure of the chamber will be in equilibrium with the external pressure in the core. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.