When a fire broke out at the reactor in May, remnants of an old welding job were discovered in the system. The Swedish Radiation Safety Authority wants to make sure this is fixed.
"We see it as a serious problem that earlier tests of the security system didn't work," says Leif Karlsson, a division head with the Authority, to news agency TT.
Sweden's other two nuclear plants, Forsmark and Oskarshmamn, will also be required to test their sprinkler systems.
Just three of the country's 10 reactors are currently operating at full capacity; a fact some blame for high electricity bills in winter.