The water through a contacting nozzle or by guide vanes is made to strike on a series of buckets. The wheel revolves free in air and water is in contact with only a part of wheel at a time and pressure is atmospheric. A casing is provided in order to prevent splashing and to guide water discharged from buckets to tail-race.

In a reaction turbine, only part of available potential energy is converted into velocity head at entrance to runner. The balance portion remains as a pressure head. In Francis turbine, re is an inward radial flow of water and in Francis turbine, re is a mixed flow of water.

Intakes should be provided with trash racks so as to prevent entry of debris into penstocks or to prevent choking up of nozzles of impulse turbines. Trash racks are made up of thin flat steel bars placed 10 to 30 cm apart and permissible velocity of water entering se is 0.6 to 1.6 m/s.

The pondage takes care of hour to hour fluctuations over a day or occasionally day to day fluctuations over a weekly cycle. It ensures that desired instantaneous rate of flow into turbines under day to day changing conditions of power load as well as of inflow. It may also be provided for weekly demands.

In impulse turbine, all potential energy or head is converted into kinetic energy or velocity head by passing water through a contracting nozzle or by guide vanes before it strikes buckets. Some types of impulse turbine are Pelton wheel, Turgo impulse wheel, and Girard turbine.

The specific speed is given by equation: N_s = N. P_t^1/2 / H^5/4
Where, N = normal speed of turbine = 500 rpm
P_t = Turbine output in metric horsepower = 10,000 hp
H = effective head = 81 m
N_s = 500 x 100 / 81^5/4 = 205.76 rpm.
The turbine which is suitable for a head between 25 to 150 m and Ns of 60 ? 300 is Francis turbine i.e reaction type turbine.

The surge tank is mostly employed in a water power plant or in a large pumping plant to control pressure variations resulting from rapid changes in flow. It is done by admitting a large mass of water in surge tank orwise it would have flown out of pipeline and returns to tank due to closure of pipe end.

The water that runs back to ocean during low tides is also utilized to generate electric power and special turbines are provided for same. Rance power station is an example of such tidal plants. The tidal range is about 11 meters and this powerhouse contains 9 units of 38,000 KW.

A dam is constructed across river and powerhouse may be located at foot of dam. The powerhouse may sometimes be located much away from dam at end of tunnels which carry water away from reservoir.

The nuclear power generation is only about 3% of total power generation including hydropower, rmal power, nuclear power, etc. The Indian Department of Atomic Energy is planning to boost its generation in next 15 years from 3% to 8% of total electricity generation.