Analysis of signal drifts at Co Self-powered neutron detectors

Self-powered neutron detectors (SPND) constitute an important part of in-core measurements in the majority of power reactors, where radial neutron flux fluctuations are normally encountered. The measurement chains with SPND provide usually input for NPP safety related systems such as the Protection System or Reactor Control Surveillance and Limitation System. For this, the output from the chains must be as accurate as possible. Operational data indicated unexpected behaviour of some of the Co SPNDs at the beginning of their life cycle in the reactor. Operational data analysis and Co SPND laboratory tests indicated that water was present in the detector insulation. The effect of this water on the signal drift was investigated by using the Geant4 simulation tool. Although the simulation revealed some interrelation between the Co SPND signal drift and the presence of water, this effect contributes but little to the drift. (orig.)