Critical parameters and measurement methods for post closure monitoring: A review of the state of the art and recommendations for further studies

Abstract

Both NRC and EPA regulations require programs of post closure monitoring to detect substantial and detrimental deviations from expected performance. The unexpected in this case would involve anomalous stress changes that might rupture the canisters or changes in the hydrologic regime that might accelerate corrosion. In the event of leakage brought about by any means transport of radionuclides to the accessible environment could occur through unexpected changes in the hydrologic flow regime caused either by the long term effects of the thermal loading by the waste or by changes in regional stress or hydrology. Studies of performance confirmation have identified six parameters or conditions that should be monitored that are associated with the thermal, mechanical and hydrologic phenomena introduced by the waste heat: temperature, stress, displacement, pore pressure, groundwater velocity and permeability. Since it is the thermal load that continues to increase after decommissioning, and which continues to alter the stress field and the hydrological regime, these same six parameters remain the critical ones in post closure monitoring. At two of the repository sites fractures have been clearly shown to be critical in modelling and performance confirmation; at the tuff site fluid saturation is also a critical parameter and for all the sites estimates of the groundwater velocity through the site are very important. Changes in fracture properties, saturation and fluid flow are thus of continuing importance in post closure monitoring. 14 refs., 19 figs.