Design Evolution Study: Thermal Operating Methodology

This study provides results supporting the conclusion that the repository can be operated over a varying range of thermal modes and therefore temperatures. In particular, this work focused on limiting the peak, postclosure waste package surface temperature to less than 85 degrees Celsius, a possible limit due to corrosion considerations. These operating modes were compared by varying the waste package in drift spacing (0.1-2.83 meters), drift pitch (drift spacing centerline to centerline of 40-120 meters), ventilation duration (75-300 years), and ventilation efficiency (50-80%). The resulting graphical representation shows where the constant temperature of the waste package (85 degrees Celsius) lies with respect to drift pitch and waste package spacing.
The waste considered in this study is the strict youngest fuel first 5 years old fuel. Using only strict youngest fuel first 5 years old fuel in the waste stream results in an average heat load per waste package of 12.48kW/Pkg. With this high average heat load, it is not possible to achieve a maximum waste package surface temperature of 85 degrees Celsius or less. By aging 63% of the strict youngest fuel first 5 years old fuel for 27 years, it becomes possible to maintain the waste package surface temperature at 85 degrees Celsius or less. The 27 years of aging comes from the fact that the repository could be closed in as little as 50 years. It takes 23 years to emplace the waste and therefore the last fuel received for emplacement is 27 years prior to closures. The strict youngest fuel first 5 years old fuel waste stream with 63% aged for 27 years, results in an average power level of 8.4kW/Pkg. This lower heat load allows the controlled parameters of drift pitch, waste package spacing, aging, and ventilation duration to be varied to achieve the desired results.
This study compares the hot strict youngest fuel first 5 years old fuel to the previous waste stream, which has an average of 26-year-old fuel. The 26-year-old fuel waste stream has an average power level of 7.2 kW/Pkg. This comparison illustrated how the parameters important to thermal performance differ for these two assumptions about the waste stream.