7.1. Introduction

A balance-of-plant thermal/hydraulic model has been developed for use with the SASSYS-1 liquid metal reactor systems analysis code. This model expands the scope of SASSYS-1 so that the code can explicitly model the waterside components of a nuclear power plant. Previously, only the water side of the steam generators could be modelled, with the remainder of the water side represented by boundary conditions on the steam generator. This chapter is organized with a structure that reflects the three major areas of the balance-of-plant thermal/hydraulics module: 1) the network model, 2) the steam generator model, and 3) the component models.

The balance-of-plant network thermal/hydraulics model represents the water side as a network of components, similar to the representation used by SASSYS-1 for the sodium side of the plant. The model will handle subcooled liquid water, superheated steam, and saturated two-phase fluid. With the exception of heated flow paths in heat exchangers, the model assumes adiabatic conditions along flow paths. This assumption simplifies the solution procedure while introducing very little error for a wide range of reactor plant problems.

The balance-of-plant steam generator thermal/hydraulics model is totally new, and completely replaces the previous SASSYS-1 steam generator model. A number of modeling improvements have been implemented in the new steam generator model to achieve better accuracy, capability, and computational performance. First, variable spatial nodalization has been introduced to permit more accurate estimates of heat fluxes. Second, an improved treatment of heat transfer regime transitions is employed to promote numerical stability. Third, donor cell spatial differencing is used with a semi-implicit time-differencing scheme to add further numerical stability and to allow larger time stops. Fourth, the steady-state initialization process has been improved to accommodate a wider variety of heat transfer and flow regimes. Finally, the new steam generator model contains a preliminary method for variable time step selection.

The steam generator model simulates the two most likely configurations for LMR plants with their high core outlet/inlet temperatures. The two configurations are: 1) the once-through system where the superheater is an integral part of the evaporator, and 2) the external recirculation type where the balance-of-plant components are used to connect the separate superheater to the evaporator through the steam drum.

The balance-of-plant components thermal/hydraulics models extend the scope of the network model, to treat nonadiabatic and two-phase conditions along flow paths, and to account for work done across the boundaries of compressible volumes. Simple conservation balances and extensive component data in the form of correlations constitute the basis of models of heaters (deaerators, steam drums, condensers, reheaters, flashed heaters, drain coolers, desuperheating heaters, and desuperheater/drain coolers), turbines, and relief valves. Except for the turbine nozzle, the mass and momentum equations for the component models are the same as the network equations. The component energy equations contain a heat source due to energy transfer across a flow boundary or to work done through a shaft. To handle two-phase conditions, the equation of state is expressed differently for each phase in terms of the quality and separate intensive properties.

The next three sections describe the thermal-hydraulic network, steam generator, and component models.