Patent Number: 039363503
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention utilizes support members formed from preselected materials with different rates of thermal expansion to compensate for the different temperature gradients encountered during reactor operation. The accompanying drawing shows a schematic illustration of a nuclear reactor 10, illustrating the relative positions of various reactor components such as the top plug 12 (top closure with control rod drive mechanisms 14), the core bottom support plate 16, which positions the core fuel assemblies 18 and a coretop hold down plate or core bundling 20. It may be seen from the figure that the aforementioned reactor components provide lateral support to the fuel sub-assembly 18 and control rod drive mechanism 14. However it is to be noted that this invention need not be limited to the specific components shown and described herein, but may be applied to any arrangement of reactor components that experience thermal gradients at reactor operation. Thus, the aforementioned components will be used as an illustration of the principles of this invention and the scope of the invention is not meant to be limited thereby. The reactor components, such as, the fuel rod and the control rod supports, are constructed out of materials having thermal expansion rates such that the total expansion of each component, under the environmental operating conditions at its respective location, substantially equals the thermal expansion of every other reactor component which functions to provide support in a parallel plane within the reactor, thus balancing the thermal expansions of the various supports to retain alignment. The balancing of the thermal expansions is achieved as illustrated in the following example. The top plug 12 is to be maintained at, for example, 400.degree.F, the bottom support plate 16 at 750.degree.F and the top "hold down" plate or "core bundling device" 20 at 1000.degree.F. These are the anticipated temperature gradients for the fast breeder reactor. The following equation is then used to calculate the per unit length thermal expansion of each component: EQU E = .alpha. (T.sub.2 - T.sub.1); where "E" equals the per unit length thermal expansion of the reactor component; PA1 ".alpha." equals a constant called the coefficient of thermal expansion, which is readily obtainable in precalculated tables; and PA1 "(T.sub.2 - T.sub.1 )" is the temperature gradient experienced by the reactor component, which is the difference between room temperature or the temperature at which alignment is originally obtained and the operating temperature. PA1 where ".DELTA.W" is the change in width of the sub-assembly 18; PA1 ".alpha." is the coefficient of thermal expansion of the sub-assembly 18; PA1 "(T.sub.2 - T.sub.1)" is the thermal gradient, which is the operating temperature of the sub-assembly 18 (i.e., 1000.degree.F), minus the loading temperature (i.e., 400.degree.F); and PA1 "W" is the width of the sub-assembly 18. PA1 where ".DELTA.S" is the change in spacing; PA1 ".alpha." is the coefficient of thermal expansion of the core bundling device 20 which would effect the change in spacing of the fuel-assemblies 18, (in this example it is assumed, the core bundling device 20 is constructed out of vanadium as calculated above); "(T.sub.2 - T.sub.1)" is the thermal quadrant of the core bundling device 20 (1000-400); and PA1 "S" is the spacing between the centers of the fuel sub-assemblies 18 at loading temperatures (approximately 5 inches). The materials to be used for the individual components may then be determined by first specifying the material to be used in the construction of one of the aforementioned reactor components. This choice is arbitrary and is only limited to a material which will satisfy the components characteristics. Thus, specifying type 304 stainless steel as the material used in the construction of the top plug 12, the coefficient of thermal expansion (.alpha.) for Type 304 stainless steel is then found to be 10 .times. 10.sup.-.sup.6 in/in. The temperature gradient as defined above is then (400-70). Substituting these values in the equation we obtain: EQU E = 10 .times. 10.sup.-.sup.6 (400-70) = 3300 .times. 10.sup.-.sup.6 = 0.0033 in./in. Then to balance the thermal expansion of the bottom support plate 16, with the thermal expansion of the top plug 12, we use this value of per unit length thermal expansion and the thermal gradient for the bottom support plate 16, which is (750-70 ) and substitute it into the equation: ##EQU1## Using this coefficient of thermal expansion we then consult the tables of coefficients of thermal expansion and select a material that has an .alpha. approximately equal to 4.86 .times. 10.sup.-.sup.6 and the other desired characteristics necessary for constructing the bottom support plate 16. Such a material is vanadium, which has a coefficient of thermal expansion equal to 5.0 .times. 10.sup.-.sup.6 in./in. The same procedure is followed in selecting a material for the top core bundling plate 20: ##EQU2## Molybdenum is an example of a material which satisfies this criteria with a coefficient of thermal expansion of 3.2 .times. 10.sup.-.sup.6 in./in. If the alignment dimensions are based on a heated assembly, for a hot sodium with a temperature of 400.degree.F and a top plug 12 at room temperature, the stainless steel plug 12 expansion will remain 3300 .times. 10.sup.-.sup.6 in./in. For the bottom support plate 16: ##EQU3## Therefore Type 304 stainless steel with a coefficient of thermal expansion of 10 .times. 10.sup.-.sup.6 may also be used for the bottom support plate 16. For the top core bundling plate 20: ##EQU4## Therefore Vanadium with a coefficient of thermal expansion of 5.0 .times. 10.sup.-.sup.6 could be used for the top core bundling plate 20. In addition to balancing the thermal expansions to retain alignment, the present invention permits the reduction of the gap between fuel sub-assemblies 18, wherein thermal bowing occurs. Some nominal gap is required at reloading temperatures (approximately 400.degree.F) so that the sub-assemblies 18, can be inserted and removed. This gap may be selected to be about 0.030 inches for a fuel sub-assembly 18 about 5 inches wide wherein the structural components of the subassembly are formed from Type 304 stainless steel. Then of a 0.030 gap exists at 400.degree.F this would be reduced at operating conditions as follows. The change in width of a fuel sub-assembly 18 is: EQU .DELTA.w = .alpha. (T.sub.2 - T.sub.1) W; Thus substituting the aforementioned values in the equation we obtain: EQU .DELTA. W = .alpha. (T.sub.2 - T.sub.1) W = 10 .times. 10.sup.-.sup.6 (1000-400) 5 = .030 inches. The change in the spacings between sub-assemblies 18 is then calculated from the equation: EQU .DELTA. S = .alpha. (T.sub.2 - T.sub.1) S; Substituting these values in the equation we obtain: EQU .DELTA.S =.alpha. (T.sub.2 - T.sub.1) S = 5.0 .times. 10.sup.-.sup.6 (750-400) 5 = 0.0084. The gap between fuel sub-assembies 18 at operating conditions is then: EQU gap = (the gap at loading temperature) - .DELTA.W + .DELTA.S EQU gap = 0.030 - 0.030 + 0.0084 = 0.0084 inches. Thus, the reduction in the gap between fuel sub-assemblies 18 reduces the amount of bow which can develop from 0.030 to 0.0084 inches per sub-assembly 18. If the gap between sub-assemblies 18 is reduced, as shown above, then the "jump movement" problem is also reduced by the same amount .