United States Nuclear Regulatory Commission - Protecting People and the Environment

Development of a Fault Injection-Based Dependability Assessment Methodology for Digital I&C Systems (NUREG/CR-7151, Volumes 1–4)

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Publication Information

Manuscript Completed: November 2011
Date Published: December 2012

Prepared by:
C. R. Elks, N. J. George, M. A. Reynolds, M. Miklo,
C. Berger, S. Bingham, M. Sekhar, B. W. Johnson

The Charles L. Brown Department of Electrical and Computer Engineering
The University of Virginia
Charlottesville, Virginia

NRC Project Managers:
S. A. Arndt, J. A. Dion, R. A. Shaffer, M. E. Waterman

NRC Job Code N6214

Prepared for:
Division of Engineering
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

Today's emergent computer technology has introduced the capability of integrating information from numerous plant systems and supplying needed information to operations personnel in a timely manner that could not be envisioned when previous generation plants were designed and built. For example, Small Modular Reactor (SMR) plant designs will make extensive use of computer based I&C systems for all manner of plant functions, including safety and non-safety functions. On the other hand, digital upgrades in existing light water reactor plants are becoming necessary in order to sustain and extend plant life while improving plant performance, reducing maintenance costs of aging and obsolete equipment, and promoting prognostic system monitoring and human machine interface (HMI) decision making.

The extensive use of digital instrumentation and control systems in new and existing plants raises issues that were not relevant to the previous generation of analog and rudimentary digital I&C systems used in the 1970's style plants. These issues include the occurrence of unknown failure modes in digital I&C systems and HMI issues. Therefore, digital system reliability/safety, classification of digital I&C system failures and failure modes, and software validation remain significant issues for the Light Water Sustainability and SMR initiatives and the digital I&C system community at large.

The purpose of the research described in volume 1 thru volume 4 is to help inform the development of regulatory guidance for digital I&C systems and potential improvement of the licensing of digital I&C systems in NPP operations. The work described herein presents; (1) the effectiveness of fault injection (as applied to a digital I&C system) for providing critical safety model parameters (e.g., coverage factor) and system response information required by the PRA and reliability assessment processes, (2) the development and refinement of the methodology to improve applicability to digital I&C systems, and (3) findings for establishing a basis for using fault injection as applied to a diverse set of digital I&C platforms. Some of the specific issues addressed in Volume 1 are:

  • Fault Injection as a support activity for PRA activities.
  • Development of the UVA fault injection based methodology.
  • Fault models for contemporary and emerging IC technology in Digital I&C Systems.
  • Requirements and challenges for realizing Fault Injection in Digital I&C systems.
  • Solutions to challenges for realizing fault injection in digital I&C systems.

Volume 1 presents the findings of developing a fault injection based quantitative assessment methodology with respect to processor based digital I&C systems for the purpose of evaluating the capabilities of the method to support NRC probabilistic risk assessment (PRA) and review of digital I&C systems. Fault injection is defined as a dependability validation technique that is based on the realization of controlled validation experiments in which system behavior is observed when faults are explicitly induced by the deliberate introduction (injection) of faults into the system [Arlat 1990]. Fault injection is therefore a form of accelerated testing of fault tolerance attributes of the digital I&C system under test.

Volumes 2 and 3 of this research present the application of this methodology to two commercial-grade digital I&C system executing a reactor protection shutdown application.

In Volumes 2 and 3, the research identified significant results related to the operational behavior of the benchmark systems, and the value of the methodology with respect to providing data for the quantification of dependability attributes such as safety, reliability, and integrity. By applying a fault injection-based dependability assessment methodology to a commercial grade digital I&C, the research provided useful evidence toward the capabilities and limitations of fault injection-based dependability assessment methods with respect to modern digital I&C systems. The results of this effort are intended to assist NRC staff determine where and how fault injection-based methodologies can best fit into the overall license review process.

The cumulative findings and recommendations of both applications of the methodology and application of the generalized results to broader classes of digital I&C systems are discussed in volume 4.

The digital I&C systems under test for this effort, herein defined as Benchmark System I and Benchmark System II, are fault tolerant multi-processor safety-critical digital I&C systems typical of what would be used in a nuclear power plant 1-e systems. The benchmark systems contain multiple processing modules to accurately represent 4 channel or division 2 out of 4 reactor protection systems. In addition, the systems contain a redundant discrete digital input and output modules, analog input and output modules, inter-channel communication network modules, other interface modules to fully represent and implement a Reactor Protection System. The application Reactor Protection System software was developed using the benchmark systems software development and programming environments.

To establish a proper operational context for the fault injection environment a prototype operational profile generator tool based on the US NRC systems analysis code TRACE [NRC 2011] was developed. This tool allowed generation of realistic system sensor inputs to the Reactor Protection System (RPS) application based on reactor and plant dynamics of the simulated model. In addition, the tool allowed creation of accident events such as large break LOCAs, turbine trips, etc., to stress the RPS application under the various design basis events.

Bibliography

[NRC 2001] Commission, U.S. Nuclear Regulatory. Computer Codes. April 2011.
http://www.nrc.gov/about-nrc/regulatory/research/comp-codes.html
(accessed 2011).
[Arlat 1990] J. Arlat, M. Aguera, et. al. "Fault Injection for Dependability Evaluation: A Methodology and Some Applications." IEEE Transactions on Software Engineering, February 2, 1990.

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