Long term safety assessment of geological waste disposal systems: issues on release scenarios

Geological insolation of high level nuclear waste is an attractive waste disposal concept. However, long term safety demonstration of this concept is a major challenge to the operators, regulators and the scientific community. Identification of the factors responsible for the release of radionuclides from geosphere to biosphere,is first step in this regard. Current understanding of the release scenarios indicates that faulting, ground after percolation, seismicity, volcanism and human intrusion are the dominating release factors for most of the candidate rock formations. The major source of uncertainties is the probability values of various release events due to random nature of catastrophic geological events and past poor historical records of the frequencies of such events. There is consensus among the experts that the waste release via human intrusion is the most unpredictable scenario at present state of the knowledge. (author)