Steven A. Arndt

A Benchmark System for Comparing Reliability Modeling Approaches for Digital Instrumentation and Control Systems

Abstract There is an accelerating trend to upgrade and replace nuclear power plant analog instrumentation and control systems with digital systems. While various methodologies are available for the reliability modeling of these systems for plant probabilistic risk assessments, there is no benchmark system that can be used as the basis for methodology comparison. A system representative of the steam generator feedwater control systems in pressurized water reactors is proposed for such a comparison. Dynamic reliability modeling of the benchmark system for an example initiating event is illustrated using the Markov/cell-to-cell mapping technique and dynamic flowgraph methodologies.

Methodologies for the probabilistic risk assessment of digital reactor protection and control systems

Nuclear power plants are in the process of replacing the existing analog instrumentation and control (I&C) systems with digital technology. Digital systems distinguish themselves from other control and instrumentation systems mainly due to the presence of active software/firmware as well as hardware. The U.S. Nuclear Regulatory Commission policy statement on the use of probabilistic risk assessment (PRA) methods in nuclear regulatory activities encourages licensees to use PRA and associated analyses to support the licensing applications to the extent supported by the state-of-the-art and data. Before digital system reviews can be performed in a risk-informed manner, PRAs will need the capability to model digital I&C systems. The available methodologies for the reliability and risk modeling of digital I&C systems are reviewed with respect to their capability to account for the features of the digital I&C systems relevant to digital reactor protection and control systems, as well as the integrability of the resulting model into an existing PRA. It is concluded that the methodologies that rank as the top two with most positive features and least negative or uncertain features (using subjective criteria based on reported experience) are the dynamic flowgraph methodology and the Markov methodology combined with the cell-to-cell mapping technique, each with different advantages and limitations.

Analysis of Random Neutron Sensor Fluctuations for Surveillance of Nuclear Instrumentation Channels in Nuclear Power Plant Protection Systems

The analysis of the random fluctuations from nuetron sensors operating in the current mode has been shown to be a feasible method for monitoring the dynamic state of neutron measurement channels. A laboratory evaluation of both boron-lined and uranium-lined neutron sensors with simulated sensor and signal cable degradation was completed. The experimental evaluation, supported by a theoretical analysis, has demonstrated the potential for using the power spectral density of the random fluctuations on the detection process as an indicator of change in transient response of a neutron measurement channel.

The application of radiation detection noise as an in situ method of surveillance and performance verification of nuclear instrumentation channels for reactor protection systems

Abstract The monitoring of “detection noise” as a method of surveillance of power plant nuclear instrumentation has been theoretically and experimentally evaluated. The evaluation employed simulated degradations and malfunctions of the neutron sensor and the signal cable to assess the change in the high frequency power spectral density of the random fluctuations in the current signal from the neutron sensor. The experimental measurements were completed with plant instrumentation channels installed in The Ohio State University Research Reactor and in an operating Pressurized Water Reactor. The results of this research demonstrate the feasibility of using detection noise “signatures” to detect degradation and malfunctions in power plant nuclear instrumentation.

The High Voltage Perturbation Technique for Testing the Response of Neutron Sensors of the Type Used in Nuclear Reactors

Perturbation of the high voltage bias supply of neutron sensors of the type used in nuclear reactor protection systems has been demonstrated in a laboratory environment to be a potential method for response time measurement. The laboratory evaluation used simulated sensor and signal cable degradation with boron lined and uranium lined chambers operating in current mode. The investigations have demonstrated the sensitivity of the technique for detecting simulated degradations of the time response of the simulated detector-cable system.

Lessons Learned from the U.S. Nuclear Regulatory Commission's Digital System Risk Research

Abstract For the past several years, the U.S. Nuclear Regulatory Commission and its contractors have been actively engaged in research to determine the capabilities and limitations of the state of the art of digital systems risk and reliability modeling. This program was developed to assess the capabilities of various modeling methods and to develop regulatory acceptance criteria for the use of digital system risk and reliability modeling in risk-informing digital system reviews. The program investigated both traditional and advanced modeling methods for the evaluation of digital system risk and reliability in the context of including these methods in current generation probabilistic risk assessments (PRAs). The methods investigated included traditional event tree/fault tree analysis, Markov modeling, and dynamic flow graph methodology. As part of the investigation into the capabilities of these methods, we have also reviewed the availability, capability, and practicality of the needed supporting data and analysis methods, including failure mode identification, data generation methods, and uncertainty analysis. The review indicated that for some digital systems traditional PRA modeling methods may be appropriate but that a number of potential issues exist that must be carefully evaluated in modeling these systems. Both the traditional and advanced modeling methods review found that the order of component failures can be important and that simulation either as part of the reliability model or as part of the supporting analysis is needed to determine the effects of combinations of component failures and the timing of digital system failures. Finally, the research showed that better data and models of fault-tolerant features of digital systems and software are needed to support more complete and accurate modeling of digital instrumentation and control for use in nuclear power plant PRAs.