Modeling and simulation of coupled flow/power behavior in low pressure natural circulation systems
Abstract
Natural circulation boiling water reactors (NCBWRs) may experience certain operational difficulties due to flow instabilities which may occur at low-pressure and low flow conditions. These are typical conditions observed during the reactor start-up. The flow oscillations coupled to reactor power via void-reactivity may cause significant power oscillations. Furthermore, at aforementioned conditions, the significant variation of saturation temperature inside reactor pressure vessel results in complicated dynamic behavior governed by thermal non-equilibrium between liquid and gas phases. Flashing in the unheated section due to gravitation head loss is one of the important phenomena and needs to be considered for the scaling and stability analysis considering the two-way coupling of the reactor power and core flow. The coupled flow/power behavior in two-phase natural circulation systems operating at low-pressure and low-power conditions are investigated via simplified field and constitutive equations for flow kinematics, energetics, dynamics, fuel heat conduction, and neutron kinetics. A dimensional analysis is introduced to obtain governing dimensionless groups. A flashing number, which is derived mechanistically from a model for vapor generation, is presented to have a unique relationship between pressure and length scales. It is demonstrated that fewer number of dimensionless groups governing both single- and two-phase natural circulation loops can be obtained via introducing appropriate forms of characteristics scales. Scaling laws for the dynamics simulation of single- and two-phase natural circulation systems are derived for the simulation of the low-pressure behavior. The derived dimensionless field and constitutive equations are also used to formulate the problem for steady-state, frequency domain, and time-domain analysis. The derived scaling criteria are used to develop simulation strategy for the start-up procedure of a typical natural circulation BWR design. Several experiments are performed to simulate the transient via a sound scaling strategy for the first time. Based on these simulation experiments, it is demonstrated that inclusion of void/power coupling is a destabilizing effect and may cause large amplitude power/flow oscillations during the early stages of the reactor start-up.
Degree
Ph.D.
Advisors
Ishii, Purdue University.
Subject Area
Nuclear physics
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