There are numerous components of industrial and utility equipment whose dimensional and geometric characteristics are of a critical nature and therefore must be manufactured to extremely strict specifications. A notable example of one such critical component is the flow channel of a nuclear fuel assembly or bundle, such as disclosed in U.S. Pat. No. 3,689,358. These channels are elongated tubular components of square cross section, which may measure approximately 6 inches on each side and on the order of 14 feet in length. Typically, the channels are created by seam welding two U-shaped channel sections together. The preferred material is a zirconium alloy, such as Zircaloy, on the order of 125 mils thick. It is imperative that these flow channels are manufactured to the proper dimensions and be free of geometric irregularities, such as face or side bulge, out-of-square cross section, non-parallelism of sides, longitudinal bow and twist, and the like. Unfortunately, the channel creating step leaves residual geometric irregularities. Thus as a final manufacturing step, the channels must undergo thermal sizing to eliminate these irregularities.
The thermal sizing step involves inserting an elongated, close-fitting mandrel into the channel and raising the temperature of the channel to about 1100.degree. F. in an inert atmosphere. The mandrel is expanded into engagement with the channel, causing the channel to yield plastically to the specified final form. Typically, the outwardly directed mandrel forces are exerted solely on the four corners of the channel throughout their lengths. When the channel is returned to room temperature, the mandrel is withdrawn, leaving the channel in a stable form substantially free of geometric irregularities. It has been found that mandrel insertion is sometimes quite difficult due to the fact that certain irregularities produce sliding friction between the channel and the mandrel as the latter progresses into the channel passage. A particularly serious problem, however, is scoring of the interior corner surfaces inflicted during mandrel insertion, mandrel withdrawal, and/or relative movements of the mandrel and channel surfaces during the heat-up/cool down cycle due to differential thermal expansion. Such surface blemishes have been found to be a source of future corrosion in the harsh environment of a nuclear reactor core, and therefore must be avoided.
It is accordingly an object of the present invention to provide improved hot-forming apparatus for the thermal sizing of elongated tubular channels to remove geometric irregularities therefrom.
A further object is to provide hot-forming apparatus of the above-character, wherein the forces required to insert and withdraw a hot-forming mandrel from the channel passage are significantly reduced.
Another object is proved hot-forming apparatus of the above-character wherein scoring of the channel interior surface is largely avoided.
An additional object is to provide hot-forming apparatus of the above-character, which utilizes an improved mandrel for developing the channel forming forces.
Other objects of the invention will in part be obvious and in part appear hereinafter.