Parametric Effects of Glass Reaction Under Unsaturated Conditions

Eventual liquid water contact of high-level waste glass stored under the unsaturated conditions anticipated at the Yucca Mountain site will be by slow intrusion of water into a breached container/canister assembly. The water flow patterns under these unsaturated conditions will vary, and the Unsaturated Test method has been developed by the YMP to study glass reaction. The results from seven different sets of tests done to investigate the effect of systematically varying parameters, such as glass composition, composition and degree of sensitization of 304L stainless steel, water input volume, and the interval of water contact are discussed. Glass reaction has been monitored over a period of five years, and the parametric effects can result in up to a ten-fold variance in the degree of glass reaction

Alpha and gamma radiation effects on air-water systems at high gas/liquid ratios

Radiolysis tests were conducted on air-water systems to examine the effects of radiation on liquid phase chemistry under high gas/liquid volume (G/L) ratios that are characteristic of an unsaturated nuclear waste repository setting. Test parameters included temperatures of 25, 90, and 200{degrees}C; gamma vs. alpha radiation; dose rates of {approximately}3500 and 50,000 rad/h; and G/L ratios of 10 and 100. Formate, oxalate, and total organic carbon contents increased during irradiation of the air-water systems in gamma and alpha tests at low-dose rate ({approximately}3500 rad/h). Increases in organic components were not observed for tests run at 200{degrees}C or high-dose rates (50,000 rad/h). In the tests where increases in organics occurred, the formate and oxalate were preferentially enriched in solutions that were rinsed from the test vessel walls. Nitrate (NO{sub 3}{sup {minus}}) is the dominant anion produced during the radiolysis reactions. Significant nitrite (NO{sub 2}{sup {minus}}) also occurs in some high-dose rate tests, with the reduced form of nitrogen possibly resulting from reactions with the test vessels. These results indicate that nitrogen acids are being produced and concentrated in the limited quantities of solution present in the tests. Nitrate + nitrite production varied inversely with temperature, with the lowest quantities being detected for the higher temperature tests. The G(NO{sub 3}{sup {minus}} + NO{sub 2}{sup {minus}}) values for the 25, 90, and 200{degrees}C experiments with gamma radiation are 3.2 {+-} 0.7, 1.3 {+-} 1.0, and 0.4 {+-} 0.3, respectively. Thus, the elevated temperatures expected early in the life of a repository may counteract pH decreases resulting from nitrogen acid production. Little variation was observed in G values as a function of dose rate or gas/liquid ratio

NNWSI Waste Form Performance Test Development

A test method has been developed to measure the release of radionuclides from the waste package under simulated NNWSI repository conditions, and to provide information concerning materials interactions that may occur in the repository. Data from 13 weeks of unsaturated testing are discussed and compared to that from a 13-week analog test. The data indicate that the waste form test is capable of producing consistent, reproducible results that will be useful in evaluating the role of the waste in the long-term performance of the repository. 6 references, 3 figures

The role of laboratory analog experiments in assessing the performance of waste package materials

There is an immediate need to begin to validate models that can be used for assessing the performance of waste package materials in an unsaturated repository environment. This paper examines available testing information and testing approaches that could support validation of models for engineering barrier system (EBS) radionuclide release. The content is presented in the context of the general methodology that has been proposed for validating performance assessment models. Available experimental observations are used to test some of the EBS release rate modeling premises. These observations include evidence of fluid film formation on waste glass surfaces in isothermal humid environments, accelerated waste glass reaction rates under repository service conditions of large glass surface area to water volume ratio, and mobilization of radionuclides as solutes and colloids. It is concluded that some important modeling premises may not be consistent with available experimental information. However, it is also concluded that future laboratory testing, which simulates the integrated waste package systems, is needed to evaluate the significance of these inconsistencies and to test the system level models. A small-scale apparatus which was developed and tested to examine the feasibility of laboratory analog testing for the unsaturated Yucca Mountain repository environment is described. 16 refs., 4 figs., 1 tab

Corroded spent nuclear fuel examined with EELS

Samples, exposed to dripping simulated groundwater, were prepared by ultramicrotomy for TEM. The alteration phase was identified as a layered Cs Mo uranyl oxide hydrate, structurally related to phases of the becquerelite group uranium minerals. Second-difference EELS of rare earth elements within the corroded particle detected low levels of TRUs

The release of technetium from defense waste processing facility glasses

Laboratory tests are being, conducted using two radionuclide-doped Defense Waste Processing, Facility (DWPF) glasses (referred to as SRL 13IA and SRL 202A) to characterize the effects of the glass surface area/solution volume (SN) ratio on the release and disposition of {Tc} and several actinide elements. Tests are being conducted at 90{degrees}C in a tuff ground water solution at SN ratios of 10, 2000, and 20,000 m{sup {minus}1} and have been completed through 1822 days. The formation of certain alteration phases in tests at 2000 and 20,000 m{sup {minus}1} results in an increase in the dissolution rates of both classes. The release of {Tc} parallels that of B and Na under most test conditions and its release increases when alteration phases form. However, in tests with SRL 202A glass at 20,000 ,{sup {minus}1}, the {Tc} concentration in solution decreases coincidentally with an increase in the nitrite/nitrate ratio that indicates a decrease in the solution Eh. This may have occurred due to radiolysis, glass dissolution, the formation of alteration phases, or vessel interactions. Technetium that was reduced from {Tc}(VII) to {Tc}(IV) may have precipitated, thou-h the amount of {Tc} was too low to detect any {Tc}-bearing phases. These results show the importance of conducting long-term tests with radioactive glasses to characterize the behavior of radionuclides, rather than relying on the observed behavior of nonradioactive surrogates

Grain boundary corrosion and alteration phase formation during the oxidative dissolution of UO{sub 2} pellets

Alteration behavior of UO{sub 2} pellets following reaction under unsaturated drip-test conditions at 90 C for up to 10 years was examined by solid phase and leachate analyses. Sample reactions were characterized by preferential dissolution of grain boundaries between the original press-sintered UO{sub 2} granules comprising the samples, development of a polygonal network of open channels along the intergrain boundaries, and spallation of surface granules that had undergone severe grain boundary corrosion. The development of a dense mat of alteration phases after 2 years of reaction trapped loose granules, resulting in reduced rates of particulate U release. The paragenetic sequence of alteration phases that formed on the present samples was similar to that observed in surficial weathering zones of natural uraninite (UO{sub 2}) deposits, with alkali and alkaline earth uranyl silicates representing the long-term solubility-limiting phases for U in both systems

Analysis of components from drip tests with ATM-10 glass

Waste package assemblies consisting of actinide-doped West Valley ATM-10 reference glass and sensitized 304L stainless steel have been reacted with simulated repository groundwater using the Unsaturated Test Method. Analyses of surface corrosion and reaction products resulting from tests that were terminated at scheduled intervals between 13 and 52 weeks are reported. Analyses reveal complex interactions between the groundwater, the sensitized stainless steel waste form holder, and the glass. Alteration phases form that consist mainly of smectite clay, brockite, and an amorphous thorium iron titanium silicate, the latter two incorporating thorium, uranium, and possibly transuranics. The results from the terminated tests, combined with data from tests that are still ongoing, will help determine the suitability of glass waste forms in the proposed high-level repository at the Yucca Mountain Site

Comparison of the Corrosion Behavior of Tank 51 Sludge-Based Glass and a Nonradioactive Homologue Glass

We are conducting static dissolution tests with a glass made at the Savannah River Technology Center (SRTC) during a demonstration of the Defense Waste Processing Facility (DWPF) process control for remote vitrification [1]. The glass was made with sludge from Tank 5 1, SRL 202 frit, and added soda. This glass is similar to waste glasses being made in the current DWPF campaign. Parallel tests are being conducted with a nonradioactive glass made at ANL having the same composition as the radioactive glass, except without the radionuclides. The radioactive and nonradioactive glasses are referred to as 5lR and 5lS, respectively. The results of these tests provide information pertinent to assessing the long-term corrosion behavior of DWPF glasses, comparing the corrosion behaviors of radioactive and nonradioactive glasses, and characterizing the disposition of radionuclides as the glass corrodes

Simulation of natural corrosion by vapor hydration test: seven-year results

We have investigated the alteration behavior of synthetic basalt and SRL 165 borosilicate waste glasses that had been reacted in water vapor at 70 {degrees}C for time periods up to seven years. The nature and extent of corrosion of glasses have been determined by characterizing the reacted glass surface with optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS). Alteration in 70 {degrees}C laboratory tests was compared to that which occurs at 150-200 {degrees}C and also with Hawaiian basaltic glasses of 480 to 750 year old subaerially altered in nature. Synthetic basalt and waste glasses, both containing about 50 percent wt SiO{sub 2} were found to react with water vapor to form an amorphous hydrated gel that contained small amounts of clay, nearly identical to palagonite layers formed on naturally altered basaltic glass. This result implies that the corrosion reaction in nature can be simulated with a vapor hydration test. These tests also provide a means for measuring the corrosion kinetics, which are difficult to determine by studying natural samples because alteration layers have often spelled off the samples and we have only limited knowledge of the conditions under which alteration occurred