[en] In Power Reactor and Nuclear Fuel Development Corporation, the hot test of the fuel reprocessing plant was carried out, starting in September, 1977. For seven months period from November, 1977, to June, 1978, during the hot test, the dispersion of low-level liquid effluent following its release in the sea was investigated. The liquid effluent was carried to a point 1.8 km offshore by a pipeline. It was then released vertically upward from a nozzle about 16 m below the sea surface. For the objectives of confirming the dilution of waste liquid till it reached to the sea surface and the subsequent diffusion in seawater, gross beta activity, and tritium and ¹³⁷Cs concentrations were measured. It the above connection, the following matters are described: methods of measuring radioactivity concentrations, release conditions and ocean observation data, radioactivity concentration/salinity/seawater temperature data, calculation data on jet-releasing dilution, distribution of seawater sampling points, respective measured results, continuous measurement data on sea current direction/velocity, background of gross beta activity in seawater. (J.P.N.)