This invention relates generally to the reprocessing of spent nuclear reactor fuel elements to recover the fissile material. Particularly the present invention is concerned with the dissolution of the fuel elements as an initial step in the separation and recovery of the fissile materials by liquid extraction techniques. More particularly, the invention concerns the dissolution of pressurized water reactor fuels containing oxide fuel wafers, and specifically concerns the dissolution of zirconium-clad ZrO.sub.2 -UO.sub.2 -CaO-type fuel.
As is well known in the art, spent nuclear reactor fuel elements are reprocessed in order to recover the valuable fissionable material for reuse as a nuclear fuel. In the very commonly used liquid extraction reprocessing techniques, the fuel elements are dissolved into solution for subsequent separation of the fissionable material from the fission products, cladding material, etc. and ultimate recovery of the fissionable material. At the head end of such reprocessing schemes, there is generally a dissolver vessel in which is carried out the initial step of dissolving the nuclear reactor fuel elements to place the materials into solution for the subsequent liquid extraction separation steps. A particular nuclear fuel reprocessing facility employing these techniques is the Idaho Chemical Processing Plant located at the Idaho National Engineering Laboratory, formerly named the National Reactor Testing Station, in southeastern Idaho. An understanding of fuel reprocessing and fissile material recovery systems in general, and specifically the system in use at this reprocessing facility, can be obtained from a more detailed discussion contained in U.S. Atomic Energy Commission Report No. IN-1471, entitled "Zirconium Fuel Reprocessing Campaign of 1960", which report is incorporated herein by reference.
One particular type of fuel which must be dissolved in order to permit recovery of the enriched uranium is calcium stabilized uranium dioxide - zirconium dioxide which is used as a pressurized water reactor fuel. This fuel consists of sintered wafers of enriched uranium dioxide - zirconium dioxide - calcium oxide which are clad in zirconium or Zircaloy. These oxide wafers are highly resistant to dissolution in hydrofluoric acid, nitric acid, and other common dissolvents acceptable to aqueous chemical reprocessing.
The dissolution processes employed to date for these wafers have not proved entirely satisfactory. Until the process of the present invention the most successful dissolution process available for these fuels was the dissolution of the cladding material with 7 M hydrofluoric acid and subsequent dissolution of the oxide wafers with concentrated hydrofluoric acid (18 M) with 0.1 M CrO.sub.3 used either simultaneously or as a succeeding treatment. However, this fuel dissolution process was less than satisfactory from the standpoint of dissolution time, waste volume and, of utmost importance, corrosion.
In addition, the exposure of the oxide fuel material to the high hydrofluoric acid solution converts the calcium oxide and some of the zirconium oxide to insoluble calcium fluozirconate and the skeletons of the wafers which had been converted to calcium fluozirconate posed a problem in waste handling. One technique for dealing with the calcium fluozirconate remaining after complete dissolution of the uranium oxide and the zirconium metal was to dissolve the material with a dilute solution of sodium hexametaphosphate. However, this required an additional step in the total dissolution process.
While other dissolution processes have been sought, other common dissolvents have been found to be ineffective in the dissolution of the highly resistant fuel wafers. Also it was found that other dissolvents which were found able to attack the fuels, even at extremely slow rates, were highly corrosive to a wide variety of materials of construction.
Consequently, it is an object of the present invention to provide a dissolution process for ZrO.sub.2 -UO.sub.2 -CaO-type reactor fuels.
It is another object of the present invention to provide a dissolution process for this type of fuel which proves satisfactory from the standpoint of dissolution time, waste volume produced and total dissolution of the materials present.
A particular object of the present invention is to provide a dissolution process for ZrO.sub.2 -UO.sub.2 -CaO-type pressurized water reactor fuels which will provide satisfactory dissolution of the fuel within satisfactory corrosion limits for the dissolver vessel and other processing equipment.
Other objects and advantages of the present invention will become apparent upon reading the following description of the invention.