[en] VTT Energy has a comprehensive and independent calculation system for reactor physics and dynamics analyses. The system is widely utilized in contract research for the nuclear safety authorities and power companies. The four Finnish reactors, some foreign plants and potential new plant concepts have been studied. The system is being modernized all the time both by own development work and by international cooperation. The reactor physics codes are upgraded and new codes and methods are developed and acquired for calculations and safety evaluations of new, increasingly complicated fuel assembly types and fuel-loading schemes, as well as for criticality and dose rate studies. The reactor dynamics methods are developed and new sophisticated models are created for tasks related to increased safety requirements. The primary aim is to realistically simulate reactor stability and complicated reactivity accidents with three-dimensional core models. For thermal hydraulics calculations, an accurate general flow model based on a new solution method has been developed. (orig.) (30 refs., 3 figs.)