Mechanical modeling of nuclear waste disposal in argillite at the Nevada Test Site

Numerical calculations for a near surface heater experiment in argillite conducted at the Nevada Test Site were performed using the finite element code ADINA assuming a two-dimensional axisymmetric geometry. The existence and extent of the region of tensional opening of joints surrounding the heater, predicted by the mechanical model, were confirmed by posttest borehole inspection, permeability measurements, and drillback. Exrapolation of near surface heater model to repository depths reveals the necessity for prior knowledge of the mechanical properties and state of stress in-situ. The extent of the joint opening zone, for example, is not altered by changes in the elastic modulus at the near surface, but is significantly decreased beyond certain depths depending upon the in-situ elastic modulus. Results of these calculations are presented. To further define the behavior at depth, and place bounds on the joint opening zone, far-field calculations were performed for a generic repository in argillite. Both spent fuel and high level waste heat sources were considered at different burial densities and depths. Results of a parametric study are presented in which the mechanical properties, in-situ stresses, and waste heat sources were varied