Side groove for operation floor

A side groove main body having a substantially V-shaped downward slope disposed on the bottom of an operation floor is formed in a building of a nuclear power plant, and an insulation material is laid on the upper surface of the side groove main body. An elongate protrusion is formed along the center line in the central portion of the insulation material. A pair of electrode plates are laid at the upper surface of the insulation material at substantially the same angle of inclination as that of the floor surface while putting the protrusion between them. An insulative plate for protecting electrodes having an inflow channel for leaked water is disposed at the upper surface of the pair of electrode plates. Leaked water, as floor drain, flows down from the inflow channel disposed in the insulative plate protecting electrodes along the slope of the electrode plates and stored on the protrusion of the insulation material insulating area between the electrodes. Then, if the quantity of the leaked water is increased to reach a portion between the electrode plates, the electrode plates are conducted with each other. With such a constitution, even a trace amount of leaked water can be detected rapidly without fail. (I.N.)