Dependability assessment of nuclear digital systems using function theory and fault injection experiment

Abstract

The use of digital systems in nuclear instrument and control system (I&C) prevails because of their increased capability and superior performance compared with the analog systems. However, it is very difficult to evaluate the reliability of digital systems because they include the complex fault processing mechanisms at various levels of the systems. Software is another obstacle in reliability assessment of the systems that requires ultra-high reliability. There are ongoing debates in industry, academia, and the international standards community on the problem whether software reliability can be quantified or not. In addition, the reliability of digital systems has to be assessed considering software, hardware and SW/HW interactions because the software consideration cannot be fully understood apart from hardware considerations and vice versa. This work describes a combinatorial model for estimating the reliability of the embedded digital system by means of discrete function theory and software control flow. This model includes a coverage model for fault processing mechanisms implemented in digital systems. Furthermore, the model considers the interaction between hardware (H/W) and software (S/W). The fault processing mechanisms make it difficult for many types of components in digital system to be treated as binary state, good or bad. The discrete function theory provides a complete analysis of multi-state systems as which the digital systems can be regarded. Through adaptation of software control flow to discrete function theory, the HW/SW interaction is also considered for estimation of the reliability of digital system. In this work, the information on the coverage factor of fault tolerance mechanisms and software masking effects on hardware faults is obtained through fault injection experiments. Based on the Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL), a simulation model for fault injection was developed. The informati...