This paper discusses the replacement of the original nuclear instrumentation system at the Zorita Nuclear Power Plant. The reasons for upgrading were reliability, availability, and problems in maintaining the original instrumentation due to a lack of qualified spare parts. In replacing the system, the plant had to address the current regulatory, design, and technical specification requirements, including changes in equipment qualification, new physical separation requirements, and adding signal isolation to analog signal outputs. The number of Source Range and Intermediate Range channels increased from two to four, adding 2 out of 4 coincidence logic to the Source Range and Intermediate Range trip outputs. The testing method for the coincidence logic changed, adding test permissive logic to prevent testing from generating a trip or causing a violation of technical specification requirements, and to allow testing in bypass mode (2 out of 3 logic) or in trip mode (1 out of 3 logic). The new designs included built-in calibration standards, and the design eliminated the need to disconnect cables and to rack-out the chassis assemblies in order to perform routine equipment testing, Fitting a new system into the relatively small space available presented several challenges. This paper discusses the motivation for the system's replacement the challenges to the plant engineers and to the equipment designers, and the manner in which the new design met the challenges.
