(Updates with details, restart plans)
LJUBLJANA, Feb 16 (Reuters) - Slovenia's sole nuclear power plant Krsko (NEK) is expected to restart on Friday after shutting down on Thursday due to a problem with a regulatory mainstream water valve, NEK said in a statement.
"Diagnostics on the impairment of the valve is under way, the impairment is expected to be corrected today. Based on the information we have at present we expect the plant will be restarted tomorrow," it said, adding there had been no impact on the environment.
The plant is stable and all security systems operated well, spokeswoman Ida Novak Jerele told Reuters.
The plant, which was built in cooperation with U.S. firm Westinghouse, now part of Japan's Toshiba Corp, started operating in 1982. It is jointly owned by Slovenia and its neighbour Croatia.
It provides about 25 percent of Slovenia's power and about 20 percent of Croatia's.
Last year the two countries prolonged the plant's lifespan by 20 years to 2043. Regular maintenance of the plant, which takes place every 18 months, was completed in November.
Reporting by Marja Novak; editing by David Goodman and Jason Neely