Since 1989, EPRI has been conducting independent assessments of the proposed deep geologic repository for the disposal of spent nuclear fuel and high level radioactive waste at Yucca Mountain, Nevada. EPRI pioneered application of the total system performance assessment (TSPA) approach for evaluating performance of geologic repository systems on a probabilistic basis. Along the way, EPRI developed the Integrated Multiple Assumptions and Release Code (IMARC) as its primary analytical tool for TSPA-based evaluations.