Laboratory ventilation tests are being conducted for the design of a potential high-level radioactive waste repository at Yucca Mountain. The tests were initiated to validate a numerical approach developed to simulate the ventilation for the repository emplacement drifts to manage temperatures during the preclosure period. The tests have been planned for four phases, each corresponding to different conditions anticipated in the eraplacement drifts. In the tests of Phase I, four air flow rates and two power output levels were used. The numerical approach used in the pretest predictive calculations and the results from the tests of Phase I are presented. Comparisons between the predicted and the measuredata indicate that the approach used to simulate the ventilation for the repository design can provide a reasonable assessment on the performance of ventilation as long as the convection heat transfer coefficient is appropriately estimateat.
Use of continuous ventilation during the preclosure period to manage the emplacement drift and waste package temperatures is an important feature in the current design for the potential repository at Yucca Mountain, Nevada. How to correctly predict the performance of ventilation using numerical simulation has become a critical issue. In order to examine and validate the numerical approach used for the ventilation modeling and evaluate the performance of the ventilation system, a ventilation test is being conducted.
The following conditions are being tested: (I) ambient air at the inlet fan with no obstruction in the simulated emplacement drift other than simulated waste packages; 0I) conditioned air at the inlet to simulate a portion of the emplacement drift toward the middle or end of a ventilation mn where the air will be heated upon its arrival at that position; hot and moist air being ventilated due to emplacemerit drift inffitration; and (IV) rapid cooling by higher than normal air flow rates to determine ventilation effectiveness under conditions that could exist after the ventilation system was temporarily inoperable or waste retrieval was required. The data collected from the test can be used to validate and, if necessary, modify the numerical model for the ventilation simulation. Discussion in this paper is limited to the tests and predictions related to the Condition I or Phase I.
Pretest predictive calculations are conducted using the computer prog!am ANSYS. The calculations use a numerical model that couples the heat transfer simulation carried out by ANSYS with the additional energy balance calculation for air flow handled by an Excel spreadsheet. With the energy balance calculation, the total heat removed by the ventilation air and its temperature can be estimated. The ventilation problem is three-dimensional in nature, and for simplicity is approximated with a twodimensional model by dividing the test section into several segments. Within each segment, the parameter variables are assumed to be constant over the segment length.
Predicted temperatures of air, simulated waste package, and concrete pipe that represents an emplacement drift to be excavated in the welded tuff at the Yucca Mountain are obtained for different heat power levels and air flow rates to examine the sensitivity of the system performance to various parameters. These temperatures can be used to compare with the measured data from tests to evaluate the validity of the numerical model. Furthermore, overall efficiency of ventilation in heat removal can be better understood.
