Derating of Instrument and Control System Cables for Small Modular Reactors

Small modular reactors (SMRs) are factory-built transportable nuclear power plants (NPPs) that can generate up to 300 MWe and support new applications for nuclear energy such as hydrogen production, industrial heat generation, and water desalination. SMRs are poised for near-term deployment in the United States and offer improvements over existing NPPs such as passive safety features, greater plant site flexibility, reduced construction costs, load-following operations, and extended refueling cycles. However, in order to realize the benefits of SMRs, there are several challenges related to instrumentation and control (I&C) system components that must be addressed. The I&C systems of an SMR ensure its safe and efficient operation. I&C sensors measure process parameters such as temperature, pressure, level, flow, and neutron flux and provide input to the Reactor Protection System (RPS) to initiate a rapid shutdown if necessary. The performance of these safety-related I&C sensors and systems must be periodically verified to ensure they meet the plant technical specifications for accuracy and response time. In addition, the associated I&C cabling (which includes low-voltage cable assemblies for sensors, Control Rod Drive Mechanisms (CRDMs), and Rod Position Indication (RPI) Systems) is tested as part of normal plant maintenance activities or in support of aging management programs. Over time, exposure to harsh environmental conditions in the plant can result in degradation and failure of I&C sensors and cables. This is especially true for the I&C sensors and cables in SMRs which will be subjected to elevated temperatures, high radiation, and a vacuum atmosphere within the containment vessel during normal plant operation. These conditions lead to excessive Ohmic heating within the cables which further accelerates damage to the cable insulation material and results in premature failure. Frequent cable replacement due to premature degradation is not practical or economical for SMR plant owners. To combat these environmental stressors, the ampacity of the cable must be derated. However, there is limited experience to date with cable derating in vacuum, and no experience with cable derating in vacuum at high temperature and radiation. As a result, Analysis and Measurement Services Corporation (AMS) is conducting a study on several common NPP I&C cables to determine how various insulation materials perform at conditions that emulate those anticipated in the containment of an SMR. The on-going effort is funded under a research and development (R&D) grant awarded to AMS by the U.S. Department of Energy (DOE). In addition, AMS is partnering with the Oak Ridge National Laboratory (ORNL) to conduct specialized testing in support of this study under a separate initiative called the Gateway for Accelerated Innovation in Nuclear (GAIN). The goal of this R&D is to develop technical guidance and technologies to support initial startup and subsequent operation of SMRs through specialized testing services and condition monitoring technologies. Preliminary results of this R&D are provided in this paper. AMS will continue to investigate challenges associated with I&C systems for SMR applications in support of the timely deployment of the first U.S.-based SMR by the mid-2020s.