An attempt to restart an idled nuclear power reactor at Sweden's Oskarshamn plant failed this week, adding to a debate over the aging plant's future.

All three of the reactors at Sweden's oldest nuclear power plant, about 200 miles south of Stockholm on the Baltic Sea coast, remained inactive Friday.

With two reactors already shut down for routine maintenance, the 1,450-megawatt Oskarshamn No. 3 reactor was also idled Sept. 1 due to a control valve in the turbine system that had stuck open, plant owner OKG said.

The stuck control valve caused rising water levels in a tank, which was closed manually and followed by a round of "ongoing troubleshooting."

After several failed attempts, the company again tried to fire up the reactor Tuesday, but that effort was abandoned due to safety concerns, OKG Chief Executive John Robbins told Radio Sweden, without hazarding a guess as to when another attempt might be carried out.

"In the current situation, I cannot do that," he said. "When we know that, we will we tell the market when we consider that we can be back online."

Radio Sweden said it was likely the Swedish Radiation Safety Authority would decide by the end of this week when another restart attempt of Oskarshamn 3 can be done.

OKG's inability to get Oskarshamn back online brought an apology from Robbins to customers in a press release, explaining it was a difficult task to get the No. 3 reactor online when the other two are already idled.

When functional, the three Oskarshamn reactors supply about 10 percent of Sweden's electricity needs. The No. 3 unit is classified as the highest-capacity boiling water nuclear reactor in the world.

"Because of our safety precautions, we knew that we may have problems restarting the reactor while the other two are out of service," Robbins said. "We are dependent on the availability of outside power generation from the external power grid, since it requires a fairly large electrical capacity to power the pumps and other equipment that is essential for our production.

"When uncertainty about the availability of other power occurred on Tuesday night, we decided for reasons of prudence to cancel the connection to the external power grid."

Oskarshamn's 494-megawatt No. 1 reactor has been idled for maintenance since July 9 and was originally expected to be back online in August. A restart was then pushed back to Sept. 12. However, by Friday, the reactor was still shuttered.

The 664-megawatt Oskarshamn 2 is out of service for an 11-month maintenance update.

The situation has brought criticism of OKG and its majority owner, Germany's E.On.

The Swedish magazine NyTeknik this week rated Oskarshamn 1 -- commissioned in 1972 -- as among the world's worst boiling water nuclear reactors because of all the downtime.

The reactor, it noted, has suffered "one failure after another," including turbine vibration and a dislocated feedwater valve producing high water level that wrecked the nearly new high-pressure turbine.

After a new turbine was installed, there were more vibrations, followed by the discovery of cracks in the feedwater system. Since then, new problems have been discovered leading to more delays in its restart.

The magazine noted the No. 1 reactor, which has been placed under special supervision by the Swedish nuclear safety agency, is approaching the maximum number of "scrams" allowed before its owner must prove that it be allowed to continue operating.

Almut Zyweck of E.On told NyTeknik there are no plans to retire the reactor early.