A thermal-hydraulic system to be simulated with the KORSAR computer code is divided into elements. Each element is classified to one of element types adopted in KORSAR [1]. For oriented elements, their orientation is defined (channel inlet/outlet, number of heat conduction surface, etc.).

All elements are numbered, consecutively or not, within their element type. For distributed elements, the number of their components is selected depending on the problem specification. For each distributed element, consecutive numbering of components from 1 onwards, with no missing numbers, is applied depending on the selected orientation.

The elements make up a nodalization scheme. Connections between the elements of the nodalization scheme are coded in the input file with LAYOUT procedure using valid element representation forms [1].

Table – Element types in the KORSAR computer code

Element name |
Element designation in input file |
Purpose of element |

Channel |
CH | Description of flow in a pipe or fuel assembly with a one-dimensional two-fluid approximation; the element consists of Channel control volumes (or simply volumes) and junctions |

Singular channel |
SCH | Simulation of critical flow from broken pipes or flow channels assuming equal velocities of liquid and vapor phases |

Collector |
COL | Simulation of a point (with non-zero volume) of connection to any number of or channels singular channels |

Boundary volume |
BVOL_T | Setting of boundary conditions for scalar fluid parameters at inlet/outlet of or channels singular channels |

Accumulator |
ACCUM | Simplified simulation of a closed gas tank partially filled with water of temperature below saturation |

Steam-liquid pressure vessel |
SLVES | Simplified simulation of pressurizer, steam generator internal space (the secondary circuit), reactor upper plenum |

Free surface tank |
TANK | Simulation of a water tank open to atmosphere |

Blocked junction |
BLJUN | Simulation of dead-end volume condition for inlet or outletchannel |

Branch junction |
JUNB | Simulation of critical flow at junction between two neighboring channelvolumes assuming equal velocities of liquid and vapor phases |

Local loss |
LR | Assigning values to coefficients of drag for channeljunctions, inlet and outlet or channelsingular channel |

Valve |
VAL | Calculation of drag (local loss) coefficients for valves depending on the stem position; the element connects to Valve channeljunction, inlet/outlet or channelsingular channel |

Controller |
CONT | Calculation of controlled element (valve stem, control rod, etc) position, accounting for the lost motion and inertia of the controller’s actuator and in accordance with the defined control law |

Mass source |
SMASS_T | Setting of the enthalpies and flow rates of phases flowing to or out of channelvolumes |

Heat conduction structure |
HCS | Simulation of the walls of coolant flow pipes, vessels and tanks, fuel rods with specified wall heat transfer boundary conditions assuming constant geometry; a heat conduction structure is transversely divided into layers that have different thermal properties; each layer contains a number of computational nodes in the transverse direction that are lengthwise divided into volumes |

Heat conduction structure power |
QHCS_T | Assignment of power levels to heat conduction structures |

Moderator |
MOD | Calculation of coolant density, void fraction, temperature, and boron reactivity feedbacks for point neutron kinetics model and calculation of core coolant power |

Fuel |
FUEL | Calculation of temperature reactivity feedback from weighted average temperature of fuel rods and calculation of power in fuel rods for point neutron kinetics model |

Heat transfer boundary condition |
BHEAT | Assigning heat transfer surfaces the first-type, second-type, or third-type heat transfer conditions |

Coolant power |
QFL | Assignment of heat input to or removal from the coolant phases in ,andchannel, steam-liquid pressure vesselelements free surface tank |

Centrifugal pump |
CPUMP | Calculation of pump head accounting for impeller inertia, cavitation, and two-phase effects. Flow parameters and head are calculated in the volume to which the pump is connectedChannel |

Shim rods |
SR | Calculation of the amount of reactivity inserted by a group of shim rods depending on the position of rods that is assumed the same for all rods in the group. The reactivity worth of shim rods is calculated in the elementcore |

Core |
CORE | Calculation of the reactor core neutron and thermal power. Neutron power is calculated with a point approximation considering delayed neutrons. Reactivity calculation includes reactivity margin, coolant density and temperature reactivity feedbacks, and worth of control rods. Xenon and samarium poison is ignored. |

Radiation heat transfer |
RAD | Calculation of radiation heat transfer between the surfaces of ; orientation of the surfaces relative to each other is considered in terms of angular coefficientsheat conduction structures |

Crossflow junction |
JN | Making a hydraulic connection of a volume of a to a channel volume of another , or to channel, collector, boundary volume. The hydraulic connection is calculated with a homogeneous approach assuming equal velocities of the liquid and gas phases. The element can be used to model critical break flow.accumulator, steam-liquid pressure vessel, and free surface tank |

Turbulent mixing |
TM | Simulation of turbulent transfer of energy, momentum, and dissolved boron and noncondensables in a single-phase flow between channelvolumes |

**Reference**:

- KORSARV/3 – User Manual. – NITI, 2019.