Patent Number: 039895904
Section: claims

1. A clad nuclear fuel element comprising an elongated hermetically sealed casing member, nuclear fuel occupying a portion of the interior of said casing member and the remainder of said casing member forming a gas filled plenum pressurized during manufacture with a predetermined volume of gas, at least one hermetically sealed elongated capsule having no extraneous internal support within the capsule, occupying at least a portion of the interior of said plenum out of contact with the casing member, said capsule formed with a depression in a wall of the capsule extending substantially less than the longitudinal dimension thereof, the depression controlling collapse of the capsule in a predetermined manner in a buckling mode, proportionally linearly with increases in gas generation that would result in an increase in pressure above a given designed level within said plenum due to fission gas accumulation generated under irradiation if the capsule did not collapse in a buckling mode, to provide additional volume in said plenum resulting from collapse of the capsule to the extent necessary to accommodate the increase in gas volume to maintain the pressure within said plenum substantially constant throughout the operation of the fuel element under irradiation. 2. The nuclear fuel element of claim 1 wherein the depression in said capsule is prefailed at manufacture past the yield point of the material from which the capsule is constructed in order that initial collapse during operation under irradiation occur in a buckling mode. 3. The nuclear fuel element of claim 1 wherein the depression is mechanically formed as a dimple in a wall of said capsule. 4. The nuclear fuel element of claim 1 wherein said depression is formed only in one end of said capsule. 5. The nuclear fuel element of claim 4 the elongated dimension of said capsule is supported along the longitudinal dimension of the fuel element and wherein the depression is formed at the end of the capsule closest the fuel. 6. The nuclear fuel element of claim 1 wherein the elongated dimension of said capsule is supported along the longitudinal dimension of the fuel element and wherein said capsule collapses in a controlled manner in response to an increase in gas accumulation within said plenum during operation of the fuel element under irradiation extending the depression in a straight line along the longitudinal dimension of the capsule in the form of a lobe at a rate sufficient to maintain said plenum pressure substantially constant. 7. The nuclear fuel element of claim 6 wherein said depression is designed to result in a plurality of said lobes around the circumference of said capsule, each of said lobes expanding the collapse of said capsule along corresponding straight lines parallel to the longitudinal dimension of said capsule in a controlled predictable manner in response to an increase in gas accumulation within said plenum during fuel element operation under irradiation to maintain said plenum pressure substantially constant. 8. The nuclear fuel element of claim 1 wherein said capsule is constructed from a tubular member hermetically sealed at both ends forming an interior chamber filled at manufacture with a predetermined volume of gas designed to control the collapse of the capsule of the desired rate to maintain a substantially constant pressure within the fuel element during fuel element operation under irradiation. 9. The nuclear fuel element of claim 1 wherein said capsule is constructed from stainless steel.