1. Field of the Invention
The invention described herein relates to gravity-feed loading apparatus and more particularly to apparatus for delivering highly frangible nuclear fuel pellets into a container, such as a sintering boat, prior to further processing the pellets in a system.
2. Description of the Prior Art
The present invention is particularly suitable for delivering green, i.e. unfired, uranium dioxide fuel pellets from the press head of a pelletizing press to a sintering boat for subsequent firing into a ceramic-like fuel pellet used in a fuel rod in a nuclear reactor.
The pellets have a generally cylindrical configuration, with slightly dished-in areas on opposite ends, ranging in size from about 0.3 inches to 0.8 inches long and 0.25 inches to 0.5 inches in diameter. The pellets are formed from uranium dioxide powder which is flowed into the die of the press.
As the power-producing capacity of the array of fuel pellets in a fuel rod is dependent upon their density, it is necessary to obtain generally uniform density among the fired, end product ceramic pellets. Such preferred density is generally on the order of 95% of the theoretical density. However, it is known that various uranium dioxide powders have sufficiently different compaction and firing characteristics so that to obtain a desired end result, the pellets must be formed in the press at different pressures depending upon the inherent characteristics of the feed powder. It is generally known that as the know-how and experience level increased in powder-producing technology, less and less pressure on feed powder in the press was required to yield the desired density of a pellet.
The experience of the corporate assignee of the present invention has typically conformed to the above sequence, thus, during early years of powder-forming technology, when the pressures were required to be relatively high, for example, in the order of 75,000 psi, the resultant green pellet was extremely hard and could be subjected to substantial abuse without chipping or other damage prior to firing. Therefore, the handling technique for removing fuel pellets from the press and depositing them into the firing vessel could be rather crude, and, for the most part, the pellets were permitted to tumble down an inclined ramp from the press head to above the boat and free-fall thereinto. Another arrangement, although more complex, is shown in U.S. Pat. No. 3,897,673, wherein an inclined ramp is used to deliver pellets to a loading zone. However, as pressing pressures decreased because of improved powder producing technology, the required pressing pressure became less and the green pellets formed thereby became softer, and transporting them via a continuous inclined ramp to a lower collecting point resulted in substantial chipping even when the inclined plane had a resilient surface. Such chipping was particularly evident at the end edges of the pellets. This loss of the material from the pellets had the ultimate effect of decreasing the energy producing capacity of the reactor and such damaged pellets were accordingly rejected. Scrap pellets were reworked through a process that permitted the material to be reintroduced into the powder for repressing. Due to the fact that there is always some damage to some of the pellets during manufacturing, all pellets are inspected for defects and classified accordingly as class "A", class "B", or "scrap". It is preferred that fuel rods be filled with class "A" pellets. However, due to the economics involved in reprocessing, a quantity of class "B" pellets (up to 10% of the quantity of class "A" pellets) can be used as "bottom end" pellets in each rod. Class "B" pellets that are produced in excess of the 10% limit are reprocessed along with the "scrap" pellets.
Thus, as the powder producing technology has advanced, it has become increasingly important to provide a mechanism for delivering the soft pellets from the pressing head to a sintering boat in a manner which minimizes damage to pellets and which requires a minimum of operator attention and adjustment of the mechanism and is relatively inexpensive and non-complex.