A study of solute transport of radiolysis products in crud and its effects on crud growth on PWR fuel pin

Axial Offset Anomaly (AOA);crud;radiolysis;diffusion

Joe, Hyun-Jong

关键词: Axial Offset Anomaly (AOA);    crud;    radiolysis;    diffusion;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/24384/Joe_Hyun-Jong.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This research examines the effect of radiolysis species (H2, O2, and H2O2) upon crud deposition and growth and also the effect of their concentration within the crud. A 3-D transient diffusion model has been developed to simulate solute transport of these radiolysis products in porous crud layer. It attempts to explain the behavior of the porous crud growth on PWR fuel pins. This model employs a system of coupled mass transport and chemical interactions as the source term, which make the problem non-linear. A Monte Carlo technique is adopted to simulate the behavior of the different species.The direct application of this study to nuclear engineering research is to aid in the design of reactors with higher performance. With the deregulation of the U.S. power market, there is an incentive to extend the life and to enhance the performance and efficiency throughout the power generation industry. This requires higher power levels for existing nuclear reactors, which can cause the occurrence of an Axial Offset Anomaly (AOA). Approximately 20 U.S. PWRs have experienced significant Crud-Induced axial Power Shift or CIPS (also referred to as AOA), an unexpected measured shift in axial power distribution from predicted values. Such abnormal performance has driven the Nuclear Regulatory Commission (NRC) to impose lower reactor power levels at the aging reactor fleet for safety reasons.While much effort has been focused on finding the causes of AOA, a detailed understanding has not yet been developed, which predicts its growth rate over the fuel lifetime, because of the complexity of its causes. Porous crud deposits in the form of the non-stoichiometric nickel-ferrite (FexNi3-xO4) have been observed on the Zircaloy cladding surface exposed to the subcooled boiling length of the fuel rods. The thermal processes in this layer, which result in the holdup of boron (10B and 11B), cause a larger than predicted decrease in the neutron flux. Research suggests that the buildup rate of metallic corrosion products from the reactor vessel and primary coolant loop can be accelerated. Boron holdup in the crud under the presence of radiation is likely higher, especially through radiolysis and irradiation damage on the cladding.For the modeling of solute transport of radiolysis products, the present analysis considered two competing solute transport mechanisms within the porous crud deposits: molecular diffusion with a source term from the radiolysis and mass convection. Initially, the analytical model for the equilibrium state using the Laplace equation is summarized from the author’s earlier work. Later, a transient diffusion equation was introduced using a random walk Monte Carlo technique coupled with a constant source term from radiolysis. This procedure by Ragheb was modified from a cartesian to a cylindrical geometry random walk. The molecular radiolysis species of hydrogen, oxygen, and hydrogen peroxide are the most stable radiolysis products that directly affect: (1) the oxidation-reduction reactions at the interface of crud/cladding and (2) the formation of nickel ferrite within the crud deposit. In a post-AOA crud, Bonaccordite (Ni2FeBO5) is also observed in the nickel-ferrite. It is recommended that further investigations be performed incorporating the electrochemical interactions of these electroactive species. More detailed consideration of convective transport should be coupled with diffusion with a source term of radiolysis.