Dong-Seong Sohn

Pore pressure and swelling in the rim region of LWR high burnup UO2 fuel

Based on measured rim characteristics of LWR high burnup UO2 fuel, the pressure of rim pores and the additional pellet swelling due to rim formation have been modeled. Using the assumption that the number of Xe atoms retained in the rim pores is the same as that which is depleted from the rim matrix, excessive pore pressure is derived as a function of temperature, pellet average burnup and pore radius. The rim pores with small radii are calculated to be highly overpressurized at high burnup. Comparison with experimental data shows that, while the pellet swelling obtained with best-estimate rim width is underpredicted, the one calculated with conservative rim width agrees well with the measured data for rim burnups between 50 and 65 GWd/tU. On the other hand, the measured swelling at 85 GWd/tU is about in-between the two calculations.

Multi-layer coating of silicon carbide and pyrolytic carbon on UO2 pellets by a combustion reaction

Abstract The coating layers of silicon carbide and pyrolytic carbon on UO 2 pellets were prepared by using a combustion reaction between the carbon and silicon layers. The pyrolytic carbon and silicon were deposited by thermal decomposition of propane at 1250°C in a chemical vapor deposition unit and microwave pulsed electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) using silane at 500°C. Microstructural observation of the layers with scanning electron microscopy (SEM) showed that an inner layer existed following the surface contour of the pellet and the outer layer had a small number of fine pores inside. Chemical analyses with Auger electron spectroscopy (AES) and X-ray diffractometry (XRD) revealed that the inner and outer layers were pyrolytic carbon and silicon carbide, respectively. From the transmission electron microscopy (TEM) observation, the silicon carbide formed during the combustion reaction was identified as fine crystalline β-SiC. The temperature distribution of the specimen during the combustion reaction was estimated by a finite element method, which showed that preheating above 1300°C was required for the combustion reaction between silicon and carbon to propagate well through the specimen.

Cosmos: A computer code to analyze LWR UO2 and MOX fuel up to high burnup

Abstract A computer code called cosmos has been developed for the analysis of UO 2 and MOX fuel during steady-state and transient operating conditions. The main purpose of the cosmos is to calculate temperature distribution in the fuel and cladding, and fission gas release from the fuel. Experimental findings regarding such areas as rim effect and thermal conductivity degradation with burnup are taken into account to analyze high burnup fuel. In addition, a mechanistic fission gas release model developed based on physical processes is incorporated into the code to calculate gas release during steady-state conditions. An empirical model developed based on the amount of fission gas stored at the grain boundary is used for transient operations. Another important feature of the cosmos is that it can analyze fuel segments refabricated from base-irradiated fuel rods. This feature makes it possible to utilize database obtained from international projects such as halden and riso , many of which were collected from refabricated fuel segments. Based on models for UO 2 fuel, MOX features such as change in themo-mechanical properties and the effect of microscopic heterogeneity on fission gas release have been taken into account. The capacity of the cosmos has been tested with a number of experimental results from some international fuel irradiation programs. This paper provides a general description of the models contained in the cosmos and some of the comparison results between the calculations and measurements.

Analysis of fission gas release and gaseous swelling in UO2 fuel under the effect of external restraint

Abstract To analyze fission gas release and gaseous swelling in UO 2 fuel, a model has been developed that can be used under both steady-state and transient operating conditions up to high burnup. With emphasis on the effect of external restraint stress on the behavior of gas bubbles at grain boundaries, the gaseous swelling due to grain edge bubbles, which affects gas release rate through the formation of release tunnels at grain edge, is described. Gas release rate at the grain edge is assumed to be proportional to both the fraction of grain edge bubbles interlinked to open space of fuel and the rate of gas atoms arriving at the grain edge bubbles. The model was compared with the data obtained from commercial reactors, Risφ-III Project, isothermal irradiation and post-irradiation annealing experiments. It is shown that the model predicts well the fractional fission gas release as well as the radial distribution of Xe gas across fuel pellet under various operating conditions. This suggests that the present model can be used for the analysis of fission gas release at high burnup fuel where strong external restraint stress may develop due to pellet cladding interaction.

FABRICATION OF GD CONTAINING DUPLEX STAINLESS STEEL SHEET FOR NEUTRON ABSORBING STRUCTURAL MATERIALS

A duplex stainless steel sheet with 1 wt.% gadolinium was fabricated for a neutron absorbing material with high strength, excellent corrosion resistance, and low cost as well as high neutron absorption capability. The microstructure of the as-cast specimen has typical duplex phases including 31% ferrite and 69% austenite. Main alloy elements like chromium (Cr), nickel (Ni), and gadolinium (Gd) are relatively uniformly distributed in the matrix. Gadolinium rich precipitates were present in the grains and at the grain boundaries. The solution treatment at 1070℃ for 50 minutes followed by the hot-rolling above 950℃ after keeping the sheet at 1200℃ for 1.5 hours are important points of the optimum condition to produce a 6 mm-thick plate without cracking.

Improvement of Fuel Performance Code COSMOS with Recent In-Pile Data for MOX and UO2 Fuels

A fuel performance code, COSMOS, was developed for an analysis of the thermal behavior and fission gas release of both mixed-oxide (MOX) and UO2 fuels up to high burnup. The models have been improved for the fuel thermal conductivity, the fission gas release, and the cladding corrosion and creep. In particular, the thermal conductivity and fission gas release models were restructured with due consideration for the inhomogeneity of the MOX fuel. These improvements enhanced COSMOS’s precision for predicting the in-pile behavior of the MOX fuel. The COSMOS code also extends its applicability to the sophisticatedly instrumented fuel test in a research reactor. With the improved models, the recent in-pile test results were analyzed and compared with the code’s prediction. The database consists of the instrumented MOX fuel test in a research reactor, the postirradiation examination results after an irradiation in a commercial reactor, and a preliminary instrumented test in the HANARO reactor. With the rigorously characterized fabrication data and irradiation information, the COSMOS code predicted the in-pile behaviors well, such as the fuel temperature, rod internal pressure, fission gas release, and cladding properties of MOX and UO2 fuels. The estimations by COSMOS also demonstrated its applicability to the instrumented irradiation test.

The irradiation test of inert-matrix fuel in comparison to uranium plutonium mixed oxide fuel at the halden reactor

Conservative modelling for pin layout shows that the relatively low thermal conductivity of Inert-Matrix Fuel (IMF) causes higher temperatures and therefore higher fission gas release than in uranium plutonium mixed oxide (MOX). According to neutronic calculations, performance differences will also arise from different evolutions of the respective radial power and burnup distributions. Modelling of these effects as well as a ∼10% greater production of Xe in the thermal spectrum of the Halden reactor is well within the capabilities of appropriate codes. Some of the data and models used for the pre-calculations are preliminary and will be revised after the first experimental data have become available.

Rim characteristics and their effects on the thermal conductivity in high burnup UO2 fuel

Characteristics of high burnup UO2 fuel such as threshold burnup for the formation of high burnup microstructure (rim), rim average burnup and rim width were estimated and then the thermal conductivity degradation due to the porous rim region was investigated. The threshold burnup for rim formation was estimated as a function of temperature and fission rate using Rests model. The calculated threshold burnup, which shows a particular dependence on temperature, ranges from 40 to 50MWd/kgU at typical fuel periphery temperatures of 400 to 600°C. In addition, the rim average burnup and the rim width were obtained by statistical analysis of the data available in open literature. To consider the additional degradation of thermal conductivity in the rim region, a formula for rim porosity was presented with the assumption that rim pores are overpressurized and that all the produced fission gases are retained in the rim pores. To estimate the thermal conductivity in the porous rim using the general correction method applicable to two-phase structure, it was assumed that the rim region consists of pores and fully dense materials composed of UO2 matrix and solid fission products. Then by combining the general model for two-phase with the rim porosity developed in the present paper and HALDENs thermal conductivity model, a thermal conductivity model for the porous rim region was developed. The predicted thermal conductivity shows an additional reduction of ∼20% due to the porous rim structure which would cause to increase the fuel temperature of high burnup fuel during steady-state operation and transient irradiation.

Modeling and parametric studies of the effect of inhomogeneity on fission gas release in LWR MOX fuel

Abstract To analyze the effect of an inhomogeneous mixture of an PuO 2 powder on fission gas release in MOX fuel, a model has been developed using the assumption that gas release mechanism in Pu-rich particles is identical with that in UO 2 fuel. A parametric study was performed to see the respective effect of the number density, size and fraction of Pu retained in the Pu-rich particles on gas release in MOX fuel. The model shows that, for the condition of all the other remaining parameters being fixed, more gas is released in a MOX fuel for lower number density of, smaller size of, and larger fraction of Pu retained in, the Pu-rich particles. However, there exists some condition or combination of parameters for which the effect of inhomogeneity on gas release is negligible depending on the characteristics of MOX fuel. Comparison with measured data for OCOM MOX fuel shows that the present model can predict the level of gas release in MOX fuel once the release mechanism in the Pu-rich particles is known.

Effects of sintering processes on the duplex grain structure of UO2

Abstract The grain structure of UO2 has been investigated under the conditions of three sets of sintering atmospheres and powder properties: (1) CO2 atmosphere and the powder derived from the AUC process, (2) H2CO2 atmosphere and powder from the AUC process, and (3) CO2 atmosphere and powder from the AU process. Powder compacts have been sintered at 1300°C for 0.1 h or partially sintered. The duplex grain structure which consists of coarse and irregular grains and fine and regular grains develops only under the conditions of both CO2 atmosphere and powder derived from the AUC process. It is found that the difference in uranium diffusivity between the U 4 O 9-z and the UO 2+x phases does not play a dominant role in the development a duplex grain structure. The microstructural features suggest that the coarse and irregular grains develop abnormally. The reason for the development of the duplex grain structure is discussed on the basis of the inhomogeneity of the powder compacts derived from the AUC process.