Patent Number: 062981080
Section: description

The Figures are not drawn to exact scale or proportion for the purpose of better illustration. DETAILED DESCRIPTION OF THE INVENTION A sketch of the fuel rod of prior art is shown in FIG. (1), where the following three components are marked. The first component is the sealed cylindrical tube (C) called the "clad" which is made of a material with low neutron absorption cross section such as a Zirconium-based alloy. The second component is a stack of cylindrical pellets (P) containing the fissionable matter in ceramic form. The stack of pellets rests on the bottom of the cladding tube (C) and is held down by a compressed spring (S), which makes the third component. The space atop the stack where the fission gas accumulates is called the fission gas plenum which is marked on FIG. (1) as (F). The clearance (G) between the pellets and the clad is called the "gap." The approximate length of the fuel rod is 4 m, and its diameter is approximately 1 cm. The height of the pellet stack is approximately 3.7 m, which leaves a height of approximately 30 cm for the gas plenum. In contrast with the fuel rod of the prior art, a sketch of the new invention is shown in FIG. (2) and illustrated herein. A crushable support tube (R) is placed under the pellet stack thus shifting the stack to a higher elevation. The exact length of the crushable support tube (R) can be varied as a design parameter, and is shown in FIG. (2) as approximately 15 cm. The placement of the crushable support tube (R) creates a new space for fission gas accumulation defined as the "lower plenum" (LP). The space atop the pellet stack, called the "upper plenum" (UP) is reduced compared with the space (F) shown in FIG. (1). The total volume of the two plena (LP) and (UP) shown in FIG. (2) remains approximately similar to the prior art gas volume (F) shown in FIG. (1) which can accommodate the same amount of fission gas release. The length of the crushable tube is .DELTA.Z, which is also the pellet stack displacement length. At any time in the fuel cycle, the fission power generated in the pellets has a fixed shape as a function of the axial position of the pellets relative to the bottom-most pellet. When the pellet stack is shifted upward by .DELTA.Z, the axial power shape in the assembly is similarly shifted upward by .DELTA.Z relative to the reference coolant entry point at the bottom of the assembly. The resulting change in the characteristics of the pressure drop shift is shown in FIG. (3). The rate of pressure drop, dp/dZ, is plotted as a function of the elevation Z along the length of the assembly. Three distinct zones are discernible. The first zone is the single phase zone Z.sub.1 to Z.sub.2 where the pressure drop rate is constant and relatively small. The second zone spans the length between Z.sub.2 and Z.sub.3 where the pressure drop rate monotonically increases with elevation, as the steam content increases by energy addition to the flowing coolant. The third zone which spans the length between Z.sub.3 and Z.sub.4 is characterized by pressure drop rate of relatively high but constant magnitude, as the said span does not contain any pellets and the steam content in the flowing coolant remains constant. The pressure drop rate is shown for the prior art fuel rod by a solid line in FIG. (3). With the shifted pellets and the axial power shape, the elevation at the onset of boiling, called the boiling boundary, is shifted to Z.sub.2 +.DELTA.Z. The elevation marking the end of the heated length Z.sub.3 is similarly shifted to Z.sub.3 +.DELTA.Z. The location of the top of the fuel rod Z.sub.4 remains unchanged. The pressure drop rate along the assembly with rods of the new design is marked in FIG. (3) by the dash line. The total pressure drop .DELTA.p is the integration of the pressure drop rate with respect to the assembly length Z according to the equation ##EQU1## It is obvious that the total pressure drop is the area under the pressure drop rate curve, which results in concluding that EQU .DELTA.p(new)&lt;.DELTA.p(old), where .DELTA.p(new) and .DELTA.p(old) are the pressure drop of the new design and the prior art design respectively. The reduction in the total and 2-.phi. pressure drop with the upward shift of the boiling boundary is one of the important objectives of this invention. The total pressure drop reduction allows useful design changes which have the side effect of inadvertently increasing the pressure drop. Examples of such design changes are 1. The use of larger diameter fuel rods containing more fissionable material. 2. Allow more spacers to be used particularly in the top section of the assembly, where the flow mixing effect of the spacer increases pressure drop but also improves the critical power performance against dryout. 3. Allow the reduction of the number of part-length fuel rods, or mitigate the need to increase same, which are used to reduce the 2-.phi. pressure drop at the expense of reducing the mass of the fissionable material. 4. Allow using higher pressure drop inlet piece (such as lower tie plate) with the effect of substantial decrease in the 2-.phi. pressure drop relative to the single phase pressure drop resulting in increasing the safety margin against density wave instabilities and more reactor operational flexibility. 5. Allows higher flow rates for the new fuel assembly which decreases the steam content in the boiling section of the flow and increases the neutron moderation efficiency and the neutron economy of the fuel assembly. This allows using lower U-235 enrichment and reduces the cost of the fuel assembly. It must be noted that the spacer locations needed to support the fuel rods against rod bow and seismic loads need not be changed due to the use of the new fuel rod design with upward-shifted pellet stack. The axial power shape is thus shifted relative to the spacer locations. Critical heat flux causing dryout occurs immediately under a spacer as it is the largest distance away from the next spacer below. This is explained as due to the spacer flow mixing effect which increases the critical heat flux, and such effect decays as the fluid travels away from the spacer. Thus, shifting the top spacer down to a lower elevation where the power is higher would help with increasing the critical power. However, the spacer location must not be shifted physically so that its mechanical support function is not adversely affected. A relative shift of the spacer location is produced by shifting the axial power shape instead. Therefore, the new invention results in improving the critical power performance of the fuel assembly without the need to increase the number of spacers. This does not preclude the increase of the number of spacers as mentioned above as a possible means to increase critical heat flux. The device for affecting the pellet stack shift is a crushable tube placed under the fuel pellet stack. While any method for supporting the fuel pellet stack produces the same pressure drop advantages of this invention, the crushable tube offers important advantages. As the fissionable material is consumed under irradiation, the fuel pellets undergo physical changes such as swelling due to accumulation of fission gas in the porous ceramic material. The pellet may also crack due to these irradiation effects and thermal stresses. The aforementioned physical changes in the pellet result in closing the small gap between the pellet and the clad, which is called pellet-clad mechanical interaction which prevents the pellet from sliding freely relative to the clad. Under unanticipated power transients, which may result in large differential thermal expansion of the pellets relative to the clad, the clad section between the bottom of the fuel rod and the first pellet experiencing pellet-clad interaction comes under large mechanical stress resulting in possible clad failure and the release of radioactive-fission products into the coolant stream. The use of the crushable tube of this invention allows the expansion of the fuel pellet stack under these severe conditions by crushing the tube and relieving the mechanical stress thus avoiding clad failure. The threshold crushing force required to compress the crushable tube must be designed to be larger than the weight of the pellet stack plus the compression force of the spring located in the upper gas plenum. The same crushing force must be lower than the minimum force required to cause clad failure. A sketch of the preferred embodiment of the crushable tube is shown in FIG. (4). In the said sketch, the tube (R) has a cylindrical cross section with outer diameter slightly smaller than the inner diameter of the cladding tube. The wall of the crushable tube is perforated at several places to make holes (H) which serve to weaken the structure of the tube and allow the crushing desired to occur above a certain design stress. The material removed by the perforation allows a larger volume for the lower plenum which is desired for accommodating fission gas released by the fuel pellets.