Further Development of the Core Simulator CORE SIM: Extension to coupled capabilities for BWRs

Rapport, 2014

In nuclear reactors, the monitoring of the nuclear core is of prime importance for guaranteeing the safety of the plant. Assuming stationary conditions, the measurement of process signals using the existing instrumentation supplemented by adequate data acquisition chains allows monitoring fluctuations of the process signals around their mean values. Even though the system does not exhibit any change in the mean values of the process signals, these fluctuations are always present and are the result of e.g. the turbulent character of the cooling flow, coolant evaporation, and/or possible anomalies (excessive vibrations, etc.). These fluctuations (often referred to as “noise”) thus carry some information about the dynamics of the system, and can be used either for core diagnostics/surveillance purposes (i.e. when an anomaly is suspected in the core) or for determining dynamical core parameters/safety coefficients. The main advantage of such techniques relies on the fact that no perturbation of the system is required and that the method is thus a non-intrusive one. The instrumentation present in nuclear core mostly consists of neutron detectors. Many neutron noise diagnostics tasks thus involve an inversion or „unfolding“ procedure, where the neutron noise measured in a few locations throughout the nuclear core is used to determine the root cause (i.e. noise source) responsible for the measured neutron noise. Such an inversion is seldom possible without the knowledge of the so-called reactor transfer function, i.e. the function giving the neutron noise induced by any arbitrary noise source. The Division of Nuclear Engineering at Chalmers University of Technology has been very active for the last ten years in developing computational methods allowing the estimation of such a transfer function for actual reactor cores, i.e. strongly non-homogeneous systems. A numerical tool, named CORE SIM, was developed to estimate the open-loop reactor transfer function. In this tool, the noise source is defined in terms of fluctuations of the macroscopic cross-sections. This report deals with the development of a thermal-hydraulic module coupled to CORE SIM, so that the closed-loop reactor transfer function can also be estimated for Boiling Water Reactors (which is the main type of reactors constituting the Swedish fleet). This module is based on solving the mass, momentum, and enthalpy conservation equations for the fluid, and on solving the heat conduction equation in the solid fuel pellets. Because of the fully coupled neutronic/thermal-hydraulic character of the tool, the noise source can be directly defined in more realistic terms such as perturbations of the flow velocity, temperature, etc. at the inlet of the core. The coupled tool, in addition to be the only one of its kind, has a wide range of applicability.