Far-field thermal analysis of a high level waste repository in tuff. Topical report RSI-0137

Thermal loadings of 25, 50, 75, 100, and 150 kW/acre were used for thermal analyses of a high-level waste repository at a depth of 800 m in tuff. Three methods of handling the presence of pore water were considered; the first assumed that pore water did not boil, the second that pore water boiled in the interval 900C to 1100C, and the third that pore water boiled at a temperature governed by hydrostatic pressure. The results of two modeling methods performed independently by RE/SPEC and Texas A and M University are presented. The RE/SPEC model was a two-dimensional finite element model while the Texas A and M model was a one-dimensional finite element model. The results of the two modeling methods were nearly identical. It was found that hydrostatic boiling did not occur for any thermal load considered and that boiling in the interval 900C to 1100C did not occur for thermal loads of 25 or 50 kW/acre. The most important result was the significant suppression of maximum temperatures because of energy consumed in boiling pore water. The peak temperature suppression ranged from zero at a thermal load of 25 kW/acre to nearly 22 percent at 150 kW/acre