Tritium effects on the performance of turbomolecular pumps

A special 150 L/s turbomolecular pump was used to pump pure tritium in closed loop mode over the course of six separate tritium runs. About 5.3 kCi of tritium was pumped during a cumulative period of 237 days. The pump, supplied by Leybold Heraeus, Germany for the purpose of tritium testing, features heavy duty bearings and motor, metal body seals, and radiation resistant coatings on electrical components. No tritium related degradation of the pump has been observed after 12,000 hours of operation. A liquid nitrogen cooled trap, used to trap the outgassing products from the system, required periodic regeneration. The loop was vacuum baked and recharged with tritium every 14 to 63 days of tritium testing. A bearing failure at 7176 hours is believed to have resulted from excessive bearing loads imposed by high system pressures. Testing was resumed after the original oil lubricated steel bearings were replaced with grease lubricated ceramic bearings. Vibration monitoring of turbomolecular pump bearings can be effectively used to predict remaining bearing life and to schedule maintenance. The study is intended to continue until the pump has undergone one year of cumulative tritium service.