Nuclear Technology / Volume 163 / Number 1 / July 2008 / Pages 47-61
Technical Paper / High-Level Radioactive Waste Management
The nuclear waste storage concept according to the baseline design of the proposed high-level nuclear waste repository at Yucca Mountain is analyzed. The high-temperature storage concept, in which the emplacement area is heated above the boiling temperature of water, is subject to criticism on the basis of uncertainties due to nonlinear multiphysics processes in the rock mass and in the storage airspace. The storage environment around the nuclear waste containers is reexamined using a new thermal-hydrologic airflow model. The complex nature of the thermal-hydraulic behavior in a superheated waste repository is described with fewer simplifying assumptions than those used in the baseline design. The emplacement area in the mountain is described as an open system, in which the air pressure is connected to the barometric pressure through fractures, faults, and partially sealed drifts. The cyclic variation of the atmospheric pressure that affects the heat and mass transport processes in the near-field rock mass is also modeled. The implications of evaporation into the drift airspace are discussed, and a hypothesis of salt accumulation in the near-field rock mass is established. Model calculation is also presented for a below-boiling temperature storage concept that is easier to predict and has fewer anomalies. The price for a below-boiling temperature storage is the extended preclosure ventilation time period. However, as demonstrated for a trade-off, it is possible to design a repository with below-boiling temperatures and doubled waste inventory at the same time.