Patent Number: 043572987
Section: description

DESCRIPTION The invention is described herein as employed in a water cooled and moderated nuclear reactor of the boiling water type, an example of which is illustrated in simplified schematic form in FIG. 1. Such a reactor system includes a pressure vessel 10 containing a nuclear reactor core 11 submerged in a coolant-moderator such as light water. The core 11, which is surrounded by an annular shroud 12, includes a plurality of replaceable fuel assemblies 13 arranged in spaced relation between an upper core grid 14 and a lower core plate 16. A plurality of control rod drive housing tubes 17 house control rod drives by which a plurality of control rods 18 are selectively insertable among the fuel assemblies 13 for control of the reactivity of the core. Each of the housing tubes 17 is fitted with a fuel assembly support member 19 each of which is formed with sockets for receiving the nose pieces 21 of four adjacent fuel assemblies. The nose pieces 21 and the support members 19 are formed with coolant passages or openings for communication with a coolant supply chamber 22. A coolant circulation pump 23 pressurizes the coolant in the supply chamber 22 from which the coolant is thus forced through the openings in support members 19 and the fuel assembly nose pieces upward through the fuel assemblies. A part of the coolant is thereby converted to steam which passes through a separator-dryer arrangement 24 to a utilization device such as a turbine 26. Condensate formed in a condenser 27 is returned as feedwater to the vessel 10 by a pump 28. A fuel assembly 13 is illustrated in elevation view in FIG. 2. The fuel assembly 13 comprises a plurality of fuel elements or rods 31 supported between a skeletonized upper tie plate 32 and a skeletonized lower tie plate 33. The fuel rods 31 pass through a plurality of fuel rod spacers 34(l)-34(n) which provide intermediate support to retain the elongated rods in spaced relation and restrain them from lateral vibration. Each of the fuel rods 31 is formed of an elongated tube containing fissile fuel and other materials, such as fertile fuel, burnable poison, inert material or the like, sealed in the tube by upper and lower end plugs 36 and 37. Lower end plugs 37 are formed with extensions for registration and support in support cavities 38 formed in the lower tie plate 33. Upper end plugs 36 are formed with extension 39 which fit into support cavities 41 in the upper tie plate 32. Several of the support cavities 38 (for example, selected ones of the edge or peripheral cavities) in the lower tie plate 33 are formed with threads to receive fuel rods having threaded lower end plug extensions 37'. Extensions 39' of the upper end plugs of these same fuel rods are elongated to pass through the cavities in the upper tie plate 32 and are formed with threads to receive retaining nuts 42. In this manner the upper and lower tie plates and the fuel rods are formed into a unitary structure. The fuel assembly 13 further includes a thin-walled tubular flow channel 43, of substantially square cross section, sized to form a sliding fit over the upper and lower tie plates 32 and 33 and the spacers 34(l)-34(n) so that the channel 43 readily may be mounted and removed. Fixed to the top end of the flow channel 43 is a tab 44 by which the channel is fastened to a standard 46 of upper tie plate 32 by means of a bolt 47. The lower tie plate 33 is formd with a nose piece 21 adapted to support the fuel assembly 13 in a socket of the support member 19 as shown in FIG. 1. Shown in FIG. 3 is curve 48 of the typical axial thermal neutron flux distribution in a boiling water reactor core with respect to the active core height; that is, with respect to the fuel containing portion of the fuel rods of the fuel assemblies. If the density of the water-moderator were axially uniform, the axial thermal neutron flux distribution would have a cosine shape, that is, maximum at the center and decreasing toward the top and bottom of the core. However, under actual reactor operating conditions, the water-moderator is heated and becomes less dense (and hence less effective as a moderator) as it flows upward through the fuel assemblies. In a boiling water reactor, the boiling creates a two-phase steam-water mixture in the upper portion of the fuel assemblies with further decreases density and moderation effect. The result is a thermal neutron flux distribution that is peaked toward the bottom of the core as shown by the curve 48. Also shown in FIG. 3 are the relative axial locations Sp(1)-Sp(7) of the seven fuel rod spacers used in the fuel assembly 13 of the illustrative example. It is noted that the positions of the spacers are evident from the local "dips" in the flux density curve 48 caused by the neutron absorption by the spacers. In accordance with the invention, low neutron absorption (composite) spacers are used in the lower three or four spacer positions while low flow resistance (skeletonized) spacers are used in the upper three or four spacer positions. A suitable fuel rod spacer of the composite type, illustrated in FIG. 4 as spacer 34(l), includes a peripheral band 50 supporting a plurality of cross-laced divider members, including divider members 51 and spring support divider members 52, spaced to form a plurality of fuel rod passages or cells 53. Supported at intersections of the divider members 52 are four-sided box spring assemblies 54 with outwardly extending V-shaped spring members 55 extending into the passages 53 whereby the fuel rods are urged into contact with oppositely positioned, relatively rigid projections 56 formed in the divider members 51. The structural members of the spacer 34(1) are formed of a material having a low neutron absorption cross section such as a zirconium alloy, for example, Zircaloy-4. The spring members 54 are formed of a material having suitable strength and resiliency characteristics such as a nickel alloy, for example, Inconel. A composite spacer of the type shown in FIG. 4 is described in greater detail in the previously mentioned U.S. Pat. No. 3,654,077 which is incorporated herein by reference. A suitable fuel rod spacer of the low flow resistance skeletonized type is illustrated as a spacer 34(n) in FIG. 5A. The spacer 34(n) is formed of a plurality of cells 61 (each for receiving a fuel rod therethrough) which are assembled in an array and welded together. One of the cells 61 which make up the spacer 13(n) is illustrated in FIG. 5B. The cell 61 is formed, for example, from a stamping from sheet metal which is then bent into the configuration shown. As thus shaped, the cell 61 includes a pair of axially aligned polygonal sleeves 62(1)and 62(2) joined together in axially spaced apart relation by a pair of laterally spaced spring members 63(1) and 63(2) having a generally W shape extending into the fuel rod passage of the cell. At their apexes the spring members 63(1) and 63(2) are formed with bosses or protuberances 64 for limiting contact area with the fuel rod in the cell. The lateral spring force of the spring members 63(1) and 63(2) on the fuel rod biases the fuel rod into contact with relatively rigid protuberances 66 formed in the sleeves 62(1) and 62(2) in the sides of the cell opposite the springs. The spacer 34(n) formed by the welded-together cells 61 can be strengthened by the addition of skeletonized peripheral plate members 67 welded to the sleeves of the peripheral cells. Since the spring members 63(1) and 63(2) are integrally formed, the entire spacer structure is formed of a material having suitable resiliency characteristics. A suitable such material is a nickel alloy such as Inconel. A spacer of the skeletonized type as illustrated in FIGS. 5A and 5B is described in greater detail in the previously mentioned British Pat. No. 1,480,649 and U.S. Pat. No. 4,190,494 which are incorporated herein by reference. In accordance with a preferred form of the invention as applied in a boiling water reactor, spacers of the low neutron absorption composite type (FIG. 4) are used in the high neutron flux region of the core, namely, in spacer locations Sp(1), Sp(2) and Sp(3) while skeletonized spacers of low coolant flow resistance (FIG. 5A) are used in the lower neutron flux region, namely, in spacer locations Sp(5), Sp(6) and Sp(7). Either type of spacer may be used in the center spacer location Sp(4) since there is no predominate advantage to the use of one type over the other in the intermediate neutron flux density at this position. Consideration of practical examples of the skeletonized and composite spacers indicates that the skeletonized spacer has a relative neutron absorption cross section about 10 times greater than the composite spacers. The consequence of this is that the reactivity penalty from use of composite spacers at all seven spacer locations would be about 50 percent of the reactivity penalty that would be incurred if the skeletonized spacers were used in all locations. On the other hand flow tests indicate that coolant flow pressure drop through the fuel assembly is decreased by about 20 percent and thermal limits are increased as much as 19 percent by use of skeletonized spacers at all locations as compared to the pressure drop and thermal limits resulting from use of composite spacers at all locations. By use of the spacer arrangement of the invention, coolant flow pressure drop is decreased in the order of 15 percent and thermal limits in the order of 10 percent while the reactivity penalty is increased by only about 20 percent (as compared to the case of all spacers being of the composite type).