The effect of aluminum corrosion on the advanced neutron Source Reactor fuel design

Abstract

The corrosion of fuel plates can have a significant impact on the performance of reactors with aluminum fuel cladding. Increased fuel temperatures result when the oxide imposes an additional thermal resistance between the fuel plate and the coolant, while spallation of the oxide film may reduce cladding integrity. Characterization of oxide growth over the fuel cycle is therefore a necessary element in the design of such reactors. This paper describes the impact of fuel plate oxide formation on the Advanced Neutron Source Reactor (ANSR) core design. An oxide layer continually grows on the fuel plate surface throughout the fuel cycle. Although the cycle is short in duration (14 days), even a thin layer ( < 25 μ m ) of low conductivity aluminum oxide causes a significant increase in fuel temperatures due to high ANSR power densities. The growth rate of the oxide is determined by many factors: impurities in the D 2O, coolant and oxide temperatures, heat flux, etc. Since several of these factors vary during the fuel cycle, prediction of oxide thicknesses must be time dependent. This is presently accomplished by using a series of core power density distributions calculated at different times within the fuel cycle in concert with an empirical oxidation model. Two specific thermal limits are imposed on the ANSR core design which are affected by oxide growth. A fuel centerline temperature limit of 400°C is established by fuel swelling behavior, while the oxide temperature drop is limited to 119°C to avoid oxide spallation. This paper discusses these limits and the constraints they impose on the core design. A recently developed oxide growth correlation is used in combination with thermal hydraulic analysis to show that fuel loading design can be tailored to minimize thermal limitations on the ANSR imposed by oxide growth. A power shape is presented which ideally causes a uniform oxide film to form over the entire plate surface, improving operating margins by several percent. In reality, this ideal shape cannot be obtained due to various aspects of the core: control rod movement, the time dependent nature of fuel burnup, the fuel/moderator relationship, etc. The design process is, therefore, iterative between thermal hydraulic and neutronic analyses. Results of additional calculations are presented which describe the performance of these more realistic fuel loadings and compare them to the ideal case.