Thermally induced acoustic emissions in salt at the Waste Isolation Pilot Plant

A series of field-based observations were conducted as a part of the Brine Availability Test in Salt (BATS) conducted at the Waste Isolation Pilot Plant (WIPP) Carlsbad, NM. BATS focuses on exploring brine availability as part of a wider investigation into the disposal of heat-generating radioactive waste in salt. Brine has the potential to transport radionuclides, corrode waste forms and packages, reduce criticality, and pressurize porosity to resist closure through salt creep. In BATS, two identical borehole arrays were constructed along a drift in WIPP. In each array, observational boreholes were installed around a centralized borehole. One array was heated, and the other array was maintained at the formation temperature. In this initial phase, the heater ran for 4 weeks, and the heated borehole reached a maximum temperature just over 80°C. In both arrays, 3 observational boreholes were dedicated to monitoring emitted acoustic emissions, AE, as a proxy for thermally induced cracking, with 8 sensors in the heated array and 6 sensors in the unheated array. Observed AE demonstrate that heating and cooling cycles have a dramatic impact on salt. In the heated borehole, AE rates surged at the start of heating, quickly reaching a peak before decaying to a steady state level. When heating stopped, AE rates again surged to levels 4x higher than the previous rate maximum. AE rates decayed as the formation cooled. Associated with these peaks, average energy and frequency bandwidth increased with heating and cooling. Locations of AE events remained centralized around the borehole, and the average radial distance from an event to the borehole increased with heating and again with cooling. In the unheated borehole, AE rates remained near background levels, but increases were detected at the onset of heating and cooling. The increased rates did not result in changes in signal energy or amplitude. Located events remained centered around the borehole. These results show that AE provide valuable information about the thermal behavior of salt, demonstrating that cooling of salt rather than initial heating is more likely to create fractures that may change near-wellbore hydrology.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

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