Fuel deposit characterization during in-bay fuel inspection

This paper presents the results of fuel deposit characterization experiments performed by Stern Laboratories (SL) under CANDU Owners Group (COG) work package WP-22218 R2, in collaboration with Bruce Power and Kinectrics Inc. One potential application of this deposit characterization is in estimating the levels of deposits generated from processes such as hot-conditioning. Hot-conditioning (HC) is often performed to prepare a given unit's heat transport system for operation following initial commissioning or following refurbishment. The process is used to mitigate flow-accelerated-corrosion (FAC) of heat transport system (HTS) piping by precipitating a thin magnetite (i.e., iron oxide) layer on internal HTS carbon steel surfaces. Some limited magnetite deposition may occur on fuel bundle surfaces if fuel is present in core during the hot-conditioning process. Previous experiments, at Stern Laboratories, have demonstrated that effective chemistry control during the conventional HC process minimizes deposits and eliminates the potential for negative impacts on the fuel thermal-hydraulic performance. Nevertheless, it is prudent to understand the nature of deposits, if and when they present themselves, and be able to characterize them. Such characterizations can be procedurally employed by on-site fuel inspectors at the various utilities. For the purposes of this work, electrically heated specimens were used in the SL single-element Corrosion Research Loop (CRL) under the following conditions: 10 MPa channel outlet pressure, 1000-4000 kg/m² deionized water mass flux, 286°C outlet temperature, 20-30 kW/m linear element power and ~7 pH. The steady-state operations took 8 days to complete with carbon steel piping valved-in during the operations, mimicking the primary in-reactor magnetite source which is the feeder pipes. The mass flux, element power and pH content were instrumental in artificially creating deposits in the out-reactor setting and deposits of varying thicknesses were successfully generated. Post-test inspections included deposit thickness measurement using a state-of-the-art optical device, on-the-bench (dry) photography, as well as underwater photography, to produce visual standards and references to support on-site, in-bay inspection activities. Inspection photos and deposit characterizations were obtained for three ranges of deposits, specifically: <20 µm, 50-90 µm and >100 µm. (author)