Police said bomb sniffer dogs detected the explosives during a routine check Wednesday afternoon by security staff at an industrial area within the power plant's enclosure. Police declined to describe the amount or type of explosive material.
Four reactors are at Ringhals, 70 kilometers (45 miles) south of Sweden's second-largest city, Goteborg. The plant is controlled by energy companies Vattenfall and E.ON.
Bomb technicians said the material lacked an ignition device, meaning there was no danger of an explosion.
Police spokesman Tommy Nyman said officers were investigating possible sabotage but had no suspects. He said the driver of the truck had been unaware of the explosives and was not suspected of being involved.
"An outsider has obviously placed them on the truck," Nyman said. "We're talking to the truck driver and are trying to map out her movements within the premises throughout the day."
The area surrounding the truck has been evacuated and cordoned off.
Ringhals officials said that an explosion on the truck would not have caused "any serious damage" to the site.
Sweden has 10 nuclear reactors providing about half of the country's electricity.
