South Dakota: Rock-Melt Monitoring

South Dakota
US Department of Energy

Campbell Scientific monitoring-and-control gear key to important nuclear-waste disposal research

The US currently has an estimated 75,000 tons of spent nuclear fuel housed at dozens of locations across the nation that are not designed for long-term storage. In 2012, the Blue Ribbon Commission on America’s Nuclear Future Report to the Secretary of Energy (2012) identified an urgent need for the US to develop a new strategy for long-term disposal of nuclear waste.

Geological disposal of nuclear waste has historically focused on mined facilities, but the deep-borehole disposal option has recently received attention because many factors suggest that this disposal method is inherently safe. A vital component in the concept is a borehole-sealing system that can successfully isolate nuclear waste. Such a system was the focus of the rock-melt borehole-sealing system (RMBSS) research that RESPEC conducted for the US Department of Energy.

From 2015 to 2017, RESPEC designed and tested an RMBSS system at its laboratory in Rapid City, South Dakota. Silicon carbide-heating elements were powered by a Spang 1051 SCR power controller was controlled by a Campbell Scientific CR1000. An NL120 Ethernet interface was also used to network the CR1000 and allowed for remote operation and data viewing.

In the testing facility, granite boulders and backfill material were used to simulate a borehole environment. Thermocouples (TCs) were used to measure heater, rock, and backfill temperatures at various locations. The CR1000 was initially programmed to operate as a proportional-integral-derivative (PID) controller based on the temperature of the heating element and nearby melt; however, stray voltages that seemed to develop in the melt at extreme temperatures were difficult to eliminate, scrambled the TC data, and caused the PID algorithm to fail.

The CR1000 was then programmed to slowly step up the heater power over several days while collecting temperature data so as not to induce thermal shock and early fracturing. Twelve tests were conducted with temperatures exceeding 1,500 degrees Celsius (°C) (2,732 degrees Fahrenheit [°F]) at the heat source, and the granite melted at 1,100°–1,200°C (2,012°–2,192°F).

Permeability tests were performed on pre- and postmelt specimens. Long-duration tests that lasted 1 to 2 months achieved a better melt, with permeability much lower than the results in pretest specimens.

In 2016, underground tests were performed at the Sanford Underground Research Facility in the former Homestake Mine in Lead, South Dakota, at 518 meters (1,700 feet) below the surface. Five boreholes were drilled between two levels (the 1550 and 1700 Levels) to create test zones in rhyolite rock. Hydraulic conductivity tests of the borehole wall were performed within the boreholes to determine hydraulic conductivity at 3-meter (10-foot) intervals. A Campbell Scientific CR300 datalogger was used to collect pressure and flow data during conductivity tests.

On the 1700 Level (below the test zone), a Campbell Scientific CR300 and NL200 network interface collected and transmitted geophone and closure-pole data. Geophones were installed to detect potential rock fracturing caused by thermal-mechanical stress, and custom closure poles were used to measure the displacement of the mine drift.

On the 1550 Level (above the test zone), a Campbell Scientific CR1000 and NL201 network link were used to control the heater power supply, measure temperatures, and measure the water level as water accumulated in the boreholes. Alarms were configured that would be triggered if the voltage, current, or temperatures drifted out of range or the connection to the NL201 was lost. Four tests were performed between 30 and 43 meters (100 and 140 feet) down the boreholes.

This research identified two primary concerns that must be addressed before the technology can be applied in a deep-borehole environment meant for waste disposal. These challenges include withstanding high pressures and understanding the duration of time needed to create a seal without damaging the rock through thermal-mechanical fracturing.