Development and characterization of coupled physics critical experiments at the Walthousen Reactor Critical Facility for use as multi-physics experimental benchmarks

Abstract

Modern reactor simulation tools provide advanced prediction capabilities by coupling multi-physics models to simulate reactor behaviors, involving thermal, neutronic, and mechanical interactions. Experimental data is needed to validate the coupled-physics models deployed in these tools, assuring the high-fidelity of their predictions. In order to provide data to benchmark coupled thermal-hydraulic and neutronic simulations, coupled-physics critical experiments were designed and performed at the Walthousen Reactor Critical Facility (RCF). The facility houses a low power and open-pool type light water reactor operated at atmospheric pressure. The reactor allows flexible reconfigurations for many unique critical experiments. As the main contribution of this thesis, a water loop system was designed and installed in the facility. The system contains an external reservoir tank of water, connected with pipes and a pump, forming a closed water loop. A section of the loop is located inside the reactor central region, surrounded by fuel pins and moderator water. Heated water can circulate inside the water loop and pass through the center region of the reactor core from top to bottom. The addition of the heated water loop broadens the range of validation experiments available for neutronics/thermal-hydraulics couplings. Direct effects of the loop water thermal dynamic change on reactor power and reactivity are demonstrated through a series of more than 100 experiments, including reactivity change over different loop water temperatures, and reactor power evolution under different flow transient conditions.

To provide the best quality experimental data, particular care is given to each of the measured data post-processing steps, with a focus on uncertainty quantification. Previously developed data analysis methods have been adapted to be used in the unique experiments conditions treated in this work. For example, signal denoising has been performed with multivariate discrete wavelet transform and multiscale principal component analysis, time-dependent reactivity has been determined through the Asymptotic Period Method based on the Inhour Equation applied to a slow-transient case and the Inverse Kinetics Method, and isothermal reactivity uncertainty has been evaluated with an equivalent uncertainty method combining the errors on both reactivity and temperature measurements. The experiments are all completed, and the steps of post-processing and of uncertainty quantification are thoroughly described. All the experiments operating conditions, results and uncertainties are gathered to provide a multi-physics data library.

The new multi-physics steady-state and transient coupling experiments are performed under specific reactor operating conditions of atmospheric pressure, low power (<15 W), sub-boiling temperature range (10 to 70 °C) and positive temperature reactivity coefficient. The results of these experiments can be used to validate neutron cross-section data models such as S(α,β) thermal scattering treatment of water and Doppler broadening of 238U or other fertile isotopes cross-sections in the fuel in the 10-70 °C range, as well as general neutronics/thermal-hydraulics multi-physics coupling schemes. The users can also apply these experiments to validate the tools that are used to model the startup reactor physics of current light water or future small-scale modular reactors, to study the reactor noise at the extremely low power, and to analyze the criticality safety of flooding accident in dry-cask storage of spent fuel, or criticality accident in spent fuel pools. The current available benchmark libraries for multi-physics modeling validation lack the type of data produced in this work, like positive isothermal reactivity coefficient experiments results, and/or transient thermal-hydraulics/neutronics coupling critical experiments, especially in the particular operating conditions of the RCF.