A reaction turbine operates with its runner fully flooded and develops torque because as a reaction to the pressure of water against the blades of the turbine.
An impulse turbine operates with its runner in the air (not submerged in water) and converts the water’s pressure energy into kinetic energy using a jet that shoots water onto the runner buckets to develop torque.
FURTHER READING: Q&A: The Choice between Reaction and Impulse Turbines, Hydro Review, Volume 16, No. 1, February 1997, pages 89-91.
