A numerical method RFPA was used to study the cracking behavior in square-shaped rock material subjected to heating from a central borehole considering heterogeneity. The model was enlarged from small scale to large scale and subsequently to engineering scale to study the scale effects for thermal cracking using stress state analysis. In the large-scale model, the numerical test results showed that the crack initiated at a spot a certain distance away from the borehole and then propagated inward and outward until cutting through the model. For the engineering scale model, when one of the in situ stresses in the two vertical directions is considerably small, tensile stress was formed at the spot a certain distance away from the borehole in the host rock, which can present a serious potential safety hazard.
For many countries concerned about the safe isolation of nuclear waste from the biosphere, disposal in a deep geological formation is considered an attractive option. The vast majority of the studies were focused on coupled processes of THM. However, nuclear wastes are usually placed in relatively compact and intact rock mass, for which the thermal load is one of the most important influential factors for cracking in rock mass in the early stages after nuclear-storage containers are introduced. Thus, the thermal cracking process, as a fundamental problem for nuclear waste disposal as well as a driving force and initial inducement for fluid migration, requires further research.
Actually, thermal cracking problems concerning rock mass in nuclear waste disposal can be simplified as a rock sample heated from the central borehole. This model has been studied by conducting small-scale laboratory experiments and numerical simulations. The model ultimately failed because of the maximum tensile tangential stress at the outer boundary (Ishida, 2004). However, when the specimen is sufficiently large, the heating front will not be able to reach the outer boundary of the model, resulting in thermally induced maximum tensile tangential stress appearing at a spot a certain distance away from the borehole. An in situ heater experiment in hard rock provided a qualitatively similar theoretical and measured result (Cook and Hood, 2008). However, such a problem has not been given sufficient attention because thermal cracking in rock materials that are heated from a central borehole has mostly been studied at small scales.
This paper describes the results of numerical tests that were conducted to further our understanding of thermal cracking in samples heated from a central borehole. Moreover, these numerical simulations and tests are focused on the scale effect for thermal cracking as the model sizes are varied from 300, 600, and 1200 mm to even engineering scale.