Thermohydrologic behavior at an underground nuclear waste repository

Abstract

[1] We present a multiscale model that simulates coupled thermal and hydrological behavior driven by radioactive decay heat from a potential nuclear waste repository at Yucca Mountain, Nevada. We use this model to evaluate repository performance for different designs with respect to major thermal design goals (e.g., keeping waste packages dry). A locally boiling or globally boiling design uses rock dry out to create dry (low relative humidity (RH)) conditions around waste packages for a long period of time. A subboiling design eliminates boiling in the host rock (possibly reducing uncertainty) but is less effective at maintaining dry conditions. The addition of backfill increases the duration of RH reduction significantly without added boiling in the host rock but at the cost of higher waste package temperatures. The interaction between engineering design variables and natural system factors that affect thermohydrologic behavior is highly nonlinear. As a consequence, designs that differ by seemingly small details can result in markedly different thermohydrologic behavior and consequently repository performance, whereas with regards to other details, large differences may have little or no effect.

A mountain-scale thermal–hydrologic model for simulating fluid flow and heat transfer in unsaturated fractured rock

“…Mountain scale TH processes have also been investigated with the multi-scale thermal-hydrologic model [6], which also incorporates the DKM approach for fracture-matrix flow. However, the 3-D model consists of integrating fully coupled 2-D TH models at multiple locations in the mountain, in which heat transfer at the mountain-scale occurs only through conduction.…”

“…However, the 3-D model consists of integrating fully coupled 2-D TH models at multiple locations in the mountain, in which heat transfer at the mountain-scale occurs only through conduction. In other words, the 3-D model [6] does not perform a fully coupled, 3-D TH analyses at the mountain-scale. In general, there has been the lack of modeling effort using the 3-D, mountain-scale, DKM approach for fully coupled TH analyses in Yucca Mountain fractured rock.…”

“…While the site characterization has been mostly carried out for analyzing unsaturated flow and radionuclide transport in ambient, isothermal conditions [30,31], the inherent nature of nonisothermal flow and transport processes, created by repository heating from radioactive decay, has also motivated many research efforts to understand thermal-hydrologic (TH) behavior and its impact on the repository performance within the UZ. In particular, significant progress has been made in quantitative TH modeling studies at Yucca Mountain [9,10,6]. …”

“…Because of the large spatial and temporal scales involved in such characterization, quantitative modeling 3 evaluation of the coupled fluid-flow and heat-transfer processes has proven to be essential. Quantitative investigation of TH processes at the Yucca Mountain repository site have motivated a continual effort to develop and apply different scale fluid and heat flow models [25,20,21,19,5,29,26,9,10,1,6,13,14,2]. These numerical models have played a crucial role in understanding coupled fluid and heat flow as well as in assessing how TH conditions affect on various aspects of the overall UZ waste disposal system.…”

Modeling Thermal-Hydrologic Processes for a Heated Fractured Rock System: Impact of a Capillary-Pressure Maximum

“…Strongly heat-driven conditions cause dryout of the host rock, affecting the thermal-hydrologic environment within the emplacement drifts. Buscheck et al (2002) developed the MultiScale ThermoHydrologic Model (MSTHM) to predict the range of thermal-hydrologic conditions within emplacement drifts, in the adjoining host rock, and across the entire Yucca Mountain repository SNL 2007). The MSTHM calculates thermalhydrologic variables, such as temperature, gas-phase pressure, saturation, evaporation/ condensation rates, etc.…”