Yaohiro Inagaki

Manufacturing of zirconia microspheres doped with erbia, yttria and ceria by internal gelation process as a part of a cermet fuel

Zirconium oxide is an inert matrix candidate for the transmutation of plutonium in light water reactor (LWR). The thermal conductivity of cubic zirconia is however lower than the conductivities of UO2 and MOX. Special designs are therefore necessary to avoid high peaking temperatures close to the melting point in the zirconia pellet. Cermet would be a favorable design to improve the thermal conductivity. The suggested cermet fuel consists of fine plutonium doped stabilized zirconia particles dispersed in a metallic inert matrix. Manufacturing tests on cubic zirconia microspheres were carried out by using the internal gelation process developed at the Paul Scherrer Institute. Gelation was conducted successfully and the sintered spheres had a homogeneous single cubic structure. The lattice parameter of the cubic zirconia was estimated as a function of the Er, Y and Ce atomic fraction using a simplified semi-quantitative formula. On the experimental side, it is necessary to further investigate the ideal fabrication conditions, because some gel spheres were opaque and fragile and most of the sintered spheres were cracked, nicked and porous.

Corrosion behavior of a powdered simulated nuclear waste glass: A corrosion model including diffusion process

Abstract Static corrosion tests were performed with a powdered simulated waste glass in deionized water at 90°C for periods of up to 130 days. It was observed that normalized elemental mass loss (NL) values for soluble elements (Li, B, Na and Mo) were larger than those for Si by a factor of three and continued to increase after saturation of Si. A corrosion model (diffusion-combined model), where a diffusion model is combined with a dissolution/precipitation model (reaction path model), was developed and applied to the analysis of experimental results. In the diffusion-combined model, it is assumed that less-soluble elements dissolve into the solution congruently with the silica glass matrix (glass matrix dissolution). On the other hand, it is assumed that soluble elements diffuse through the glass to the surface and dissolve into the solution, in addition to the glass matrix dissolution. The diffusion-combined model can explain the experimental results well, and it is found that the diffusion coefficient is the most effective parameter determining the corrosion behavior.

Volumetric change of simulated radioactive waste glasses irradiated by the 10B(n, α)7Li Reaction as simulation of actinide irradiation

Abstract The density change of simulated radioactive waste glasses irradiated by the 10 B(n,α) 7 Li reaction was determined by a sink-float method as a function of irradiation exposure. Simulated waste glasses P0500, P0798 and GP98/12 swelled, while P0504 shrinked. The magnitude of the density change was less than 0.6% up to a fluence of 6.6 × 10 25 reactions/m 3 , which corresponds to the cumulative irradiation during a few tens of thousand years after disposal of the waste glass from the spent fuel irradiated up to 33000 MWD/MTU. The processes which play an important role on the density change have not been clarified, but it is likely that one of the processes is helium bubble formation which was clarified by a carbon replica technique, in association with transmission electron microscopy.

Oxidation kinetics of Zircaloy-2 between 450°C and 600°C in oxidizing atmosphere

Abstract The oxidation kinetics of Zircaloy-2 have been studied in the temperature range 450–600°C under the atmosphere of flowing Ar/5%H2, CO2/1%CO, and CO2. Using the micro-balance technique, the weight change of the specimen has been measured as a function of time. The results showed that the oxidation kinetics of Zircaloy-2 obeyed the cubic rate law rather than the parabolic one. The effect of oxygen partial pressure on the rate constant was not found under the present experimental conditions. On the other hand, the activation energies of the oxidation were 145, 171, and 188 kJ/mol for Ar/5%H2, CO2/1%CO, and CO2 atmospheres, respectively. It was shown from the X-ray diffractometry that the specimens oxidized under the conditions of this study consisted mainly of monoclinic zirconia and, to a minor degree, of tetragonal one. It is suggested that the lateral cracks observed with scanning electron microscopy (SEM) may cause the slow diffusion of oxygen in the oxide phase.

Review of waste glass corrosion and associated radionuclide release as a part of safety assesment of entire disposal system

Abstract Current knowledge on high-level nuclear waste glass corrosion is summarized, and remaining problems are discussed for meaningful predictions of the glass corrosion and associated radionuclide release as a part of safety assessment of entire disposal system. In recent years, much progress has been made in understanding the mechanism of waste glass corrosion in aqueous environments. Glass corrosion models based on the mechanism have been developed for predicting the long-term glass performance, and they are incorporated as part of radionuclide source term in safety assessments of the disposal system. However, these results have not yet allowed meaningful predictions for the long-term release of individual radionuclides from the glass in repository environments, because mechanism of the long-term glass corrosion has not been fully understood and solubilities of actinoids and fission products under disposal conditions are rather uncertain. In addition, the most serious problem is that the effects of various reactions and interactions occurring in the engineered barrier system, such as corrosion of overpack, alteration of backfill and chemical interactions of the released glass constituents with them have not been fully coupled with the glass performance. These reactions may be dominant processes controlling the glass corrosion and associated radionuclide release for the long-term. For the meaningful predictions, we must evaluate the waste glass performance in combination with the effects of various reactions and interactions occurring in the engineered barrier system on the basis of fully understanding of the chemical and geochemical mechanisms.

The oxidation kinetics and the structure of the oxide film on Zircaloy before and after the kinetic transition

Abstract Oxidation kinetics of Zircaloy-4 have been measured using a micro-balance technique in CO–CO 2 gas mixtures between 450°C and 600°C. Oxidation kinetics of Zircaloy-4 obeyed a cubic rate law with time at 450–600°C up to 24 h. At 600°C, the kinetic transition occurred after about 36 h. After the transition, oxidation kinetics obeyed a linear rate law. X-ray diffraction patterns for the samples oxidized at 600°C showed that the volume fraction of tetragonal phase of zirconia decreased with time until the kinetic transition occurred and was almost constant after that. In addition, stresses in the oxide films were found to be larger for the pre-transition samples than for the post-transition ones.

Initial dissolution rate of a Japanese simulated high-level waste glass P0798 as a function of pH and temperature measured by using micro-channel flow-through test method

Aqueous dissolution tests were performed for a Japanese type of simulated high-level waste (HLW) glass P0798 by using a newly developed test method of micro-channel flow-through (MCFT) method, and the initial dissolution rate of glass matrix, r 0, was measured as a function of solution pH (3–11) and temperature (25–90°C) precisely and consistently for systematic evaluation of the dissolution kinetics. The MCFT method using a micro-channel reactor with a coupon shaped glass specimen has the following features to provide precise and consistent data on the glass dissolution rate: (1) any controlled constant solution condition can be provided over the test duration; (2) the glass surface area actually reacting with solution can be determined accurately; and (3) direct and totally quantitative analyses of the reacted glass surface can be performed for confirming consistency of the test results. The present test results indicated that the r 0 shows a “V-shaped” pH dependence with a minimum at around pH 6 at 25°C, but it changes to a “U-shaped” one with a flat bottom at neutral pH at elevated temperatures of up to 90°C. The present results also indicated that the r 0 increases with temperature according to an Arrhenius law at any pH, and the apparent activation energy evaluated from Arrhenius relation increases with pH from 54 kJ/mol at pH 3 to 76 kJ/mol at pH 10, which suggests that the dissolution mechanism changes depending on pH.

Oxidation behavior of modified SUS316 (PNC316) stainless steel under low oxygen partial pressure

Abstract Oxidation behaviors of modified SUS316 (PNC316) and SUS316 stainless steels were investigated under the low oxygen partial pressure of 10 −31 −10 −22 atm at 600–800 °C. Oxygen uptake by these materials parabolically increased with time, and the kinetic rate constants depended on both oxygen partial pressure and temperature. Thus, semi-empirical equations of the parabolic rate constants were obtained to be 2.70×10 4 exp(−109/ RT ) P O 2 0.279 for PNC316 and 9.23×10 4 exp(−98/ RT ) P O 2 0.313 for SUS316. For the duplex layer formed under the low oxygen partial pressure, the inner layer consisted of such oxides as Cr 2 O 3 and FeCr 2 O 4 , while the outer layer consisted of non-oxidized α-Fe. Furthermore, oxidation along the grain boundaries was observed for samples oxidized for a long time. From the point of view of fuel cladding chemical interaction evaluation at high burn-up fuel for fast reactors, it is interesting that formation of non-oxidized α-Fe was observed under the low oxygen partial pressure.

Molecular dynamics analysis of diffusion of uranium and oxygen ions in uranium dioxide

Diffusion behaviours of oxygen and uranium were evaluated for bulk and grain- boundaries of uranium dioxide using the molecular dynamics (MD) simulation. It elucidated that oxygen behaved like liquid in superionic state at high temperatures and migrated on sub- lattice sites accompanying formation of lattice defects such as Frenkel defects at middle temperatures. Formation energies of Frenkel and Shottky defects were compared to literature data, and migration energies of oxygen and uranium were estimated by introducing vacancies into the supercell. For grain-boundaries (GB) modelled by the coincidence-site lattice theory, MD calculations showed that GB energy and diffusivities of oxygen and uranium increased with the misorientation angle. By analysing GB structures such as pair-correlation functions, it also showed that the disordered phase was observed for uranium as well as oxygen in GBs especially for a large misorientation angle such as Σ5 GB. Hence, GB diffusion was much larger than bulk diffusion for oxygen and uranium.

Concentrations of radiocarbon and isotope compositions of stable carbon in food

The 14C activities and/or á13C values were measured for tree rings, rices, edible oils, flavors, beers, chickens and eggs. The 14C activity in 1999 ranged from 14.4 to 14.8dpm/g.C for those of tree rings and rices. The 14C activities of oils were in good agreement with the present atmospheric one. The 14C activities of flavors could differentiate petrochemically derived compounds from natural products. On the other hand, the Δ13C value could distinguish between C3 and C4 plants for the main ingredient of oils, flavors and beers. For chickens and eggs, & Delta;13C values reflected their diet. In addition, large variation of Δ13C values was observed among components of eggs.