Jong Man Park

Reaction layer growth and reaction heat of U–Mo/Al dispersion fuels using centrifugally atomized powders

Abstract The growth behavior of reaction layers and heat generation during the reaction between U–Mo powders and the Al matrix in U–Mo/Al dispersion fuels were investigated. Annealing of 10 vol.% U–10Mo/Al dispersion fuels at temperatures from 500 to 550 °C was carried out for 10 min to 36 h to measure the growth rate and the activation energy for the growth of reaction layers. The concentration profiles of reaction layers between the U–10Mo vs. Al diffusion couples were measured and the integrated interdiffusion coefficients were calculated for the U and Al in the reaction layers. Heat generation of U–Mo/Al dispersion fuels with 10–50 vol.% of U–Mo fuel during the thermal cycle from room temperature to 700 °C was measured employing the differential scanning calorimetry. Exothermic heat from the reaction between U–Mo and the Al matrix is the largest when the volume fraction of U–Mo fuel is about 30 vol.%. The unreacted fraction in the U–Mo powders increases as the volume fraction of U–Mo fuel increases from 30 to 50 vol.%.

USE OF A CENTRIFUGAL ATOMIZATION PROCESS IN THE DEVELOPMENT OF RESEARCH REACTOR FUEL

A centrifugal atomization process for uranium fuel was developed in order to fabricate high uranium density dispersion fuel for advanced research reactors. Spherical powders of and U-Mo were successfully fabricated and dispersed in aluminum matrices. Thermal and mechanical properties of dispersion fuel meat were characterized. Irradiation tests at the research reactor HANARO confirm the excellent performance of high uranium density dispersion fuel.

PERFORMANCE EVALUATION OF U-Mo/Al DISPERSION FUEL BY CONSIDERING A FUEL-MATRIX INTERACTION

Because the interaction layers that form between U-Mo particles and the Al matrix degrade the thermal properties of U-Mo/Al dispersion fuel, an investigation was undertaken of the undesirable feedback effect between an interaction layer growth and a centerline temperature increase for dispersion fuel. The radial temperature distribution due to interaction layer growth during irradiation was calculated iteratively in relation to changes in the volume fractions, the thermal conductivities of the constituents, and the oxide thickness with the burnup. The interaction layer growth, which is estimated on the basis of the temperature calculations, showed a reasonable agreement with the post-irradiation examination results of the U-Mo/Al dispersion fuel rods irradiated at the HANARO reactor. The U-Mo particle size was found to be a dominant factor that determined the fuel temperature during irradiation. Dispersion fuel with larger U-Mo particles revealed lower levels of both the interaction layer formation and the fuel temperature increase. The results confirm that the use of large U-Mo particles appears to be an effective way of mitigating the thermal degradation of U-Mo/Al dispersion fuel.

Modeling of Interaction Layer Growth Between U-Mo Particles and an Al Matrix

Interaction layer growth between U-Mo alloy fuel particles and Al in a dispersion fuel is a concern due to the volume expansion and other unfavorable irradiation behavior of the interaction product. To reduce interaction layer (IL) growth, a small amount of Si is added to the Al. As a result, IL growth is affected by the Si content in the Al matrix. In order to predict IL growth during fabrication and irradiation, empirical models were developed. For IL growth prediction during fabrication and any follow-on heating process before irradiation, out-of-pile heating test data were used to develop kinetic correlations. Two out-of-pile correlations, one for the pure Al matrix and the other for the Al matrix with Si addition, respectively, were developed, which are Arrhenius equations that include temperature and time. For IL growth predictions during irradiation, the out-of-pile correlations were modified to include a fission-rate term to consider fission enhanced diffusion, and multiplication factors to incorporate the Si addition effect and the effect of the Mo content. The in-pile correlation is applicable for a pure Al matrix and an Al matrix with the Si content up to 8 wt%, for fuel temperatures up to 200 ℃, and for Mo content in the range of 6 ? 10wt%. In order to cover these ranges, in-pile data were included in modeling from various tests, such as the US RERTR-4, -5, -6, -7 and -9 tests and Korea’s KOMO-4 test, that were designed to systematically examine the effects of the fission rate, temperature, Si content in Al matrix, and Mo content in U-Mo particles. A model converting the IL thickness to the IL volume fraction in the meat was also developed.

Diffusion reaction behaviors of U-Mo/Al dispersion fuel

The uranium (U)-molybdenum (Mo)/aluminum (Al) dispersion fuel that is currently under development for high-performance research reactors has shown complicated diffusion reaction behavior between the U-Mo particles and the Al matrix. Diffusion reactions in U-Mo/Al dispersion fuels were characterized by out-of-pile annealing tests and in-pile irradiation tests in the HANARO research reactor. The effect of the addition of a third element such as silicon (Si), Al, or zirconium (Zr) to U-Mo fuel, and the addition of Si to the Al matrix on the diffusion reaction were also investigated. The growth rate and activation energy for the reaction phases of U-Mo/Al dispersion fuels were obtained. The effect of alloying a small amount of a third element in U-Mo and of Si in the Al matrix on diffusion reaction kinetics was negligible in annealing tests conducted at ∼550 °C. γ phase stability in the U-Mo alloy was enhanced by the addition of 0.1 to 0.2 wt.% Si. The Si accumulated in the interdiffusion layer of U-Mo/Al-Si dispersion fuel annealed at ∼550 °C, whereas Zr migration to the interdiffusion layer of U-Mo-Zr/Al was negligible.

The effect of Si-rich layer coating on U-Mo vs. al interdiffusion

Si-rich-layer-coated U-7 wt%Mo plates were prepared in order to evaluate the diffusion barrier performance of the Si-rich layer in U-Mo vs. Al interdiffusion. Pure Si powder was used for coating the U-Mo plates by annealing at 900 ℃ for 1 h under vacuum of approximately 1 Pa. Si-rich layers containing more than 60 at% of Si were formed on U-7 wt%Mo plates. Diffusion couple tests were conducted in a muffle furnace at 560-600 ℃ under vacuum using Si-rich-layer-coated U-Mo plates and pure Al plates. Diffusion couple tests using uncoated U-Mo plates and Al-(0, 2 or 5 wt%)Si plates were also conducted for comparison. Si-rich-layer coatings were more effective in suppressing the interaction during diffusion couple tests between coated U-Mo plate and Al, when compared with U-Mo vs. Al-Si diffusion couples, since only small amounts of Al in the coating could be found after the diffusion couple tests. Si-rich-layer-coated U-7wt%Mo particles were also prepared using the same technique for U-7 wt%Mo plates to observe the microsturctures of the coated particles.

POST-IRRADIATION ANALYSES OF U-MO DISPERSION FUEL RODS OF KOMO TESTS AT HANARO

Since 2001, a series of five irradiation test campaigns for atomized U-Mo dispersion fuel rods, KOMO-1, -2, -3, -4, and -5, has been conducted at HANARO (Korea) in order to develop high performance low enriched uranium dispersion fuel for research reactors. The KOMO irradiation tests provided valuable information on the irradiation behavior of U-Mo fuel that results from the distinct fuel design and irradiation conditions of the rod fuel for HANARO. Full size U-Mo dispersion fuel rods of 4-5 g-U/cm3 were irradiated at a maximum linear power of approximately 105 kW/m up to 85% of the initial U-235 depletion burnup without breakaway swelling or fuel cladding failure. Electron probe microanalyses of the irradiated samples showed localized distribution of the silicon that was added in the matrix during fuel fabrication and confirmed its beneficial effect on interaction layer growth during irradiation. The modifications of U-Mo fuel particles by the addition of a ternary alloying element (Ti or Zr), additional protective coatings (silicide or nitride), and the use of larger fuel particles resulted in significantly reduced interaction layers between fuel particles and Al.

DEVELOPMENT OF HIGH-DENSITY U/AL DISPERSION PLATES FOR MO-99 PRODUCTION USING ATOMIZED URANIUM POWDER

Uranium metal particle dispersion plates have been proposed as targets for Molybdenum-99 (Mo-99) production to improve the radioisotope production efficiency of conventional low enriched uranium targets. In this study, uranium powder was produced by centrifugal atomization, and miniature target plates containing uranium particles in an aluminum matrix with uranium densities up to 9 g-U/cm 3 were fabricated. Additional heat treatment was applied to convert the uranium particles into UAlx compounds by a chemical reaction of the uranium particles and aluminum matrix. Thus, these target plates can be treated with the same alkaline dissolution process that is used for conventional UAlx dispersion targets, while increasing the uranium density in the target plates.

Radiation-Induced Recrystallization of U-Mo Fuel Particles and Radiation-Induced Amorphization of Interaction Products in U-Mo/Al Dispersion Fuel

Two kinds of radiation-induced structural changes were observed in U-Mo/Al dispersion fuel: radiation-induced recrystallization of U-Mo fuel particles and radiation-induced amorphization of interaction products. During irradiation, U-Mo fuel showed refined microstructures of submicron-size grains due to dynamic recrystallization, occurring initially from pre-existing grain boundaries. The interaction products formed by interdiffusion between the U-Mo particles and Al matrix in U-Mo/Al dispersion fuel transformed from crystalline to amorphous during irradiation. In this paper we deal with both of the phenomena simultaneously.

Development of advanced research reactor fuels using centrifugal atomization technology

Rotating disk centrifugal atomization technology has been utilized to fabricate uranium silicide (U3Si, U3Si2) and U-Mo nuclear fuel powders having high uranium content per unit volume for high performance research reactor fuels. Atomized nuclear fuel powders have characteristics of a spherical shape with narrow size distribution, small specific surface area and high purity. The heat treatment time for the formation of U3Si by a peritectoid reaction was reduced from about 72 hours for comminuted powders to about 6 hours for atomized powders due to more rapid solidification of atomized powder with a finer microstructure. The homogeneity of fuel particles in fuel meats was improved by mixing atomized fuel powders with Al powders using a V-shaped tumbler mixer. The atomized powders with a spherical shape and smooth surface were extruded under lower force than the comminuted powders with angular rough surfaces. The dispersed fuel meat with atomized powders resulted in an increased fuel powder loading density and higher thermal conductivity in the heat flow direction. The thermal swelling of dispersed fuel meat decreased due to the reduced specific surface area of spherical atomized nuclear powder. The atomized U-10wt.%Mo dispersion fuel showed less bubble formation than the comminuted fuel after an irradiation test with 40% burnup.