Understanding the Irregular Data from the Large Block Test

Abstract

The Large Block Test (LBT) is one of three thermal tests at Yucca Mountain, Nevada. While two of those three tests (the Single Heater Test and the Drift Scale Test) were large underground tests, the LBT was above ground. The test block, with a size of 3 x 3 x 4.5 m, has a highly heterogeneous permeability field, with at least four orders of magnitude variation in permeability. Some of the temperature data from the LBT appear irregular at first glance, but on closer examination can be attributed to this heterogeneous nature of the fractured block. In this project, we attempt to develop a better understanding of these irregular temperature data from the LBT. The test block in the LBT was excavated from the surrounding Topopah Spring fractured nonlithophysal tuff in the Fran Ridge area of Yucca Mountain. The bottom of the block remained attached to the underlying rock, while the top and the four side faces were kept open to the atmosphere. Mapping of all the fractures from the four sides and the top surface of the block revealed the existence of some large fractures in the block. The presence of such large fractures indicates that the flow of water and heat-induced vapor can be very different in different zones of the block, with vapor and water flowing more easily through the highly permeable fractures than elsewhere. It is our hypothesis that the irregular temperature data arise out of thermal-hydrological processes taking place in the heterogeneous fracture system. We show, through simulations performed with a 3D dual-permeability numerical model based on the TOUGH2 simulator, that such a heterogeneous fracture system can indeed give rise to irregular temperature data.