Decontamination of Zircaloy Cladding Hulls From Spent Nuclear Fuel

The feasibility of decontaminating spent fuel cladding hulls using hydrofluoric acid (HF) was investigated as part of the Global Energy Nuclear Partnership (GNEP) Separations Campaign. The concentrations of the fission product and transuranic (TRU) isotopes in the decontaminated hulls were compared to the limits for determining the low level waste (LLW) classification in the United States (US). The {sup 90}Sr and {sup 137}Cs concentrations met the disposal criteria for a Class C LLW; although, in a number of experiments the criteria for disposal as a Class B LLW were met. The TRU concentration in the hulls generally exceeded the Class C LLW limit by at least an order of magnitude. The concentration decreased sharply as the initial 30-40 {micro}m of the cladding hull surface were removed. At depths beyond this point, the TRU activity remained relatively constant, well above the Class C limit. Reprocessing of spent nuclear fuel generates a cladding waste which would likely require disposal as a Greater than Class C LLW in the US. If the cladding hulls could be treated to remove a majority of the actinide and fission product contamination, the hulls could potentially meet acceptance criteria for disposal as a LLW or allow recycle of the Zr metal. Discard of the hulls as a LLW would result in significant cost savings compared to disposal as a Greater than Class C waste which currently has no disposition path. During fuel irradiation and reprocessing, radioactive materials are produced and deposited in the Zircaloy cladding. Due to short depths of penetration, the majority of the fission products and actinide elements are located in the ZrO{sub 2} layer which forms on the surface of the cladding during fuel irradiation. Therefore, if the oxide layer is removed, the majority of the contamination should also be removed. It is very difficult, if not impossible to remove all of the activity from spent fuel cladding since traces of U and Th in the unirradiated Zircaloy adsorb neutrons generating higher actinides in the bulk material. During fuel irradiation, {sup 92}Zr is also converted to radioactive {sup 93}Zr by neutron adsorption. Methods for decontaminating and conditioning irradiated Zircaloy cladding hulls have been investigated in Europe, Japan, and the US during the last 35 years; however, a method to decontaminate the hulls to an activity level which meets US acceptance criteria for disposal as a LLW was not deployed on a commercial scale. The feasibility of decontaminating spent fuel cladding hulls was investigated as part of the GNEP Separations Campaign. Small-scale experiments were used to demonstrate the removal of the ZrO{sub 2} layer from Zircaloy coupons using dilute solutions ({le}1.0 M) of HF. The most effective conditions resulted in dissolution rates which were less than approximately 2 mg/cm{sup 2}-min. With dissolution rates in this range, uniform removal of the oxide layer was obtained and a minimal amount of Zircaloy metal was dissolved. To test the HF decontamination process, experiments were subsequently performed using actual spent fuel cladding hulls. Decontamination experiments were performed to measure the fission product and actinide concentrations as a function of the depth of the surface removed from the cladding hull. The experimental methods used to perform these experiments and a discussion of the results and observations are presented in the following sections