“They have all raised the threat level as a precaution,” said Maria Stråhle at the Swedish Radiation Safety Authority (Strålsäkerhetsmyndigheten) to news agency TT.

The explosive device was found on a large truck that had been on the industrial estate belonging to the plant and was returning into the reactor area, which is more heavily guarded.

“Under the first step onto the truck there is a fire extinguisher and that is where the explosive had been placed,” said Gösta Larsen of the Ringhals plant to TT.

He said that police suspect that a civilian has prepared the explosive and that it was the size of a “small fist”.

The device was not primed and was not large enough to do damage to a reactor, according to Larsen. However, he confirmed that the threat levels had been raised as a result of the find and said that any discovery of explosives on the plant’s premises was “worrying”.

According to the county police, the device was in the shape of “plastic explosive” and the truck where it was discovered never leaves the plant’s immediate environs.

“The truck was entering the guarded area when the device was found by the plant’s own sniffer dogs,” said police spokesman Tommy Nyman.

The entire premises were searched with sniffer dogs over night but no other discoveries have been found and police so far have no suspects.

However, a preliminary investigation is under way and the incident has been classified as a suspected sabotage attempt.

Police will question truck drivers and officers are currently trying to find out where and when the suspected explosive could have been placed on the vehicle.

“We will speak to everyone we think could have information about the incident,” Nyman said to TT.

The device was sent off to the National Laboratory of Forensic Science (Statens kriminaltekniska laboratorium – SKL) in Linköping for testing soon after the discovery and scientists have since confirmed that it was an explosive found on the plant premises.

TT/The Local/rm