Deposition behavior of TiO₂ on fuel cladding surface in boiling water under simulated BWR operating conditions

The susceptibility of stainless steel structural materials in boiling water reactors (BWRs) to intergranular stress corrosion cracking (IGSCC) can be reduced by lowering the electrochemical corrosion potential (ECP). A technique for reducing the ECP by injecting TiO₂ solution into BWRs under operating conditions ("On-Line TiO₂ Injection") has been developed by the authors. One of the key considerations in the development and implementation of the On-Line TiO₂ injection is a possible impact on fuel performance. From results of in-core corrosion tests, the integrity of the fuel is ensured when the amount of TiO₂ deposited on a Zircaloy-2 cladding surface was less than 1000 μg/cm². In order to investigate the TiO₂ deposition behavior in boiling water, TiO₂ deposition tests on a heated Zircaloy-2 cladding surface in boiling water were conducted in a laboratory loop under simulated BWR operating conditions. The effects of quality and heat flux on the deposition rate were evaluated. At a constant heat flux, the deposition rate was proportional to the quality within a certain distance from the inlet of an autoclave. At a constant quality, the deposition rate was proportional to the heat flux within the experimental range of heat fluxes used, namely, 150 to 250 kW/m². The morphologies of the deposited TiO₂ were characterized by scanning electron microscope (SEM) observation. Moreover, the profile of the deposition rate vs. axial length of fuel in an actual BWR was calculated from the experimental data. As a result, the maximum deposition rate was calculated to be 23.0 cm/h when the axial distance from the inlet core was 2.3 m, the quality was 8.1%, and the heat flux was 598.5 kW/m². (author)