Hydraulic properties of Beishan granite after different high temperature treatments

Abstract

Disposal of high-level radioactive waste (HLW) is one of the most challenging subjects across the world. And it is reported that China intends to build an HLW repository in granite strata in Beishan, Gansu Province. In this research, to explore hydraulic properties of Beishan granite considering thermal damage, specimens were heated up to 300, 400, and 500 °C, respectively. Then, hydro-mechanical (HM) coupled tests were conducted after specimens were cooled to room temperature. It was observed that a large amount of acoustic emission (AE) is monitored during the initial compaction and elastic stage in the HM coupled tests. For mechanical response, under each fluid pressure, the peak strength and crack damage threshold gradually decrease when the treatment temperature is within 400 °C. However, they show increase after 500 °C treatment compared with that of 400 °C. In addition, peak strength and crack damage threshold reach their maximum under fluid pressure of 4 MPa for each treatment temperature. The change tendency of strain with treatment temperature is roughly the same as that for the strength. And it is noticeable that the crack damage threshold strain at 500 °C is even higher than that of specimens without heat treatment. As for the permeability of Beishan granite, the initial permeability does not change much after different high temperature treatment and maintains at about 10−18 m2 under confining pressure of 20 MPa. It is revealed that before peak stress is reached, the change of permeability with volumetric strain can be expressed by two linear relations, taking the expansion point as the dividing point. And a permeability evolution model of Beishan granite is proposed. The obtained results are of great importance for the safety of an HLW repository.