Bomb technicians said the material lacked a detonating device, meaning there was no danger of an imminent explosion.
"But even if it would have been equipped with a detonator, a potential blast would have had pretty limited effects - the truck would have received some damage and perhaps some passers-by would have been injured, but it wouldn't have harmed the plant in any way," police spokesman Tommy Nyman said.
Four nuclear reactors are at Ringhals, 45 miles south of Sweden's second-largest city, Goteborg, which has a population of 550,000. The plant is controlled by energy companies Vattenfall and E.ON.
Police said the driver of the forklift truck had been unaware of the explosives and was not suspected of being involved. Nyman said authorities had no suspects and were searching to see if there was more suspicious material.
The government declined to comment, saying it was a police matter.
Martina Kruger, head of the Climate and Energy Division at Greenpeace in the Nordic region, said that even though the explosive material lacked a detonator, the incident was serious, showing how vulnerable nuclear plants are to attacks.
"It doesn't matter if it was outside the protected area or not, it shouldn't have made itself within the premises at all," she said, accusing authorities and plant operators of playing down the incident.
Magnus Norell, a terrorism expert at the Swedish Defense Research Agency, said the incident unlikely was a planned attack. "They never got into the security area. . . .To me that indicates that it's not very professional." But he said that someone might have been testing the systems to "do something serious later on."