Earlier this month, routine inspections revealed that at least five out of 90 control rods examined revealed cracks at the Oskarshamn facility, which is situated in southeastern Sweden on the Baltic Sea, the SMM said. Control rods control the rate of fission of uranium and plutonium.

On Tuesday, officials at the Forsmark plant north of Stockholm, also on Sweden’s east coast, said they had shut down one of its three reactors, because it was of the same type as the Oskarshamn reactor with the control rod problem. The Forsmark reactor would be closed for 10 days for inspections, officials said.

Oskarshamn 3 also remains closed for further inspections. About half of Sweden’s energy is accounted for by its three nuclear power plants.

SSM Spokesman characterised the incident as serious, as control rods are the central component in containing nuclear reactions, regulating how quickly atoms are split within a reactor.

"The control rod is sort of like the nuclear reactor’s gas pedal," agency spokesman Anders Bredfall told news agencies, adding " We want to know what has caused this – it definitely serves an important safety function."

The SSM felt the finding was so disturbing it ordered all boiling water reactors in Sweden, six in all, to hand in reports by last Thursday this week indicating whether they have observed similar problems. SSM officials contacted by Bellona Web on Sunday said the reports were still under review, and declined comment.

Bredfall did acknowledge, however, that the Forsmark 3 facility responded well to the discoveries at Oskershamn.

"(On Tuesday), Forsmark 3 closed down so they could see if they have the same problem, and that’s because the two reactors are in principle identical," to the Oskarshamn reactor, Bredfall said.

"Forsmark made absolutely the right decision. If there is uncertainty you definitely shut down and make sure you don’t have the problem," he said.

In September, the SSM issues a criticism of the Forsmark plant for devoting insufficient attention to safety issues. During an inspection on August 8th, it was revealed that the emergency cooling ventilation system for the plant’s No. 2 reactor has been clogged for a year – exactly a year after an inspection revealed the same problem.

The emergency cooling system is designed only to work in emergencies. If any cooling issues had arisen during the past year, while the vent system was blocked, it could have lead to a very serious accident, or even a full scale catastrophe.

“We consider it very serious that Forsmark is not following its own safety procedures,” Bredfall said. “ We want to understand how such a thing could have occurred. This is a question of safety culture.

A representative of the Forsmark plant said the vent blockage was an “isolated incident.”

Forsmark had already been under special observation for several incidents and serious events that occurred on June 25th 2006.

The plant’s No. 1 reactor was shut down after it was discovered that two of the five generators backing up its emergency cooling system were out of order. Many experts said that a catastrophic fusion in the reactors active zone was avoided only as a result of this chance discovery.

“The problem of Sweden’s nuclear power plants appears to be an endless story said Per Hegelund of the The Swedish Environmental Movement’s Nuclear Waste Secretariat. “All else aside, the Oskarshamn and Forsmark are contaminating the Baltic Sea with usual dumps and  emissions of radionuclides. Such things are a safe and clean source of energy don’t exist.”

The Oskrashamn facility made news earlier this year, in May, when two contract workers were detained when it was discovered during a random check that one of the workers’ bags contained traces of explosive material. The men were questioned by Swedish police and released.

Andrei Ozharovksy, a physicist with Ecodefence, and frequent contributor to Bellona Web, wrote from St. Petersburg and staff writer Charles Digges wrote from Oslo.