A Review of Sensor Calibration Monitoring for Calibration Interval Extension in Nuclear Power Plants

Abstract

Currently in the United States, periodic sensor recalibration is required for all safety-related sensors, typically occurring at every refueling outage, and it has emerged as a critical path item for shortening outage duration in some plants. Online monitoring can be employed to identify those sensors that require calibration, allowing for calibration of only those sensors that need it. International application of calibration monitoring has shown that sensors may operate for longer periods within calibration tolerances. This issue is expected to also be important as the United States looks to the next generation of reactor designs (such as small modular reactors and advanced concepts), given the anticipated longer refueling cycles, proposed advanced sensors, and digital instrumentation and control systems. The U.S. Nuclear Regulatory Commission (NRC) accepted the general concept of online monitoring for sensor calibration monitoring in 2000, but no U.S. plants have been granted the necessary license amendment to apply it. This report presents a state-of-the-art assessment of online calibration monitoring in the nuclear power industry, including sensors, calibration practice, and online monitoring algorithms. This assessment identifies key research needs and gaps that prohibit integration of the NRC-approved online calibration monitoring system in the U.S. nuclear industry. Several needs are identified, including an understanding of the impacts of sensor degradation on measurements for both conventional and emerging sensors; the quantification of uncertainty in online calibration assessment; determination of calibration acceptance criteria and quantification of the effect of acceptance criteria variability on system performance; and assessment of the feasibility of using virtual sensor estimates to replace identified faulty sensors in order to extend operation to the next convenient maintenance opportunity. v Summary Transmission of accurate and reliable measurements is central to safe, efficient, and economic operation of nuclear power plants (NPPs). Current instrument channel calibration practice in the United States utilizes periodic assessment and adjustment, if necessary, of sensors to maintain sensor calibration within some prescribed tolerance. In performing calibration, intrusive techniques are used to determine the calibration condition-instruments are isolated from the system, sometimes through physical removal, and exercised through a series of known inputs. This sensor performance assessment is performed periodically, as required by the plant technical specifications (TS). Non-safety-related sensors also undergo recalibration, although not as frequently. Typically, calibration occurs during refueling outages (about every two years). The current approach to sensor calibration in operating light water reactors is expensive and time consuming, resulting in longer outages, increased maintenance cost, and additional radiation exposure to maintenance personnel, and it can be counterproductive, introducing e...

Uncertainty Quantification Techniques for Sensor Calibration Monitoring in Nuclear Power Plants

“…A previously published assessment (Coble et al 2012) identified several technical gaps that preclude the widespread adoption of OLM for calibration monitoring. Of the gaps identified, better approaches to UQ were identified as important to both current approaches to OLM as well as in any future advanced methods for OLM.…”

“…Widespread utilization of traditional OLM approaches is lacking, and application of signal validation and virtual sensors has not yet entered the commercial space for nuclear systems. Previously, several technical gaps were identified relative to the deployment of OLM in nuclear power systems (Coble et al 2012). These technical gaps will need to be addressed to provide the technical and regulatory bases for wider adoption of these technologies.…”

Materials Degradation and Detection (MD2): Deep Dive Final Report

“…Similar studies have been conducted, though on a limited scale, for piezoelectric materials commonly used for ultrasonic measurements. While recent advances (Coble et al 2012) may be used to monitor sensor drift, techniques to compensate for decreasing sensitivity may be needed to maintain the ability to monitor the materials/components over the long term. An assessment of the current state-of-the-art (SOTA) for detection of degradation and degradation precursors in various materials of interest to nuclear plants is shown in Table III.…”

Uncertainty Quantification Techniques for Sensor Calibration Monitoring in Nuclear Power Plants

“…Several modeling methods have been proposed for evaluating sensor performance of redundant and non-redunant sensor groups (Coble et al 2012). Modeling methods developed for non-redundant sensor groups require that the sensors in a single model contain related information (e.g., temperature and pressure of a gas), typically identified by linear correlations or a physical understanding of the measurement relationships.…”

“…Previously, several technical gaps were identified relative to the deployment of OLM in nuclear power systems (Coble et al 2012). These technical gaps will need to be addressed to provide the technical and regulatory bases for wider adoption of these technologies.…”