Alternative Conceptual Model for Colloid Generation from Commercial Spent Nuclear Fuel

Abstract

SummaryYucca Mountain is the Department of Energy's potential geologic repository designed to store and dispose of commercial spent nuclear fuel (CSNF) and other high-level radioactive waste forms. If approved, the site would be the Nation's first geologic repository for disposal of this type of radioactive waste. Yucca Mountain is located in a remote desert on federally protected land within the secure boundaries of the Nevada Test Site in Nye County, Nevada. It is approximately 100 miles northwest of Las Vegas, Nevada. CSNF makes up most of the spent nuclear fuel that requires disposal at Yucca Mountain.The waste-form degradation process models provide technically defensible methods for predicting the long-term capability of the waste forms to control radionuclide release from the immediate vicinity of the waste. These models were provided as eight key feeds to the Total Systems Performance Assessment: radionuclide inventory, in-package chemistry model, cladding degradation model, commercial spent fuel degradation model, high-level waste glass degradation model, other waste-form degradation models, solubility model, and colloid model. Colloid generation from CSNF has been deemed to require further qualification by the present waste-form colloid model. The present colloid model considers the effects of pH and ionic strength on the stability of ideal colloid phases. The location of the boundaries between pH and ionic strength regimes in which colloids are stable or unstable are defined together with the colloid mass concentration in each regime.Colloids may have the potential to transport strongly sorbing radionuclide contaminants in soils and groundwater aquifers (McCarthy and Zachara 1989). Recent studies from the Nevada Test Site (NTS), a site with a similar geology to Yucca Mountain, have indicated the enhanced mobility of plutonium in the saturated zone, albeit in minute quantities, in association with various silicate minerals (Kersting et al. 1999). However, calculated flow rates for the NTS saturated zone are 1000 to 10,000 times higher than those calculated using 36 Cl migration analysis for the proposed Yucca Mountain Repository wastepackage environment under unsaturated conditions (Tompson et al. 2000; CRWMS M&O 1999). Significant colloidal transport of thorium and rare-earth elements (REE), considered to be chemical analogs for plutonium, are postulated to be rare in unsaturated environments (Wood et al. 1997). The current Yucca Mountain models for colloids would have predicted extensive thorium and REE migration, given these phases' association with clay minerals and the colloidal stability of the minerals in both unsaturated and saturated environments.Several studies have pointed to the effect of pore velocity on colloid and particulate migration. An extension of the Ryan and Gschwend (1994) empirical relationship for colloid release indicates that colloid production rates within the waste package should be negligible owing to the predicted low hydraulic conductivities. However, the most important...

Effects of Time, Heat, and Oxygen on K Basin Sludge Agglomeration, Strength, and Solids Volume

“…Based on observations, both the floating of the agglomerate and the formation of voids were caused by gas bubbles. Similar flotation of uranium oxides, including metaschoepite, has been observed because of bubble capture and a high affinity of the solids scum for the air-water interface (Buck et al 2004). Because the UO 2 /UO 3 ·2H 2 O contained no uranium metal, the gas bubbles likely were created when dissolved and entrained air was displaced and separated from the solution.…”

“…Floating solids also were not found in the 22-day test. Similar but more conspicuous observations of floating solids have been made in tests done at PNNL of non-irradiated UO 2 particles that were left under aerated water cover (Buck et al 2004). In those tests, the UO 2 originally prepared as reactor fuel pellets had first been crushed, sieved to 44-to 105-μm particle size, washed to remove fines, and the granules fired under reducing conditions to restore the 2:1 oxygen:uranium stoichiometry of UO 2 .…”

The formation of schwertmannite colloids and natural remediation of toxic elements from Shojin River, Hokkaido, Japan

“…The trends of dissolved and colloidal As are very similar to those of Fe. Based on the characterization of colloids by Baikt and Hahn (1997) and Buck et al (2004), the Fe colloids are the primary colloids, while the As colloids are pseudo colloids, whose behavior is determined by the primary Fe colloids. Consequently, colloid formation is evident at Shojin River, and their formation is relevant for the removal of the elements from the drainage.…”

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

“…Although UO 2 particles have been shown to persist in the environment for many years (Oughton and Kashparov, 2007), oxidative dissolution and subsequent migration of U through the geosphere is possible, with UO 2 forming secondary U(VI) phases, such as metaschoepite (UO 3 •nH2O), under oxidizing conditions (Buck et al, 2004a;Finch and Ewing, 1992). Indeed, the production of metaschoepite colloids from UO 2 powders (44-105 μm) has also been reported following exposure to aerated deionized water, and aged metallic depleted uranium munitions are known to form metaschoepite as a major corrosion product (Buck et al, 2004b;Handley-Sidhu et al, 2009;Wang et al, 2016).…”