Recent activities within Nuclear Electric related to the qualification of digital systems and the extension of maintenance intervals

Nuclear Electric is pursuing a program of modernization and enhancement of existing control and safety (C ampersand I) systems to improve power plant availability and increase power output. This paper describes recent experience in qualifying digital systems for safety applications. Also, described is the work currently in hand to reduce the frequency of maintenance periods for safety C ampersand I equipment. The qualification of digital systems is shown to be a more rigorous and much more extensive (and thus costly) exercise than the qualification of conventional equipment. This is illustrated by brief references to two software-based safety systems, the Single Channel Trip System at the Dungeness open-quotes Bclose quotesAGR and the Primary Protection System for the new PWR at Sizewell open-quotes Bclose quotes. The emphasis of the paper is on the use of Application Specific Integrated Circuits (ASICs) in safety systems, to reduce, or eliminate dependence on software for the fulfillment of requirements which need implementations that are complex or of a high functional density. Recent experience in the development of a prototype safety system based on ASICs, The Gaseous Activity Monitor and Trip Equipment (GAM), is used to illustrate their possible benefits and potential problems. When using complex integrated circuits like microprocessors or ASICs in digital systems a difficulty arises in analyzing the failure modes of the system. Efforts to overcome this difficulty led to the development of a method for performing failure analyses based on functional blocks. The techniques and benefits of such an approach are outlined. Finally, efforts to reduce maintenance costs, yet at the same time enhance safety, are leading towards an extension of maintenance intervals, for some safety equipment, from 3 months to 4 months and also towards lengthening the period between AGR outages from 2 years to 3 years