[en] 1 - Description of program or function: A Computer Program for Analyzing Thermal-Hydraulics transients in Research Reactors. THACT-RR is a channel code. It analyses the transient response of a research reactor core after power excursions or coolant flow and/or coolant temperature changes. The THACT-RR code provides a homogeneous one-dimensional compressible fluid flow capability with an optional voiding model that estimates the void produced by sub-cooled boiling. It allows flow reversal and sub-cooled nucleate boiling. It also includes a selection of flow instability, departure from nucleate boiling, single and two-phase heat transfer correlations, and a physical properties library adapted to pressures, temperatures, and flow rates encountered in research reactors. 2 - Method of solution: The conservation laws are solved by the method of Characteristics coupled with an implicit finite difference technique to insure stability and convergence of the numerical scheme. The conduction equation is solved by an implicit finite difference method. 3 - Restrictions on the complexity of the problem: The code is not adapted to very fast transient problems