Blazenka Maslovaric

Prediction and Control of Steam Accumulation

Steam accumulators are applied as buffers between steam generators and consumers in cases of different steam production and consumption rates. The application of the steam accumulator saves energy, reduces pressure fluctuations, and prevents aging of tubes and pressurized vessels in steam generators. In this paper, modes of the steam accumulator operation are analyzed and the general design of the steam accumulator control system is defined. Equilibrium and nonequilibrium thermodynamic models of the steam accumulator are presented with the aim of predicting the steam accumulator capacity and as support to the design of the control system. The equilibrium model is based on the mass and energy balance equations of the total water and steam content in the accumulator, while the nonequilibrium model is based on the mass and energy balance equations for each phase and closure laws of nonequilibrium evaporation and condensation rates. The steam accumulator pressure transients are simulated for constant steam charging and discharging flow rates, and the influence of the nonequilibrium condensation and evaporation rates on the steam accumulator capacity is shown. It is concluded that the commonly used equilibrium thermodynamic approach to the steam accumulator design does not provide accurate results in cases of rapid charging and discharging transients; therefore, there is a need for the application of the nonequilibrium approach.

CFD simulation of steam generator tube rupture thermal-hydraulics

The tube rupture accident is one among the most risk-dominant events at the nuclear power plants. Several steam generator tube rupture accidents have occurred at the plants in the past. In this paper the Computational Multi-Fluid Dynamics (CMFD) investigation of the horizontal steam generator thermal-hydraulics during the tube rupture accident is performed. A guillotine of a steam generator U-tube is assumed with choked flow from the primary to the secondary side of the steam generator. Predicted are water and steam velocity fields, steam volume fraction distribution on the steam generator secondary (shell) side, as well as the swell level increase. Obtained multidimensional results are a support to the safety analyses of the steam generator tube rupture accident. Also, they serve as benchmark tests for an assessment of the applicability of one-dimensional horizontal steam generator models, developed by standard safety codes. Numerical simulation is performed with the multidimensional multi-fluid modelling approach. The two-phase flow around steam generator tubes in the bundle is modelled by the porous media approach. Interfacial mass, momentum and energy transfer is modelled with the closure laws, where some of them are specially developed for the conditions of the two-phase flow across tube bundles. The governing equations are solved with the SIMPLE type pressure-correction method that is derived for the conditions of multi-phase flow conditions.Copyright

Numerical Simulation of Condensation Induced Water Hammer

A numerical model for the simulation and analyses of condensation induced water hammer is presented and applied to the prediction of intensive pressure waves propagation in a two-phase pipe system. The modelling approach is based on the one-dimensional homogeneous two-phase flow model, tracking of the water column–steam bubble interface and modelling of the direct steam condensation on the subcooled liquid. The mass, momentum and enthalpy conservation equations are solved with the method of characteristics. The explicit time integration is performed along the three characteristic paths, where two of them are determined with the pressure waves propagation and third by the fluid particle flow. Fluid particle and water-steam interface tracking is achieved through the third-order accurate solving of the energy equation in space, where the thermodynamic quality determines the presence of water, two-phase mixture or steam. The heat conduction term is included into the enthalpy equation with the appropriate effective heat conduction coefficient in the two-phase region. The model is applied to the simulation and analyses of condensation induced water hammer in laboratory test case.© 2004 ASME