A new type of flow-through test method using micro-reactor was developed and applied to measurement of the dissolution/alteration kinetics for a Japanese type of simulated HLW glass, P0798. In this test method, a face of coupon shaped glass specimen (30mm × 10mm × 4mm size) is in contact with a micro-channel (20mm length, 2mm width, 0.16mm depth) constructed on a PTFE (Teflon®) plate, and a solution is injected into the inlet of micro-channel at a constant rate. The injected solution, which flows through the micro-channel reacting with the glass to the outlet, is retrieved at certain intervals to be analyzed for determination of the glass dissolution/alteration rate. After the test, the glass specimen removed from the micro-reactor is subjected to surface analyses. This test method has major features as follows, 1) any controlled solution condition can be provided over the test duration, 2) a relatively high S/V ratio can be provided by use of micro-reactor in spite of using coupon shaped glass specimen, which results in precise and consistent analyses of both the solution and the reacted glass surface, 3) the test apparatus is simple with compact size and easy operation, which allows a flexible setup of test conditions. By use of this test method the dissolution/alteration rate for P0798 glass was measured as a function of pH, temperature, and time, and the results indicated that this test method is applicable and suitable for evaluation of the dissolution/alteration kinetics.

We applied a new type of flow-through test method using micro-reactor consisting of a simple test apparatus with compact size to measurement of the dissolution rate of a Japanese type of simulated waste glass (P0798 glass). In this test method, a solution flows through a micro-channel (20 mm length, 2 mm width, 0.16 mm depth) in contact with a face of coupon shaped glass specimen, and the output solution is retrieved at certain intervals to be analyzed for determination of the glass dissolution rate. By using this test method the initial dissolution rate of glass matrix or forward dissolution rate was measured as a function of pH (3 to 11) and temperature (25°C to 90°C). The present test results indicated that the initial dissolution rate has ‘V-shaped’ pH dependence, and the effect of pH on the dissolution rate decreases with increase in temperature similar to the results measured by using the Single-pass flow-through (SPFT) method. The present test results also indicated that the dissolution of B is controlled by diffusion process and that of Si is controlled by surface reaction process.

In a simulated engineered barrier system consisting of a simulated HLW glass doped with Pu, a compacted bentonite and water under reducing conditions, migration tests of Pu from the glass into the bentonite through water were performed. The following results were obtained: (1) The presence of bentonite enhances the glass dissolution while it has no remarkable influence on the concentration of Pu dissolved species in the solution. (2) Only a small part of the leached Pu, which may correspond to the dissolved species, can diffuse into the bentonite with the apparent diffusion coefficient of 3×10−14m2/s. (3) Interactions between the glass and the bentonite have no remarkable influence on the diffusion of Pu.

The objective of this study is to evaluate the effects of water redox conditions and of the presence of iron corrosion products (magnetite) on the release of actinides from HLW glasses. Static corrosion tests were performed on a simulated HLW glass doped with Pu and Np in deionized water in the presence of magnetite under oxidizing and reducing conditions. The tests under oxidizing conditions were performed in air, while the tests under reducing conditions were performed in mixed gas (Ar+5%H2) atmosphere.

The following results were obtained: (1) The presence of magnetite enhances formation of colloids containing Pu and Np in the leachates under both redox conditions. (2) Redox conditions have no remarkable influence on the release of Pu. (3) The reducing conditions combined with the presence of magnetite cause a decrease in the concentrations of Np dissolved species, which is probably the result of the reduction of Np(V) to Np(IV) at the magnetite surface.

Static corrosion tests were performed on the simulated waste glass doped with Pu and Cm (PuO2; 0.22wt%, Cm2O3; 0.09%) in deionized water at 90 °C with S/V ratio of 2600 m−1 under oxidizing and reducing conditions, respectively. The corrosion tests under oxidizing conditions were performed in air, while the corrosion tests under reducing conditions were performed in an airtight stainless steel container purged with mixed gas (Ar+5%H2), where the Eh of the solution was maintained at approximately -0.45 V vs SHE. The results showed that the redox conditions have no remarkable influence on the leaching behavior of Pu and Cm. It was also observed that the Pu and Cm concentrations in the 1.8nm filtered solutions were one or two orders of magnitude lower than those in the 450nm filtered solutions under both redox conditions, which suggests that the insoluble suspended fractions (colloidal particles) with the size from 1.8nm to 450nm are dominant phase for Pu and Cm in the solution under both redox conditions.

Static corrosion tests of a powdered simulated waste glass were performed in the presence of magnetite under oxic and anoxic conditions, respectively. The corrosion tests under oxic conditions were performed in air, and those under anoxic conditions were performed in a glove box purged with mixed gas(Ar+5%H2). The experimental results showed that the presence of magnetite can enhance glass corrosion under both oxic and anoxic conditions, and the enhancement under anoxic conditions was small compared to that under oxic conditions. A large portion of the Si and insoluble elements were sorbed onto the magnetite surface, and the sorption under anoxic conditions was small compared to that under oxic conditions.

It was suggested that the enhancement of glass corrosion by magnetite results from the sorption or precipitation of silica on the magnetite surface, which can be greatly affected by redox condition. Under anoxic conditions, it was suggested that precipitation of amorphous silica on the magnetite surface may be the dominant process for enhancement of glass corrosion in addition to sorption.

Static corrosion tests were carried out with a powdered simulated waste glass in deionized water at 90 °C under oxic and anoxic conditions, respectively. The corrosion tests under anoxic condition were performed in a globe box purged with mixed gas (N2+5%H2), where the pH and Eh of deionized water were maintained at 6.9 and −0.35 V vs.S.H.E. at 25 °C, respectively.

It was observed that there is a little difference between NL values under oxic and anoxic conditions. For soluble elements (Li, B, Na, Mo) and Ca, NL values under anoxic condition were slightly smaller than those under oxic condition. For Fe and Al, NL values under anoxic condition were slightly larger than those under oxic condition. For Si, Ce, La and Nd, there was no difference between NL values under oxic and anoxic conditions. The experimental results were analyzed on the basis of a corrosion model (Diffusion-combined Model), in which the solution concentration of elements associated with solution pH and Eh can be derived thermodynamically by use of PHREEQE. The analysis suggests that precipitating minerals controlling the solution concentrations of Fe and Ca under anoxic condition are different from those under oxic condition.

An accelerated experiment using the actinide doping technique was performed to investigate the effects of alpha decay on the properties of actual nuclear waste glass at high radiation doses. A fully radioactive borosilicate waste glass, containing the actual high-level radioactive liquid waste generated from the Tokai Reprocessing Plant of PNC, was prepared by JAERI, and a powder mixture of the ground fully radioactive glass and 244CmO2 was melted at 1200°C for 2 hrs. The radioactivity concentration of 244Cm was 1.0 } 1010 Bq/g-glass at the date of preparation. The homogeneity of curium-doped glass samples was confirmed by the density measurement, heat load measurement and alpha autoradiography. The properties of the irradiated samples were investigated by the mass spectrometer for helium determination, the optical microscope, the electron probe microanalyzer, the densitometer, the Soxhlet and MCC-1 leach testing apparatus. By measuring the amount of helium released from the curium-doped glass samples, more than 99% of helium remained in the matrix at room temperature. The density of the sample slightly decreased with the increase of cumulative alpha decays and the decrease of 0.77% was observed at a dose of 1.55 } 1018 alpha decays/g, corresponding to an equivalent age of 150000 yrs. Optical and scanning electron micrographs showed that no cracks were observed on all samples having up to a dose of 1.5 } 1018 alpha decays/g. The leach rates, based on weight loss, in both the Soxhlet (100°C, 7 days) and MCC-1 (90°C, 28 days) tests did not significantly change with alpha decay dose.

The sorption experiments of carbon-14 on the mortar grain (grain size : 0.50 – 1.0 mm) focused on the chemical form of the carbon-14 were carried out by the batch method. Three kinds of carbon-14 chemical form were used for the experiments : sodium carbonate (Na2 14CO3) as the inorganic radiocarbon, and sodium acetate (CH3 14COONa) and acetaldehyde (14CH3 14CHO) as the organic radiocarbons. 0.30 gram samples of mortar were soaked in the solution with carbon-14 at 15°C for periods of up to 160 days. At the end of each run, carbon-14 concentrations in the supernatants were determined before and after centrifugation (3,500 rpm., 1 hr.).

In the mortar-sodium carbonate system, the retention process of carbon-14 related to reaction on the surface of the mortar was speculated as follows. First, 3CaO-SiO2. and 2CaO-SiO2, of the mortar components contact with water and produce Ca(OH)2. Ca(OH)2 produces Ca2+ and OH− in the solution. Then, calcite forms from Ca2+ and CO3 2− in the solution. Thus, the sorption ratio of carbon-14 onto mortar will be high until mortar nas been completely carbonated because Ca2+ is rich in the mortar and the solubility of calcite is low.

In the mortar-organic carbon system, the soluble organic carbon-14 is hardly sorbed on the surface of the mortar. Therefore, the cementitious materials may not inhibit the release of organic radiocarbons from the low-level radioactive wastes, contrary to the case of inorganic radiocarbon.

This paper presents the results of analyses and measurements performed on an active glass sample taken from the French vitrification facility R7.

A simulated nuclear waste glass was self-irradiated by doping with short-lived actinides of 238Pu and 244Cm. Changes in the hardness, the Young's modulus and the fracture toughness, as a function of irradiation dose, were measured by use of indentation techniques. The irradiated glass was annealed at temperatures from 573K to 723K for periods of up to 48hours, and the recovery of these changes were measured as a function of annealing temperature and time.

It was observed that the hardness and the Young's modulus decreased, while the fracture toughness increased exponentially with the cumulative dose. The maximum values of the relative changes in the hardness, the Young's modulus and the fracture toughness were about −25%, −30% and +45%, respectively. The results of the annealing show that the hardness and the Young's modulus were almost recovered to the original values at temperatures above 673K within 10 hours, while the recovery of the fracture toughness was minimal in this region of temperature and time. The changes in the hardness and the Young's modulus can be well explained by the model, in which the changes is proportional to the volume fraction of damaged zones, F, and the recovery of F is first order. On the other hand, the changes in the fracture toughness cannot be explained by the model, which suggests that the mechanism of the change in the fracture toughness is different from that in the hardness and the Young's modulus.

The microstructural changes of a simulated waste glass irradiated with doping of 238pu and 244Cm were observed by use of a preshadowed carbon replica technique in combination with scanning electron microscopy (SEM). The irradiated glass was annealed and its microstructural changes after annealing were observed by use of the sametechnique.

In the glass irradiated at a dose of 2.75x1025 α-decays/m3, bubbles with a radius from 0.15μrm to 0.35μm were observed. The average bubble radius and the bubble density were 0.23μm and 1x1017 bubbles/m3, respectively. Using these observed values, the volume change of the glass resulting from the bubble formation was estimated to be + 0.51 %. This valuoof + 0.51 % was close to the volume change of the irradiated glass measured in our previous study, which suggests a large portion of the volume change by α-irradiation results from bubble formation. In the glass annealed after irradiation the average bubbleradius was observed to decrease with annealing time. The bubble radius as a function of annealing time was analyzed on the basis of the helium diffusion model with two chemicalprocesses, i.e. trapping at bubbles and re-solution from bubbles into glass matrix. The values of the diffusion coefficient of helium, the trapping parameter and the re-solution parameter, which had been obtained experimentally in our previous study on the helium release, were applied to the calculation. The calculated curve was in good agreement with the observed data.

MCC-1 static leaching experiments were carried out for a simulated waste glass in deionized water for up to 364 days at 90°C in order to examine the relationship between alteration layer thickness and elemental mass losses. The thickness of the layer increased linearly at a rate of 0.63,μm/day for the first 91 days, and thereafter it remained constant (about 60 μm) independent of time. The change in normalized elemental mass losses of B, Na, and Si with time shows the same trend as that of the change in the layer thickness. It is also demonstrated that the growth rate of the layer can be calculated from the released boron in the leachates. The shrinkage of the alteration layer during air-drying is not significant in spite of the porous structure of the layer, which results from the depletion of cations and the recrystallization reported previously. Although the apparent release of elements almost ceases after a 91-day leaching experiment, reactions in the layer such as crystal growth continue.

An experimentally altered (1–14 days, deionized water, 90°C), synthetic borosilicate glass and naturally altered (104 years, fresh water, 0–100°C), basalt glass were characterized by analytical electron microscopy to evaluate the development of alteration layers. Layers formed on borosilicate glass are initially amorphous. With increasing age, nontronite, chlorite, septechlorite, and/or stilpnomelane appear to precipitate and grow within the amorphous matrix and on the surface of the layer. Crystals are enriched in Fe, Co, and Ni, and depleted in Si relative to the surrounding amorphous matrix. Alteration layers on basalt glass are texturally and mineralogically similar to layers formed on borosilicate glass, but the degree of crystallinity is less.

The leaching experiments of Soxhlet type have been carried out[l, 2] and on the basis of the results we developed the mathematical leaching model which used one dimensional diffusion and could treat the growth of surface layers. The model adopted the following assumptions: 1) The velocity of the bulk glass-surfacé layers boundary depends on time alone. 2) Some of the diffusing substances are immobilized in the surface layers by an irreversible first-order reaction. 3) A fictitious film exists at the solution-surface layers interface. The fundamental equations were established based on these assumptions and the numerical solutions were obtained by the Crank-Nicholson implicit method. The values of the diffusion coefficient, the reaction rate and the film mass transfer coefficient were obtained by a trial-and-error method. The applicability of the model was confirmed by the fact that the leaching mechanisms deduced from the calculated results were consistent with those mechanisms deduced from the experimental results. The present study showed the proposed model was valid for calculation of the leach rates of waste glasses when surface layers grew during leaching, and the study also indicated which parameters should be measured experimentally to predict the leach rates.