RELAP5 Modelling of PWR Reactor RTD Bypass

Resistance Temperature Detectors (RTDs) are used for measurement of coolant temperature in primary system of PWR reactor. There are two possible configurations for realization of that measurement: first has RTD embedded in thermo well structure as part of pipe wall and second requires separate small piping to divert part of the coolant flow to manifold where RTD is located (usually known as RTD bypass). Both approaches have benefits and drawbacks. From point of modeling, using thermal-hydraulic system codes, first situation is simpler and requires simple model of thermal inertia for additional metal mass attached to RTD. In second situation, detailed model of connecting pipes is required what can cause modeling problems due to needed additional data, small pipes diameter and need for estimation of pressure drops. In this paper, the development of RTD bypass model is shown using NPP Krsko as an example. The model is added to existing NPP Krsko Relap5/mod3.3 nodalization. The transient calculations are performed to show behavior of the RTD bypass lines and corresponding temperature response. The behavior of the model is studied during and after RCP trip in more detail to show why that kind of measurement is not reliable/usable in that case.(author).