Stability Analysis of the High Performance Light Water Reactor

In the Generation IV international advanced nuclear reactor development program, the High Performance Light Water Reactor (HPLWR) is one of the most promising candidates. Important features are its inherently high thermodynamic efficiency (of approximately 45 %) and the ability to use existing supercritical water technology which previously has been developed and deployed for fossil fired power plants. Within a HPLWR core, the fluid experiences a drastic change in thermal and transport properties such as density, dynamic viscosity, specific heat and thermal conductivity, as the supercritical water is heated from 280 °C to 500 °C. The density change substantially exceeds that in a Boiling Water Reactor (i.e., HPLWR: density changes from 780 kg/m 3 to 90 kg/m 3 ; BWR: density changes from 750 kg/m 3 to 198 kg/m 3 ). Due to this density change, the HPLWR can be - under certain operation parameters - susceptible to various thermal-hydraulic flow instabilities, which have to be avoided. In this thesis a stability analysis for the HPLWR is presented. This analysis is based on analytical considerations and numerical results, which were obtained by a computer code developed by the author. The heat-up stages of the HPLWR three-pass core are identified in respect to the relevant flow instability phenomena. The modeling approach successfully used for BWR stability analysis is extended to supercritical pressure operation conditions. In particular, a one-dimensional equation set representing the coolant flow of HPLWR fuel assemblies has been implemented in a commercial software named COMSOL to perform steady-state, time-dependent, and modal analyses. An investigation of important static instabilities (i.e., Ledinegg instabilities, flow mal-distribution) and Pressure Drop Oscillations (PDO) have been carried out and none were found under operation conditions of the HPLWR. Three types of Density Wave Oscillation (DWO) modes have been studied: the single channel DWO, the core-region-wide out-of-phase DWO, and the in-phase DWO. As a first step, the linear stability characteristics of a typical fuel assembly were computed by evaluating the eigenvalues of the thermal-hydraulic model. The results of the analysis are presented in stability maps to define stable and unstable operation points of the HPLWR. This stability maps are expanded by new characteristic numbers which have been derived for fluids at supercritical pressure conditions. For subcritical pressures, these new non-dimensional numbers are related to the well known non-dimensional groups of phase change systems. The sensitivity on various design and operation parameters of the stability limits have been investigated, and the results are summarized in a table. Non-linear phenomena were investigated in the time domain. Complicated mixed supercritical bifurcations were found and the resulting limit cycles were evaluated. In a HPLWR core, nine fuel assemblies form one functional unit: the fuel assembly cluster. This special design feature can be seen as an array of nine coupled parallel flow channels with a common intermediate inlet plenum. By extending the thermal-hydraulic model, it has been shown that a common inlet orifice has almost no effect on the onset of density wave oscillations. Furthermore, the thermal-hydraulic model was coupled with a point-kinetic neutronic model via a heat transfer model. It was found out that the threshold of instability is approximately at the same values of Pseudo-Phase-Change-Numbers for all three types of DWO modes. As a consequence of the various analyses, it lias been shown, while no inlet orifices are required for the fuel assemblies of the superheaters, the fuel assemblies of the evaporator must have single inlet orifices at the entrance of each fuel assembly (in respect to avoid DWOs). To design these inlet orifices, the stability criteria for BWRs have been extended for the HPLWR.