Long term stability of waste emplacement drifts is of interest in assessing the safety of disposal of high-level nuclear waste (HLW) in the proposed repository at Yucca Mountain (YM). The repository preclosure operation period is about 100 years, and the decision regarding the use of backfill in the emplacement drifts during the postclosure period of about 10,000 years has not been finalized. Two significant factors that can induce drift instability include thermal loads generated by the decay of emplaced waste and repeated seismic loads from earthquakes. One of the failure mechanisms for an excavation in a jointed rock mass subjected to repeated seismic loading is through accumulation of shear displacements along joints. These dynamic ground motions at YM due to earthquakes (and possible nearby weapons testing) will take place in the presence of in situ stresses, excavation induced stresses, and thermal-mechanically induced stresses. This paper presents a discrete element analysis using the code UDEC (Version 3.0) to investigate the effect of repeated seismic loading on cumulative joint slip and failure around a heated, unsupported, emplacement drift in the proposed repository at YM. A 100 MTU/acre thermal loading was used. Computer results indicate that slip along joints and closing of the drift take place with both single and repeated episodes of seismic loading at peak accelerations of 0.4g if yielding has occurred in the rock around the drift during the thermal-mechanical loading stage prior to seismic loading. This cumulative effect of repeated seismic loading confirms the findings of a field investigation (Hsiung et al. 1992) and a laboratory study (Hsiung et al. 1997). However, if the rock remains in the elastic state after the thermal loading is applied, no measurable cumulative slip on joints or drift convergence is evident with repeated episodes of seismic loading up to 0.4g, aside from that which occurs during the first sequence of seismic loading.