The Single Heater Test (SHT) is one of two in-situ thermal tests included in the site characterization program for the potential underground nuclear waste repository at Yucca Mountain. The heating phase of the SHT started in August 1996, and was completed in May 1997 after 9 months of heating. The coupled processes in the unsaturated fractured rock mass around the heater were monitored by numerous sensors for thermal, hydrological, mechanical and chemical data. In addition to passive monitoring, active testing of the rock mass moisture content was performed using geophysical methods and air injection testing. The extensive data set available from this test gives a unique opportunity to improve the understanding of the thermal-hydrological situation in the natural setting of the repository rocks. The present paper focuses on the 3-D numerical simulation of the thermal-hydrological processes in the SHT using TOUGH2. In the comparative analysis, they are particularly interested in the accuracy of different fracture-matrix-interaction concepts such as the Effective Continuum (ECM), the Dual Continuum (DKM), and the Multiple Interacting Continua (MINC) method.

Operations and maintenance continued this month at the Electric Power Research Institute`s (EPRI`s) Environmental Control Technology Center (ECTC). Testing for the month involved the Dry Sorbent Injection (DST) test block with the Carbon Injection System. The 1.0 MW Cold- Side Selective Catalytic Reduction (SCR) unit, the 0.4 MW Mini- Pilot Wet Scrubber, and the 4.0 MW Pilot Wet Scrubber remained idle this month in a cold-standby mode and were inspected regularly. These units remain available for testing as future project work is identified.

This closure report provides the documentation for closure of the Cactus Spring Waste Trenches Corrective Action Unit (CAU) 426. The site is located on the Tonopah Test Range,approximately 225 kilometers (140 miles) northwest of Las Vegas, Nevada. CAU 426 consists of one Corrective Action Site which is comprised of four waste trenches. The trenches were excavated to receive solid waste generated in support of Operation Roller Coaster, primarily the Double Tracks Test in 1963, and were subsequently backfilled. The Double Tracks Test involved the use of live animals to assess the biological hazards associated with the non-nuclear detonation of plutonium-bearing devices (i.e., inhalation uptake of plutonium aerosol) (DOE, 1996). The remedial alternative proposed Nevada Division of Environmental Protection proposed the capping method. The closure activities were completed in accordance with the approved Corrective Action Plan and consisted of constructing an engineered cover in the ar ea of the trenches, constructing/planning a vegetative cover, installing a perimeter fence and signs, implementing restrictions on future use, and preparing a post-closure monitoring plan. Closure activities for CAU 426 have been completed in accordance with the Nevada Division of Environmental Protection approved Corrective Action Plan as documented in this Closure Report.