Lessons learned while updating NUREG-1537

Non-power reactor (NPR) licensees have expressed interest in upgrading their existing analog instrumentation and control (I&C) systems with digital systems; therefore, the U.S. Nuclear Regulatory Commission (NRC) is updating NUREG-1537, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, to reflect applicable current positions and increase clarity of requirements for digital systems. The update to NUREG-1537 incorporates lessons learned from new digital I&C designs and upgrades. The Atomic Energy Act of 1954, as amended, states that "utilization facilities⋯ should be regulated to the minimum extent consistent with protecting the health and safety of the public." Historically, the much lower public risk associated with NPRs has been one of the bases upon which the NRC has accepted that less stringent and less prescriptive measures provide reasonable assurance for the protection of the public, workers, and the environment. For example, because of their lower thermal power levels, the decay heat in many NPRs is insufficient to cause cladding damage under any cooling condition. Furthermore, the generally intermittent operation and the smaller cores result in significantly smaller inventories of fission products in the fuel. Thus, an understanding of the design, risk, and regulatory differences between NPRs and nuclear power plants (NPPs) is necessary to assess conformance with the regulations and to apply to NPRs the experience in updating the guidance for digital systems at NPPs. In addition, there are issues unique to NPRs such as varied thermal power level, no or minimal cooling required after shutdown, and other diverse design features that may allow a graded approach within the fleet of NPRs. Insights while updating NUREG-1537 were gained from comparing the regulations and guidance for NPPs and NPRs on a system-by-system basis. Lessons learned include, but are not limited to, evaluating and assessing: • similarities and differences of the applicable regulations (including terminology); • unique characteristics and advantages of digital technology; • requirements for software development; • commercial-grade dedication process; • independence, redundancy, and the single-failure criterion, including an evaluation of software commoncause failures; and • guidance on human factors engineering. Each of these is explored in this paper.