Gee-Yong Park

AN ANALYSIS OF TECHNICAL SECURITY CONTROL REQUIREMENTS FOR DIGITAL I&C SYSTEMS IN NUCLEAR POWER PLANTS

Instrumentation and control systems in nuclear power plants have been digitalized for the purpose of maintenance and precise operation. This digitalization, however, brings out issues related to cyber security. In the most recent past, international standard organizations, regulatory institutes, and research institutes have performed a number of studies addressing these systems cyber security.. In order to provide information helpful to the system designers in their application of cyber security for the systems, this paper presents methods and considerations to define attack vectors in a target system, to review and select the requirements in the Regulatory Guide 5.71, and to integrate the results to identify applicable technical security control requirements. In this study, attack vectors are analyzed through the vulnerability analyses and penetration tests with a simplified safety system, and the elements of critical digital assets acting as attack vectors are identified. Among the security control requirements listed in Appendices B and C to Regulatory Guide 5.71, those that should be implemented into the systems are selected and classified in groups of technical security control requirements using the results of the attack vector analysis. For the attack vector elements of critical digital assets, all the technical security control requirements are evaluated to determine whether they are applicable and effective, and considerations in this evaluation are also discussed. The technical security control requirements in three important categories of access control, monitoring and logging, and encryption are derived and grouped according to the elements of attack vectors as results for the sample safety system.

Safety analysis of safety-critical software for nuclear digital protection system

A strategy and relating activities of a software safety analysis (SSA) are presented for the software of a digital reactor protection system where software modules in the design description are represented by function blocks (FBs). The SSA, as a part of the verification and validation activities, was activated at each phase of the software lifecycle. For the SSA of the FB modules, the software HAZOP was performed and then the SFTA (Software Fault Tree Analysis) was applied. Both methods are redundant and complementary because the software HAZOP is a forward broad-thinking analysis method and the SFTA is a backward step-by-step local analysis method. The software HAZOP with qualitative properties for a deviation evaluated all the software modules and identified various hazards. The SFTA with well-defined FB fault tree templates was applied to some critical modules selected from the software HAZOP analysis and it identified some hazards that had not been identified in the prior processes of the document evaluation and the formal verification.

Testing of Timer Function Blocks in FBD

Testing for time-related behaviors of PLC software is important and should be performed carefully. We propose a structural testing technique on function block diagram (FBD) networks including timer function blocks. In order to test FBD networks including timer function blocks, we generate templates for timer function blocks and transform a unit FBD into a flow-graph using the proposed templates. We apply existing testing techniques to the generated flowgraph and describe how the characteristics of timer function blocks are reflected in the testing process. By the proposed method, FBD networks including timer function blocks can be tested thoroughly without the intermediate code which was essential in the previous FBD testing. To demonstrate the effectiveness of the proposed method, we use a trip logic of bistable processor of digital plant protection systems which is being developed in Korea.

Defense-in-depth and diverse qualification of safety-critical software

In the Korea Nuclear instrumentation and control (I&C) System (KNICS) project, a digital safety system including Reactor Protection System (RPS) and Engineered Safety Features-Component Control System (ESF-CCS) is developed. It is based on a safety grade Programmable Logic Controller (PLC) as a platform for the safety critical I&C systems. The software used in the digital safety system is classified as safety-critical, and it is qualified according to an appropriate lifecycle. This lifecycle includes design and qualification activities for the components and the system. In the KNICS project a defense-in-depth organizational structure for qualification is implemented based on different teams including 3rd party review teams. Each team uses diverse techniques, methods, and tools for their qualification tasks. Using the Korean KNICS project as an example, the results of Defence-in-Depth and Diversity (D3) qualification of safety-critical software are presented.

Application of On-Line Signal Recovery to Feedwater Control System

An online signal recovery from a high noise environment is applied to a feedwater control system in nuclear steam generators, which is based on the wavelet denoising method. The wavelet denosing method proposed by Mallat, et al., has shown a superior performance for a signal extraction under a severe noise effect, but this method requires massive computations and a manual inspection of the trajectory of the modulus maxima. This paper proposes a systematic approach for finding the modulus maxima of a signal component under a dyadic decomposition, which is much cheaper for the computation than the other existing methods. From this method, the signal is extracted from a highly fluctuating fluid-flowing signal induced by a measurement uncertainty in a steam generator. The feedwater controller based on this method can control the water level safely through a water level control simulation

Pipe Corrosion Analysis by Time-Frequency Distribution and Ridge Pattern

A time-frequency analysis (TFA) is applied to the analysis of the vibration signals from a pipe where some chemical corrosion is likely to occur by an acidic material being mixed in the coolant of nuclear power plants. The spalling out of internal material pieces by the so-called flow-accelerated corrosion (FAC) is expected to change the structural vibration of a local point in the pipe, but this effect is too tiny to be recognized in the result of the Fourier transform. From the analysis by TFA, it is identified that the TFA can provide important information such as the amplitude fluctuations in the instantaneous frequency of each characteristic frequency. The analysis results show that the peak or ridge pattern of the TFA varied according to the status of the chemical corrosion within the pipe