Focus ’93, Site Characterization and Model Validations, Las Vegas, Nevada, 26–29 September 1993 Authors: L. S. Costin (Sandia National Laboratories), and M. P. Hardy and C. E. Brechtel (AAI) Thermomechanical models are being developed to support the design of an Exploratory Studies Facility and a potential high-level nuclear waste repository at Yucca Mountain, Nevada. These models are used for preclosure design of underground openings such as access drifts, emplacement drifts, and waste emplacement boreholes; and in support of post-closure issues such as performance of the waste canister, disturbance of the hydro-geological properties of the host rock, and performance of the overall system. Therefore, validation is an important process that must be pursued in conjunction with the development and application of models. This paper proposes a process for thermomechanical model validations; however, the process is general and could be extended to models for other phenomena. Both empirical models used in rock engineering design and more complex numerical models used for design and performance assessment are addressed. The validation process itself is viewed as having three main components: evaluation relative to experimental data obtained from in situ and laboratory tests, evaluation relative to empirical evidence and case histories (including natural analogues), and evaluation by peer review.