The Yucca Mountain Site Characterization Project is currently evaluating the future performance of the proposed high-level nuclear waste repository. Using the Total System Performance Assessment (TSPA) model, a stylized analysis was conducted to evaluate the relative importance of natural and engineered barriers to transport of radionuclides from the potential repository. The stylized analysis consists of sequentially adding barriers and evaluating the effect on mean annual dose. Barrier is defined as “any material, structure, or feature that.…..prevents or substantially reduces the rate of movement of water or radionuclides from the Yucca Mountain repository to the accessible environment” (10 CFR Part 63.2).

Barriers were grouped into three broad categories: (1) Waste-form barriers—treats waste as if it were deposited on land surface with no protection and includes such effects as slow degradation, limited solubility in water, and protection by cladding; (2) Natural-system barriers—considers the effects of waste isolation by the unsaturated zone above and below the repository and the saturated zone between the repository and the accessible environment; (3) Engineered system barriers—considers the effect of the major engineered systems (drip shield, waste package, and invert) and the effect of radioactive decay heat and a capillary-induced reduction of water flow due to the emplacement tunnels.

Barriers were added sequentially in the TSPA model to estimate the reduction in peak mean-annual dose and to evaluate their relative importance. Waste-form barriers were added first followed by the natural-system barriers and then by the engineered-system barriers. The results show that adding all the barriers reduces the peak mean dose by 14 orders of magnitude for the sequence evaluated, and the most influential barrier is the unsaturated zone between the repository and the saturated zone. Other barrier sequences may yield slightly different results.