Assessment of Error from the Use of Point Kinetics When Analyzing Transients in the Large Size Fast Reactor

^Point kinetics approximation is widely used in the stage of reactor design justification primarily for analysis of transients and emergency modes. Analytical study of error caused by the use of point kinetics approximation was performed on test model of the large size fast reactor with ^flat core with the use of coordinated space dependent and point kinetics models within the framework of UNICO multi-physical code (3D neutronics in diffusion approximation + 3D thermalhydraulics). Three test transient problems were studied: change of coolant temperature at the inlet of reactor core diagrid (in one of four primary loops), safety rods drop at nominal power (example of fast running process), and unauthorized withdrawal of one control rod. It is shown that in case of unauthorized movement of one control rod the error of point kinetics estimation of the fuel element cladding temperature as compared to that from space dependent model can be as high as 100 ºÐ¡. (author)