The plant is the largest in the country, with four reactors supplying one-fifth of Sweden's electricity, according to the Wall Street Journal.
Vattenfall, the power company that owns the plant, said in a Thursday statement that the substance was in fact an explosive paste but was not attached to a detonation device.
The discovered material could not have induced serious damage at Ringhals. Ringhals nuclear power plant is still in operation.
The security level at Ringhals and two other Swedish plants has been raised to the second-to-lowest level.
During the night, the police have searched the Ringhals premises with bomb searching dogs and not found anything else, said the Vattenfall statement. Investigations into the suspected sabotage are ongoing, and the Swedish Radiation Safety Authority has been alerted.