Two security guards who patrol the plant after 6pm often leave their post unmanned, local newspaper Oskarshamn-Tidningen reports.
The company that runs the plant, OKG, has said it plans to remedy the situation in October.
Despite the fact that other plants such as Ringhals and Forsmark carry out checks on everybody passing through the gates, OKG has secured a dispensation allowing it to delay security improvement by a few more months.
“But from October 1st everyone who enters OKG will be checked,” said Anders Österberg, a spokesman for the company.
Two welders were arrested last Wednesday after a routine security control at the entrance to the plant detected traces of highly explosive material on the handle of a plastic bag one of the men was carrying.
Both men were later released but the suspicions against them remain.