EXCURS-3-RR, Kinetics of Research Reactor Reactivity Transient Analysis

1 - Description of program or function: EXCURS-3-RR is a kinetic computer code for simulating reactivity and loss of flow accidents in MTR research reactors and in sodium cooled fast reactors. EXCURS-3-RR is a new version of the EXCURS-3 code. It is based on a coupling of neutron point kinetics and thermal hydraulics equations, with adjusted reactivity feedbacks. The thermohydraulics reactor behavior is determined by solving the one-dimensional heat conduction equation, with forced heat convection boundary condition. The calculations are restricted to a single equivalent unit cell which consists of fuel, clad and coolant. 2 - Method of solution: The transient power is calculated by solving a set of neutron kinetics equations, with known reactivity. The numerical is performed using the modified Runge-Kutta-Gill method. The external reactivity insertion specified as a function of times, is given in the input data file, and the amount of feedback and control reactivity contributions calculated by the code are then added to the total system reactivity. Following this, the neutron density (or power), the energy release and delayed neutron precursor densities (of a maximum of six groups) are obtained at each time step. The reactivity Feedbacks taken into account are Doppler, fuel, clad and moderator expansion effects. To EXCURS-3-RR, we have introduced new models to determine the transient TRIP time and the control rod reactivity insertion as function of time. A new thermal-hydraulics model specially tailored to fuel plate elements was also added, including temperature dependent cell material physical properties. The resolution of the corresponding set of conduction equations is performed using the Gauss elimination method. Assessment studies of the code have been made using the International Atomic Energy Agency 10 MW Benchmark cores for protected transients. These studies have shown a quite good agreement between the results predicted by the EXCURS-3-RR code and more sophisticated codes, such as PARET. 3 - Restrictions on the complexity of the problem: The coolant is considered in liquid phase during all the transient. Cell axial subdivision number can not exceed 20 nodes.