The KORSAR functionality is comprised of several modules. The thermal-hydraulic module is the basic module of the code that calculates thermal-hydraulic processes in the reactor coolant system. This module uses a numerical algorithm to solve six conservation equations for the liquid and vapor phases, assuming equal phase pressures and one-dimensional flow.

Two auxiliary program modules are provided in KORSAR to support operation of the thermal-hydraulic model:

- module for calculation of closure relations;
- module for calculation of thermodynamic properties of water and water vapor.

Closure relations in the code are a set of empirical correlations that account for the effects of mass transfer, thermal and mechanical interphase interactions, and interactions of the phases with the walls in various two-phase flow regimes.

Flow regime and two-phase flow heat transfer maps form the basis to obtain closure relations for calculating interphase and wall friction, wall heat transfer, and interfacial heat- and mass transfer.

The process of developing a system of closure relations involved:

- selection of relations with consideration of models and methods published in the literature and well-assessed correlations used in TRAC, RELAP5, CATHARE, and ATHLET thermal-hydraulic codes;
- successive verification of the system of closure relations during a separate-effect validation of the code: firstly, verification of correlations for interphase and phase-wall friction without phase transitions in the coolant and secondly, verification of correlations for interfacial and wall heat transfers;
- development of new correlations for selected physical phenomena by modifying the existing models and correlations and subsequent verification of the new correlations during separate-effect validation.

The KORSAR system of closure relations is described in detail in [3].

The module that is designed to compute the thermal properties of water and water vapor also supports:

- calculation of a complete set of continuous thermodynamic potential derivatives with respect to thermodynamic variables for both water and vapor phases in the whole computational region;
- extrapolation of the properties in the region of metastable states of water (superheated water) and vapor (subcooled vapor);
- minimization of the properties computing cost in the budget of thermal-hydraulic calculations.

The KORSAR code contains several specific component modules designed for calculating the following processes and equipment:

- reactor neutron kinetics;
- heat conduction structures;
- steam-liquid pressure vessel (pressurizer);
- accumulator;
- free surface tank open to atmosphere;
- centrifugal pump;
- valve;
- controller.