Safety issues for digital upgrades in operating NPPs

The objective of this paper is to address safety and licensing issues connected with the replacement of the existing analog protection and control systems in nuclear power plants with the new, digital equipment. While digital systems have characteristics that are potentially beneficial to overall plant operation and maintenance, they also have the potential to jeopardize nuclear safety, if the upgrade is not performed correctly through a well planned and carefully implemented process. The paper first discusses the reasons for moving from analog to digital safety applications, the benefits and potential pitfalls. Before discussing safety assessment principles, a four level safety categorization as generally accepted throughout the nuclear community is briefly addressed. Special emphasis is put on licensing considerations and regulatory involvement in general. At the end the importance of learning from experience is briefly discussed