Microwave drying of zirconium molybdate-99Mo gel in a shielded tong box: evaluation with respect to radiation safety aspects

99mTc is the workhorse of nuclear medicine. A project to take up regular processing of 99mTc gel generators on a medium scale is being executed at the Radiopharmaceuticals Programme, BRIT. Drying the zirconium molybdate-99Mo (Zr99Mo) gel-cake is a critical step in Zr99Mo gel granule preparation for use as column matrix in 99mTc gel generators. A domestic microwave oven was suitably modified, made amenable to remote operations, housed in a lead shielded tong box and used for drying the Zr99Mo gel-cake prepared with low levels of 99Mo activity (2.26 GBq to 18.5 GBq). Special arrangements were made for releasing the moisture-laden air from the microwave oven through the fume-hood exhaust line through the stack. Radiation safety aspects studied include radiation field around the facility, activity in laboratory air and fume-hood exhaust. The maximum air activity measured (at 1.5 m from the microwave oven) in the exhaust duct was 73.0 Bq/m3 when 18.5 GBq 99Mo was handled. The spot sampling during the operation showed 1.05 Bq/m3 (0.0026 DAC) in laboratory. No contamination in the working environment was detected during these operations. The adaptation of the microwave oven for safely drying Zr99Mo gel-cake in a shielded tong box with adequate personnel, laboratory and environment safety provisions has been established in the preliminary low activity experiments. (author)