The generator protection system disconnected generating unit No 1 of the Khmelnytskyy nuclear power plant (NPP) from electricity at 3.02 p.m. Kyiv time on 3 April.
Reasons are being investigated, the national atomic generating company Enerhoatom said on its website.
Generating unit No 2 is in its regular mode.
Radiation level at the industrial site and adjacent areas has not changed.
The Khmelnytskyy NPP located in Netyshyn has two generating units (VVER-1000) with the total capacity of 2,000 MW, commissioned in 1987 and 2004.
The plant covers the deficit of power capacity in western Ukraine.