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AbstractThis study, which develops a safety assessment code for radioactive waste disposal, consists of two-dimensional analyses of underground water infiltrated flow and near-field radionuclide migration, one-dimensional analyses of far-field migration, and the dose equivalent. The study takes into account the influence of a finite absorption amount of radionuclides in an engineered barrier system (EBS).The safety assessment code is applied to 14C migration calculations. The near-field cylindrical model consists of an equally mixed region of wasteforms and backfill, bentonite, and rock. Carbon-14 coexists with 3.1 × 106times as much 12C in the wasteforms. The distribution coefficient, maximum absorption amount, and solubility of CO32-against the equally mixed region are assumed to be 2.0 m3/kg, 3.06 mol/kg, and 544 mol/m3, respectively. Then, the release rate from the wasteforms (10-4to 10-6/yr) and underground water detachment period from the wasteforms are examined to lower the dose equivalent by the intake of well water.The 14C concentration on the EBS boundary is 20 times as large in the case of EBS finite absorption as in the case of infinite absorption. So, the EBS finite absorption leads to absorption saturation and accelerated release of the radionuclides. The influence of the absorption saturation could not be prevented by lowering the release rate. A 3 × 104/yr detachment lowered the dose equivalent to 1/40 of the original case.

Radioactive laundry waste water generated in nuclear power plants includes organic compounds to be removed, for which ozone oxidation treatment is a possibility. To verify the applicability, its total organic carbon concentration (TOC) lowering rate improvement was examined in 0.5x10-3m3batch and 3x10-2m3pilot equipment experiments.In the batch experiments, ozone at a concentration of 200g/Nm3was dispersed into 0.5x10-3m3of Simulated Laundry Waste Water (SLWW) with TOC of 11 mM. Total organic carbon concentration was measured every hour to see the effects of the temperature, and the initial concentrations of both H2O2and NaOH which were added to urge OH radical generation from ozone. In the pilot equipment experiments, 1x10-2to 3x10-2m3of the SLWW were circulated using an ejector to disperse the ozone. The influences of the flow rate and the SLWW volume on lowering TOC were examined, because they were related to the ejector dispersion performance and the appropriate ozone addition per SLWW volume.Appropriate initial H2O2and NaOH concentrations in the batch experiments were 14.7mM and ImM, respectively. Lowering of TOC became faster at higher temperatures, because ozone self-decomposition and OH radical diffusion to the organic compound molecules were promoted. Lowering of TOC also became faster at higher flow rates, while the influence of the volume became saturated.

After their land disposal, LiNO3added to cement solidified miscellaneous wastes inhibits hydrogen gas generation due to alkaline corrosion of aluminum contained in the wastes. We considered the presence of an Li-Al preservation film prevents hydrogen gas generation, and then, we assumed a scenario in which the amount of LiNO3included in the waste packages is lowered by underground water penetration, resulting in dissolution of the Li-Al preservation film. This dissolution allows the alkaline underground water to reach and corrode the aluminum materials. The loss of Na2O and K2O in cement by underground water penetration lowers the pH, so that the aluminum corrosion in the waste packages with LiNO3, expected when the Li-Al preservation film dissolves, is less than that without LiNO3.To test this scenario, we measured solubility of the Li-Al preservation film, Li+ion concentration, pH variation by underground water penetration, and aluminum corrosion when the Li-Al preservation film had dissolved. The measured solubility of the Li-Al preservation film was 3 × 10−4Mat 283 K. At that time, pH was lowered from 12.9—13.0 to 12.2—12.3. As a result, with LiNO3addition the aluminum corrosion amount was reduced to 10% of that without LiNO3addition, because of the pH decrease.

AbstractThe influence of increased temperature from cement hydration was checked on aluminum corrosion prevention when LiNO3was added to the cement used for aluminum waste cementation.At first, the temperature at the center of a 0.2-m3cement or mortar form was measured. Then, because the reaction mechanism of LiNO3involves formation of insoluble LiH 2AlO25H2O (Li-Al) preservation film on an aluminum surface, the Li-Al film solubility was measured in a 0.1 M KOH aqueous solution at temperatures from 283 to 353 K. In a second experiment, an aluminum specimen was soaked in a 0.1 M KOH solution with 3 wt% of dissolved LiNO3, and the volume of generated hydrogen gas was measured. Finally, aluminum plates were solidified with mortar in a full-scale test. The mortar mixture contained ordinary portland cement (OPC), blast furnace slag (BFS), and sand with a 1.5 wt% LiNO3addition, and the volume of generated hydrogen gas was measured.When only OPC was used, the temperature increased to ~363 K. With the BFS and sand addition, this temperature increase was reduced by ~40 to 323 K. The Li-Al film solubility became larger as the temperature of the solution increased. The volume of hydrogen gas generation became large as the temperature increased, especially over 323 K. When the mortar consisted of OPC, BFS, sand, and LiNO3, the volume of hydrogen gas generation from aluminum was reduced, becoming <10% of that without the LiNO3addition. Thus, it appears that the temperature did not have much influence on the ability of LiNO3to prevent aluminum corrosion, although the ability was gradually lessened as the temperature increased.

LiNO3addition to the cement solidified miscellaneous wastes has been proposed for preventing hydrogen ges generation caused by the corrosion of aluminum materials contained in the wastes. To determine an additive among alkaline metal ions, galvanic current was measured in 0.1 M alkaline metal hydroxide solution between aluminum and platinum electrodes. The volume of hydrogen gas generated from an aluminum specimen was measured in a KOH solution with LiNO3, LiCl, LiBr, Li2CO3or Li2SO4to decide the best additive. Applicability of the chosen additive to cement was confirmed by hydrogen gas generation measurement from an aluminum specimen in cement paste. The prevention mechanism was analyzed by X-ray diffraction, SEM and SIMS.The current measured in LiOH solution decreased with time, then reached 0 μA/mm2, while the current was detected in other alkaline metal hydroxide solutions. The least volume of hydrogen gas generation was measured in a KOH solution with LiNO3. The volume of generated hydrogen gas in cement paste with LiNO3was less than 10% of that without LiNO3. The results of analyses showed that an insoluble film of LiH(AlO2)25H2O was formed on the aluminum surface.These results suggested that LiNO3addition to cement is effective to prevent hydrogn gas generation by formation of the insoluble film on aluminum.

AbstractAs an alternative treatment system for laundry waste water in nuclear power plants, a system was chosen in which such organic compounds as surfactant would be oxidized by ultraviolet (UV) light and ozone. The system compatibility, UV light source, and dissolved ozone concentration were examined through experiments. First, ozone gas was absorbed in the waste water. After the dissolved ozone concentration equilibrated at the desired value, the waste water was irradiated by a mercury lamp. Then, the time dependence of the concentrations of the organic compounds, the dissolved ozone, and the hydrogen peroxide were measured to estimate the treatment rate of the system. The mercury lamp with a 105-Pa vapor pressure achieved large UV radiation and a treatment rate increase, leading to a compatible system without secondary waste generation. The effect of the dissolved ozone concentration on the treatment rate was saturated when concentration was >3.3 × 10-4mol/10-3m3at the time UV radiation was started. Numerical results indicated the saturation was due to hydrogen peroxide generation, which prevents hydroxyl radical generation.

The LiNO3effect on aluminum corrosion prevention after land disposal of cement-solidified dry active wastes was examined quantitatively, in the event that the LiH (AlO2)2·5H2O (Li-Al) preservation film was not formed on aluminum surfaces during the solidification process. It is especially probable for these bare surfaces to be left when the wastes include components of complex shapes. LiNO3dissolves from the waste forms into underground water to form the Li-Al preservation film. So, we thought that the LiNO3addition would prevent the corrosion. We measured the volume of hydrogen gas generation in mortar-soaked water during the Li-Al preservation film formation, as functions of LiNO3addition amount, the weight ratio of water to mortar when the mortar-soaked water was produced, and the aluminum surface area, to quantify the effect.We found that aluminum corrosion was inversely proportional to the LiNO3addition. For the corrosion to be less than 10−5m in 103h, the initially added amount of LiNO3must be 1.5wt% of the sum of cement and sand. Regardless of the weight ratio of water to mortar when the mortar-soaked water was produced, hydrogen gas generation with LiNO3was 10% as much as that without it, in 5 x 103h. Because of the Li-Al preservation film formation reaction, hydrogen gas generation was proportional to the cubic root of the aluminum surface area.

Experiments were conducted to examine the enhancement of hydrogen gas generation when the LiH 2AlO2middot;5H2O (or “Li—Al”) preservation film formed on aluminum materials was gradually depleted after land disposal of cement-solidified dry active wastes.In these experiments, an aluminum specimen overlaid with the Li—Al preservation film was placed in a reaction beaker and exposed to flowing alkaline water (pH 12.3) to deplete lithium content of the Li—Al preservation film in lithium content gradually. Then, the volume of hydrogen gas generation from the specimen and lithium concentration in the reaction beaker outflow were measured. The specimen surface was also examined by X-ray diffraction and SIMS to see if there was any change in its structure.As water flow rate was lowered, depletion of the Li—Al preservation film became slower, so that the aluminum corrosion rate, which was calculated from results of the hydrogen gas generation measurements, was small. Because the actual flow rate was very small, it did not seem a quick increase in gas generation occurred. In addition, hydrogen gas generation was smaller, too. This was because part of the surface of the Li—Al layer dissolved to form a fresh Al(OH)3layer which covered the Li—Al preservation film left and prevented its dissolution.