RTDBE – RTD Bypass Elimination

Nuclear Power Plant Krško in Slovenia is 2-loops Westinghouse design PWR (pressurized water reactor) plant. Its primary coolant system is provided with temperature measurements based on high quality resistance temperature detectors (RTDs). Temperature information feed into Westinghouse ACS 7300 racks in the control room for monitoring, reactor control and protection.

Sipro participated with Westinghouse in redesign of the original narrow range temperature measurement system for NPP Krško and thus contributing to plants continuous update and development. Short history overview shows that early PWR designs were based on assumption that the hot leg temperature is uniform across the pipe, so that temperature was measured directly in loop piping at only one location.

Temperature streaming effects were identified in late 1960s caused by incomplete mixing of the coolant leaving regions of the reactor core at different temperatures, so the location of measurement became important. The RTD bypass system was then designed as a good solution for effects from temperature streaming, but operating experience has now shown new problems.

Two of the problems should be highlighted: (1) failures of equipment causing primary leakage and are large contributor to unscheduled plant downtime costs, and (2) RTD bypass becomes location with high radiation level causing very high exposures for maintenance people.

Westinghouse has been contracted to modify the system. Their vision was to remove the entire bypass piping system and insert the temperature detectors directly into the coolant loop. Thus removing all of the operating, maintenance issues and the crud deposits associated with the piping. Also by increasing the number of RTD detectors and improving the protection and control logic, the 7300 protection and control system was to gain redundancy, operating margins and stability.

Although the 7300 is of 70's vintage it is safety certified and reliable analogue computer solution used in many NPPs and Westinghouse still provides support. The design change as provided has not been implemented by Westinghouse on any of two loop plants so far, so that Krško become top-design reference in this category of PWRs. Sipro was tasked with doing detailed site walk downs and recording of as found status of systems, structures and components (mechanical, piping, civil, structural, cabling etc.).

Our engineers inspected sites in the reactor building, auxiliary buildings and main control room, where the 7300 system is located. Major project milestones before the implementation of the change were the delivery of project documentation needed for installation, testing and licensing. Design descriptions, Analyses and calculations, Bill of material, Parts lists, Component lists, Installation and Test instructions and more than 600 detailed technical drawings have been included.

Picture: RTD-bypass elimination cabling design for NPP Krško

Sipro designed detailed cable routing from the new temperature detectors in the field to the Main Control Room. An example of this is shown on the picture of conduit layouts around the reactor primary coolant loop No. 1. The cabling solution considered all required nuclear standards and technical requirements in regards to various aspects of safety, including plant specific seismic design criteria.

During the outage and while installation works took place, our engineers were engaged during implementation of changes in the plant and supported all project partners to resolve any last minute issue, as well as final commissioning and preparation of final plant documentation and project closeout. The new measurement system with RTD-bypass eliminated is now in use and plant is at 100 % power.

Project Manager: Mladen Debeljak, B.Sc.M.E.