The Drift Scale Test (DST) in Yucca Mountain in Nevada, USA, was a large scale, long-term field thermal test being conducted for the United States Department of Energy (DOE). The heating phase of the DST, an integral part of DOE’s program of site characterization at Yucca Mountain to assess whether the site is suitable for a repository for the disposal of high level nuclear waste (HLW) and spent nuclear fuel (SNF), was initiated in December 1997. Heating was terminated in January 2002—ushering in the cooling phase of the test that was expected, at that time, to continue for four years. The overarching objective of the DST was to study coupled thermal, hydrological, mechanical, and chemical processes caused by the decay heat from HLW and SNF emplaced in an underground geologic repository. The layout of the DST test is shown in Fig. 3.
The DST was included as a test case in the DECOVALEX III project, i.e., Task 2 with four sub-tasks: Task 2A, Task 2B, Task 2C, Task 2D.
Task 2A was to mathematically simulate and study the thermal-hydrological (TH) responses of the rock mass in the DST. The modelling objectives included numerical predicting the time-evolution of temperature distribution and saturation changes in the test block at suitable time Intervals, with gradual developments of blind prediction and model calibration with measured data support, and further modelling and results comparison. Tasks 2B and 2C dealt with the mechanical processes in the DST during the heating and cooling phases of the test. The difference between Task 2B and 2C was that in 2B the mechanical effects were simulated using modelled temperature distributions as inputs; while, in 2C, the mechanical effects were calculated using measured temperature distributions as inputs. The provided data were basically the same as that for Task 2A, but the objectives were predicting the time-evolution of the displacements in the test block measured in the MPBX hole and changes in the (fracture) permeability of the rock due to thermal-mechanical processes at suitable time intervals. The general objective of Task 2D was to develop a thorough understanding of the coupled TMHC processes in the rock mass immediately surrounding the proposed repository because of the decay heat from the nuclear waste. This included a series of specific studies concerning the four principal processes (thermal, hydraulic, mechanical and chemical) and involved measurements, evaluations, investigations, monitoring, property evaluations, observations, data collections and analysis.
Various conceptual models were evaluated by comparing simulated and measured temperatures. The dual continuum model (fracture and matrix) and active fracture concept reflecting actual heat load yielded the best agreement. The effect of dimensionality (i.e., 2D versus 3D) on temperature was evaluated. A maximum difference of temperature of about 10oC near the wing heaters was calculated after four years of heating when a 3D model was compared to a 2D model. The TH calculations indicated that it was possible to choose appropriate hydrological parameters to obtain a distribution of saturation similar to the ones measured in the field. The good agreement between simulated and measured air permeability indicated that the adopted conceptual model was sound, and that the model of coupling stress with permeability was appropriate for predicting TM-induced permeability changes at Yucca Mountain. All the above achievements were published in the Task D reports and journal papers (cf. Section 3.3).
The DST was a large and complex field test in which a multitude of measurements were made, and its realistic simulation required multidiscipline approaches with multiple research teams with different concepts and approaches—so as to achieve more in-depth understanding of the coupled THMC processes involved. DECOVALEX III researchers studying the DST for Task 2 performed well in their individual studies, but more concentrated efforts with more teams involved were needed for further studies, assisted through more active co-ordination. Although the effects of the chemical processes were investigated in-depth by DOE over the years, the works performed in Task 2D for this effect was not adequate for furthering the scientific understanding due simply to the fact that only two teams participated in Task 2D. The presence of fractures in the DST site and the long periods of heating-cooling phases of the test could cause residual and long-term variations in the physio-chemical properties of the fractured tuff, due to the irreversibility of the coupled THMC processes, for example the residual fracture deformation and its impact on permanent changes in permeability of the near-field. Such residual and permanent changes will be important factors affecting design and performances of post-closure monitoring works and final safety assessment. Study on these effects was not considered in Task 2 of the DECOVALEX III project, but remains an important outstanding issue for further research.
Besides the reports published, two research papers by the participating research team members of Task 1 were produced and published in a special issue of the Journal of Rock Mechanics and Mining Sciences, Volume 42, Number 5-6, published on 10 August 2005, as follows.