SIGNIFICANCE OF 14C AND 228Ra IN TERMS OF THE PROPOSED YUCCA MOUNTAIN HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY

Abstract

C and Ra are two of the radionuclides that have either been identified as being potentially significant in terms of releases from the proposed Yucca Mountain high-level radioactive waste repository, or are specifically cited for consideration and evaluation in the regulations promulgated by the U.S. Nuclear Regulatory Commission. The purpose of this study was to estimate the concentrations and associated doses for these two radionuclides, if released under conditions of a scenario assumed to apply to a repository containing some of the features of the one proposed at Yucca Mountain, NV, and to compare these estimates to the regulatory limits for that facility. For C, the postulated condition was that an annual fractional release of 10 of its total remaining inventory occurs beginning at 10,000 y after repository closure. For Ra, the same fractional release rate was assumed, but in this case it was presumed to occur when the Ra inventory was projected to reach a maximum at more than 10 y after repository closure. The estimated concentrations and doses were, in turn, compared to the concentration limit, specified in the Ground Water Protection Standards (GWPSs) in the case of Ra, or derived, in the case of C, on the basis of the regulatory dose rate limit. Due to the small inventory of C in the waste, and its short half-life relative to the performance period evaluated, its estimated concentration in the ground water would be slightly more than 4% of the derived GWPS. Due to the relatively small initial inventory of Th, the precursor of Ra, and the correspondingly small quantities of higher atomic number actinides that could, through decay, produce additional quantities of Th, its estimated concentration in the ground water would be less than 3% of the GWPS, leaving the remaining portion of the limit for potential contributions from Ra. At the same time, however, it must be recognized that, in this case, the regulations require that any contributions of naturally occurring Ra and Ra already present in the ground water must be included in the determination of compliance. If this is done, the total concentration of Ra, combined with the naturally occurring concentration of Ra, would be about 10.5% of the limit. In a similar manner, the committed doses due to the annual consumption of each of these two radionuclides in ground water and food, produced in the local biosphere, were evaluated in terms of the Individual Protection Standard (IPS). Based on these analyses, the estimated effective dose for C, using the coefficients in Federal Guidance Report (FGR) No. 13, was 4.15 muSv y, less than 3% of the IPS. For Ra, the comparable estimate at the time of maximum inventory, excluding in this case the contributions from naturally occurring Ra and Ra, was 7.39 muSv y, representing about 5% of the IPS. Based on the value assumed for the fractional release rate (10 y), it was concluded that neither C nor Ra will be significant in terms of either the applicable GWPS or the IPS. While it was recognized that, due to the time spa...

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…The migration of volatile radionuclide-containing species such as 14 CO 2 can occur in near-surface repositories located in the unsaturated zone [59,60]. However, this is very unlikely in the case of a deep drillhole repository located within the saturated zone, where high fluid pressures inhibit the generation of a free gas phase and its migration through the rock.…”

“…Even given that the liquid saturation of the tuff matrix is typically above 80% [78], air flows primarily within the fracture network [79,80]. One consequence of ubiquitous airflow is that volatile radionuclides may be transported to the atmosphere [59,60]. However, the main consequence of the presence of air convection is that the repository is in an oxidizing environment, substantially affecting water chemistry and its impact on canister corrosion and radionuclide solubility.…”

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…The migration of volatile radionuclide-containing species such as 14 CO2 can occur in near-surface repositories located in the unsaturated zone [Sullivan and Pescatore, 1994;Moeller et al, 2006].…”

“…Even given that the liquid saturation of the tuff matrix is typically above 80% [Bandurraga and Bodvarsson, 1999], air flows primarily within the fracture network [Ahlers et al, 1999;Unger et al, 2004]. One consequence of ubiquitous airflow is that volatile radionuclides may be transported to the atmosphere [Sullivan and Pescatore, 1994;Moeller et al, 2006]. However, the main consequence of the presence air convection is that the repository is in an oxidizing environment, substantially affecting water chemistry and its impact on canister corrosion and radionuclide solubility.…”

Factors Affecting Dose Estimates for Long-Term Performance Assessments: Case Study???armagosa Valley

“…Topics previously addressed include a review and evaluation of the applicable regulations, with emphasis on their apparent inconsistencies; the sensitivity of the dose estimates as a function of the source of the relevant coefficients; and differences in the estimated doses to various age groups (Moeller and Ryan 2005). Supplementing these efforts were papers on the effects of the intakes of stable carbon and iodine on doses from 14 C and 129 I , and on the importance of 14 C and 228 Ra in terms of releases from the proposed repository (Moeller et al 2006). The primary conclusion of these latter evaluations was that, in the case of the proposed repository, for 14 C and 228 Ra and, perhaps in some cases, 129 I, the doses to potentially exposed groups appear not to be significant.…”