Second performance assessment iteration of the Greater Confinement Disposal facility at the Nevada Test Site

Abstract

The Greater Confinement Disposal (GCD) facility was established by the Nevada Operations Office of the Department of Energy (DOE) in Area 5 at the Nevada Test Site for containment of waste inappropriate for shallow land burial. Some transuranic (TRU) waste has been disposed of at the GCD facility, and compliance of this disposal system with Environmental Protection Agency (EPA) regulation 40 CFR 191 must be evaluated by performance assessment calculations. We have adopted an iterative approach in which performance assessment results guide site data collection, which in turn influences the parameters and models used in performance assessment. The first iteration was based upon readily available data, and indicated that the GCD facility would likely comply with 40 CFR 191 and that the downward flux of water through the vadose zone (recharge) had a major influence on the results. It was seen, however, that very large recharge rates, such as might occur under a cooler, wetter climate, could result in noncompliance. As a result, a site characterization project was initiated to study recharge in Area 5 by use of three environmental tracers. This study concluded that the recharge rate is so small that the nearest groundwater aquifer will not be contaminated in less than 10,000 years. Thus upward liquid diffusion of radionuclides remained as the sole release pathway. This second performance assessment iteration refined the upward pathway models and updated the parameter distributions based upon new site information. A new plant uptake model was introduced to the upward diffusion pathway; adsorption and erosion were also incorporated into the model. Several modifications were also made to the gas phase radon transport model. Plutonium solubility and sorption coefficient distAbutions were changed based upon new information, and on-site measurements were used to update the moisture content distributions. The results of the performance assessment using these models indicate that the GCD facility is likely to comply with all sections of 40 CFR 191 under undisturbed eonditions. A complete estimate of event and process probabilities was not done, although a simple probability model was used to estimate probabilities of human intrusion. For reasonable drilling rates, these probabilities were found to be very small.

Preliminary evaluation of the use of the greater confinement disposal concept for the disposal of Fernald 11e(2) byproduct material at the Nevada Test Site

“…Previous PA analyses of the GCD site indicate that, without human intrusion, upward diffusion is the most likely process by which contaminants will reach the accessible environment under existing hydraulic conditions (Baer et at., 1994). Based on the work of Baer et al (1994), it was assumed that surface and near surface pathways are the most likely pathways of exposure for the MOP and intruder.…”

“…Based on the work of Baer et al (1994), it was assumed that surface and near surface pathways are the most likely pathways of exposure for the MOP and intruder. Although recharge is not significant for the climate conditions that have existed at the site for the last 15,000 to 260,000 years,'o these conditions are not be expected to continue over the time periods of concern for this waste.…”

“…Consequently, the bulk solubilities of actinium and protactinium that were used in the second PA for the GCD site (Baer et al, 1994) were used in this evaluation. These solubilities are believed to be conservatively high.…”

“…The conceptual model of contaminant transport at this site is based on the interpretation of site characterization data collected for the GCD PA (see Baer et al, 1994 andPrice et al, 1993 for specific references and data) and general concepts of unsaturated flow and transport.…”

“…The liquid phase diffusion of the modeled isotopes ( Figure 3) is simulated using the SWIFT-I1 computer code (Reeves et al, 1986), in the same manner presented in Baer et al (1994). For liquid phase diffusion in unsaturated sediments, the advective velocity is set to zero and the effective diffusion coefficient is a function of the molecular diffusion coefficient in water (D,.,,), volumetric water content (e), sorption coefficient (Kd) and tortuosity (T).…”