For the second time, officials found small cracks are in the control rods used to control the fission process at two nuclear reactors in Sweden.
Officials detected cracks last year at one reactor at Forsmark, north of Stockholm, and one reactor at the Oskarshamn plant, in south eastern Sweden.
Several control rods ended up replaced at the end of the year, but some of the new rods now appear to have faults.
Chief executive Lars Turing of the Oskarshamn plant said the new rods may have suffered damage at production, but a probe was underway.
Reactor 3, one of three at Oskarshamn, will remain offline due to maintenance work and the new finds were likely to delay the scheduled start-up in June.
The Swedish Radiation Safety Authority said it would review the report from the operators, but did not rule out allowing the reactors to restart for a limited time.
Sweden operated 12 nuclear reactors at the peak of its nuclear activity. Two at the Barseback plant in southern Sweden are now decommissioned, the most recent in May 2005.