Patent Number: 039895904
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the exemplary embodiment of this invention illustrated in FIG. 1, there is shown a clad nuclear fuel element 10 constructed in accordance with the principles of this invention which includes a tubular clading member 26 closed at its ends by a pair of end plugs 18 and 28 hermetically secured to the cladding 26 by suitable means such as by welding. The fuel element 10 has a fuel free zone or gas plenum 12 normally located above the fuel pellets 24. The fuel pellets 24 are positioned in a tandem arrangement that extends longitudinally between the lower end plug 28 of the fuel element 10 and the gas plenum 12. The fuel pellets 24 and gas plenum 12 are sealed within the fuel element cladding 26 formed from a suitable material such as zirconium alloy, which extends longitudinally between the two end plugs 18 and 28. A collapsible capsule 14, constructed out of a short length of thin walled tubing, such as 304 stainless steel, crimped and sealed at both ends, is desirably disposed within the plenum 12 and can be exemplary supported by either the retaining spring 20, which functions to hold the pellets 24 compactly in place against the lower end plug 28 or suspended from the upper end plug 18 by the weldment 19 shown in FIG. 1. For a better understanding of the function and purpose of the retaining spring reference may be had to the H. M. Ferrari patent application Ser. No. 850,198, filed Aug. 14, 1969, and assigned to the Westinghouse Electric Corporation. The object of this invention is to provide a collapsible capsule 14 which slowly collapses at a rate controlled by the internal rod pressure within the gas plenum 12 to maintain a near constant pressure within the rod. The design and function of the collapsible capsule can best be understood by reference to the illustration shown in FIG. 2. The exemplary capsule illustrated is formed from a short length of thin walled tubing which is crimped and sealed on both ends. In one embodiment, a dimple 16, which initiates collapse, is mechanically formed in one end of the capsule. It should be undertstood that other embodiments may be provided in which the dimple is formed at various locations intermediary of the capsule ends. The dimple is so designed to assure that collapse initiates in a buckling rather than a yield mode. Failure in the buckling mode occurs as an elastic instability which is a function of the modulus of elasticity of the material and not the yield strength. Small deformations are recoverable, however, larger deformations dependent upon the amount of pressure and time applied result in plastic collapse. It is important that the collapsed diameter of the capsule be equal or less than the original diameter in order for the capsule to be most effective. Failure in the buckling mode satisfies this criteria while if the capsule failed in the yield mode an increase in diameter of the collapsed portion would most likely be experienced. It is also important that collapse be predictable under general operating conditions in a reactor. Failure in the buckling mode ensures the desired design predictability inasmuch as the modulus of elasticity is only subject to an approximate one percent change in value due to irradiation over the life of the fuel element. Even this small variation can be minimized by positioning the dimpled portion of the capsule close to the fuel so that collapse propagates away from the fuel and thus away from the source of irradiation. In contrast, failure dominated by the yield mode is dependent on variables that can change fifty to sixty percent over the life of the fuel. Thus, it can be appreciated that providing a capsule collapsible in a buckling mode is an important contribution of applicant's invention. The initial dimple manufactured into the capsule not only controls the mode of failure as previously described, but also the number of failure lobes 17 that result from collapse of the material adjacent the dimple 16, effectively expanding the dimples longitudinal length. FIGS. 2, 3 and 4 illustratively show the collapse propagating along a single lobe as it would under the influence of an increase in pressure within the plenum of a fuel element in a reactor during power operation. A single dimple can result in the formation of one, two, or three lobes dependent on the design of the dimple. For example, the dimple shown in FIGS. 2 and 2A will result in the three lobes shown in FIG. 4A even though only one is apparent from the view shown in FIG. 4. The lobe, or lobes, propagate in a straight line down the longitudinal length of the capsule at a rate sufficient to accommodate the additional volume of fission gas accumulated. Furthermore, the elastic nature of initial deformation in the buckling mode accommodates temperature excursions and resulting pressure spikes experienced at the beginning of the operating life of the fuel element in a reactor. Fuel rod pressure control is maintained by the capsule in the following way. When the fuel rod pressure equals the initial collapse pressure of the dimpled capsule end, initial collapse occurs as seen in FIG. 3. Accordingly, a slight reduction in the fuel rod pressure results (50-100 psi). As more fission gas is generated the fuel rod pressure will increase until it equals the running collapse pressure of the capsule 14. The fuel rod pressure will then be maintained at a constant level, because as more fission gas is released, the collapse will propagate along the length of the capsule at a rate sufficient to maintain constant rod pressure as indicated in FIG. 4. Thus, it will readily be recognized by those skilled in the art, from the aforegoing disclosure, that the running collapse pressure of the capsule 14 can be desirably designed to meet the specification of any particular fuel rod by the judicious choice of material for the capsule tubing, the wall thickness of the tubing, the capsule length or pressurization of the capsule during manufacture. Accordingly, this invention maintains a substantially constant pressure within a pressurized fuel element by utilizing a collapsible capsule which slowly collapses at a rate controlled by the internal pressure of the fuel element and thereby maintains a near constant pressure within the fuel rod throughout operating life. The advantages of a collapsible capsule, other than its ability to maintain a substantially constant pressure, are the cost advantage provided by its ease of manufacture and its avoidance of cyclic differential pressure patterns which might otherwise subject the fuel rod cladding to rupture. The total required plenum volume, when utilizing such collapsible capsules can be reduced in view of the fact that a "ballast" or "sacrifice" volume is no longer needed to help reduce pressure excursions as previously required by the prior art. Thermal spikes encountered during reactor operation, which result in pressure spikes, cause only a small reduction in pressure within the fuel rod plenum after the spike is completed when utilizing collapsible capsules. However, when employing prior designs, the spike would normally result in a much lower pressure within the plenum than desired after the spike has receded. Additionally, normally only one capsule is required per fuel element reducing the fuel element costs over prior designs. Thus, this invention provides a device for controlling the internal pressure of nuclear fuel rods which makes it possible to maintain a near constant pressure within the fuel rod during its operating life. The simplicity of the design results in a relatively low manufacturing cost and greater reliability. Furthermore, plenum volume required to accommodate fission gas accumulation within the fuel rod during reactor operation is less than that required by most prior designs.