Proposed Yucca Mountain Repository Research Articles

Transport of Weapons-Grade Plutonium and Boron Through Fractured Geologic Media

Redistribution of vitrified weapons-grade plutonium placed in the proposed Yucca Mountain repository is investigated based on the pure-colloid transport model for plutonium and the pure-solute transport model for plutonium, uranium, and boron. In the pure-colloid model, colloids carrying plutonium are assumed to settle out of groundwater in the fractures by floccu-lation. In the pure-solute model, 239Pu, 235U, and boron are transported through fractures by advection and diffuse into the rock matrix with sorption retardation.Both models show that 239Pu stays in the vicinity of the repository and decays there to 235U. All the 239Pu that originally exists in the repository reaches the bottom end of 200-m fractures as 235U. Boron spreads in the geologic medium during the glass leaching period and quickly disappears after the end of the leach time. Concentrations of 239Pu and 235U are found to be too small for autocatalytic criticality.

Micromorphology of Calcite-Silica Deposits, Yucca Mountain, Nevada

High-level nuclear waste placed at the proposed Yucca Mountain repository must remain isolated for many thousands of years. Concern remains that calcite-silica vein deposits along fault fractures near the proposed repository are not solely pedogenic, but rather record upwelling of groundwater and thus possible routes for transporting radionuclides to the land surface. The purpose of this study was to provide an additional test for whether the calcite-silica deposits have a groundwater (phreatic) origin or a pedogenic origin. This test involved comparing the micromorphic features of the controversial vein deposits with the micromorphic features of known pedogenic and phreatic deposits. Using a combination of thin sections, scanning electron microscopy, x-ray diffraction, and cathodoluminescence, the following micromorphic features were compared : argillans, calcified microorganisms, detrital grains, aquatic microfossils, calcite crystal size and sorting, and opal content. Based on these comparisons, the controversial vein deposits are more similar to pedogenic deposits than phreatic deposits, and thus appear to be of pedogenic origin. 71 refs., 4 figs., 3 tabs.

Can Nuclear Waste Keep Yucca Mountain Dry—and Safe?

Scientists don't wonder whether nuclear waste will leach out of its containers at the proposed Yucca Mountain repository in Nevada, but when. Researchers have recently proposed a new twist on an old idea: using the waste's own heat to boil corrosive moisture out of the mountain rock and keep the containers dry. By placing storage tunnels more widely apart than has previously been suggested and making some other storage changes, the scientists suggest the tunnels might stay dry for 1 million years. But skeptics point to a few leaks in the current models behind this plan.

Improved Radionuclide Transport Models for a Nuclear Waste Repository

AbstractNRC is developing improved models for transport of chain decay radionuclides in porous and fractured/porous media using finite-difference for spatial discretization, and numerical inversion of Laplace transforms for time. This hybrid technique allows great flexibility for spatial variability, flow rate changes and boundary conditions that are difficult with analytical solutions, and are much faster than solutions fully in the time domain. This paper presents the status of NRC's development of a method for transport through fractured porous media at the proposed Yucca Mountain repository, building on the work of a number of authors.

Long-Term Results from Unsaturated Durability Testing of Actinide-Doped DWPF and WVDP Waste Glasses

AbstractResults from durability drip tests designed to simulate the unsaturated conditions in the proposed Yucca Mountain Repository are reported for two actinide-doped glasses used as model waste forms. These tests are being conducted with reference glass compositions doped with neptunium, plutonium, and americium from the Defense Waste Processing Facility (DWPF) and the West Valley Demonstration Project (WVDP), and have been ongoing for over 8 years. Solution compositions, including transuranics, have been periodically determined, and selected analyses of colloid formation and composition, glass corrosion layers, and solid alteration phases have been obtained by scanning and analytical transmission electron microscopies. The importance of integrated testing has been demonstrated, as complex interactions among the glass, the groundwater, and the sensitized stainless steel have been observed. The cumulative releases of both glassforming and dopant elements are presented along with identification of reaction phases and their partitioning between solution and solid phases. Alteration phases, including smectite clay, iron silicates, uranium silicates, and calcium thorium phosphate, have been observed forming on the glass and stainless steel and have occasionally been found suspended in solution as colloids. Actinides, except neptunium, concentrate into alteration phases or sorb onto the stainless steel. The subsequent transport of the actinides is then controlled by these phases.

Alteration of uraninite from the Nopal I deposit, Pen˜a Blanca District, Chihuahua, Mexico, compared to degradation of spent nuclear fuel in the proposed U.S. high-level nuclear waste repository at Yucca Mountain, Nevada

At the Nopal I uranium deposit, primary uraninite (nominally UO 2+ x ) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5–10 μm) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H 2O) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated UO 2 under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO 2) 2Si 2O 7·6H 2O] with lesser amounts of soddyite [nominally (UO 2) 2SiO 4·2H 2O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical—chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.

Preliminary Analysis of Potential Chemical Environments Inside Failed Waste Containers at the Proposed Yucca Mountain Repository

ABSTRACTPrediction of radionuclide release rates for high-level waste requires estimates of the rates of waste form alteration and formation of secondary minerals inside the failed canister. Unsaturated repository sites may promote development of a variety of chemical environments related to two phase (liquid/ vapor) transport and temperature gradients caused by radiogenic decay. A mass balance (shell balance) approach is used to estimate the effects of dripping water, evaporation, and condensation on the waste canister and the presence of saline water inside the failed waste canister. The simplified calculations predict large variability of water chemistry over spatial scales of a few centimeters. The effects of the predicted aqueous chemistry on waste form alteration, secondary mineral formation, and radionuclide solubility are examined.

Time trend analysis of basaltic volcanism for the Yucca Mountain site

Abstract The possible recurrence of volcanic activity near the proposed nuclear waste repository at Yucca Mountain, Nevada, U.S.A. is evaluated by estimating the instantaneous recurrence rate using a nonhomogeneous Poisson process with Weibull intensity and by using a homogeneous Poisson process to predict future eruptions. Analysis on the post-6-Ma volcanism near the Yucca Mountain region indicates a moderate developing time trend ( p -value = 0.01) of volcanic activity. A similar time trend is obtained by trimming the observation period to 3.7 Ma and younger (period of the youngest episode). Data from the Quaternary basaltic volcanism also show a slight developing time trend, although the developing volcanic activity is not significant at the 0.05 level. Thus, it would oversimplify the assessment of the volcanic risk to the proposed Yucca Mountain repository site if a simple Poisson model were used to model the volcanism. Based on the Quaternary data, the estimated instantaneous recurrence rate is about 5.5 × 10 −6 /yr. An estimate of the mean time to the next eruption is about 1.8 × 10 5 years from now, if it is assumed that the intensity remains constant thereafter. Also, the risk (probability of at least one major eruption during the projected time frame) increases approximately linearly with the time frame chosen as the required interval for radioactive waste to decay to an acceptable level. Our study concludes that the estimated risk for an isolation time of 10 4 years is about 5%, which increases to 42% if 10 5 years is used as the required isolation time.

An approximate calculation of advective gas-phase transport of 14C at Yucca Mountain, Nevada

A quasilinear partial differential equation, which describes gas-phase transport of a 14C kinematic wave through a porous medium, is derived, its sensitivity to system variables is analyzed and it is applied to one possible release scenarion at the porposed Yucca Mountain, Nevada high-level radioactive waste repository. Advection, isotope exchange between CO 2 in a flowing gas phase and HCO 3 − in a static aqueous phase, and radioactive decay are incorporated. The governing equation is solved analytically by the method of characteristics. The mass fraction of 14C in the gas phase,X 14 g, is controlled by radioactive decay. The relatively long half-line of 14C, about 5720 years, and the relatively shallow proposed burial depth of the radioactive waste, about 350m, requires significant retardation of the 14C wave velocity for significant reduction in X 14 g. 14C wave velocity is most sensitive to temperature and pH which control the distribution of total carbon between gas and liquid phase; the greater the partitioning of carbon into the liquid phase, the greater the retardation of the 14C wave velocity and the greater the ultimate reduction in X 14 g from initial conditions. Partitioning of total carbon into the liquid phase is greatest at low temperatures, < 100° C, and high pH values, > 8. Increasing water saturation also tends to retard 14C wave velocity but to a lesser extent. The governing equation has been applied using conditions that may possibly occur at the proposed Yucca Mountain repository. Calculations indicate that the 14C wave takes about 5900 years to reach the surface with a X 14 g equal to 25 ppm. Diffusion and dispersion are not of major importance for these conditions. These calculations are approximate due to the number of assumptions involved. Discharge of 14C into the gas before the selected time would accelerate wave arrival and increase the amount of 14C reaching the surface.

Container Materials for High-Level Nuclear Waste at the Proposed Yucca Mountain Site

ABSTRACTCandidate container materials for high-level nuclear waste packages to be emplaced at the proposed Yucca Mountain repository site are being considered for their long-term resistance to corrosion, oxidation, embrittlement and other kinds of degradation. Selection criteria have been established, and a method has been developed for recommending a material for advanced container design activities. An extensive compilation of the degradation phenomena for six candidate materials is complete, and further studies have begun on the degradation modes affecting additional candidate materials. Phenomenological models for predicting container degradation rates are being advanced for environmental conditions applicable to Yucca Mountain. An experimental program is underway to evaluate the susceptibility of container materials to localized corrosion, stress corrosion cracking, and enhancement of corrosion and oxidation attack by gamma radiation. Initial evaluations of container fabrication and welding processes have identified some processes that appear to alleviate some long-term corrosion susceptibility concerns.

Identification of Secondary Phases Formed During Unsaturated Reaction of UO2 with EJ-13 Water

ABSTRACTA set of experiments, wherein UO2 has been contacted by dripping water, has been conducted over a period of 182.5 weeks. The experiments are being conducted to develop procedures to study spent fuel reaction under unsaturated conditions that are expected to exist over the lifetime of the proposed Yucca Mountain repository site. One half of the experiments have been terminated, while one half are ongoing. Analyses of solutions that have dripped from the reacted UO2 have been performed for all experiments, while reacted UO2 surfaces have been examined for the terminated experiments. A pulse of uranium release from the UO2 solid, combined with the formation of schoepite on the surface of the UO2, was observed between 39 and 96 weeks of reaction. Thereafter, the uranium release decreased and a second set of secondary phases was observed. The latter phases incorporated cations from the EJ-13 water and include boltwoodite, uranophane, sklodowskite, compreignacite, and schoepite. The experiments are continuing to monitor whether additional changes in solution chemistry or secondary phase formation occurs.