Development of Methodology for In-service Measurement of Nuclear Instrumentation System Response

Abstract

In a reactor plant, reactor power is the most important parameter for plant protection, control and monitoring. Hence, it is necessary to verify that the nuclear instrumentation system (NIS) satisfies the design criteria of response as well as accuracy. As the response depends on the condition in which the instrumentation is operated, it is desirable to develop an in-service method for the response measurement. Thus, applicability of auto-regressive (AR) model to in-service measurement of the response was examined for the NIS at MONJU, the prototype fast breeder reactor in Japan. In order for AR model to be applied to measurement of sensor response time, it is necessary that the physical quantity to be sensed has sufficiently higher frequency fluctuations than the sensor's frequency response limit.
Response of a NIS can be measured by use of AR model fitting when the noise fluctuations from the neutron detection process inside the detector have higher frequency and magnitude than those inherent in the reactor dynamics. This can be confirmed by checking that there is no coherence between outputs from any two detectors. In case of the wide range monitoring (WRM) system and the source range monitoring (SRM) system of Monju, the fluctuations caused by detection process are indeed greater (both frequency and magnitude) than those from the reactor dynamics. The WRM time constant was measured at 60 to 70ms by AR model fitting process while that of SRM could not be measured precisely because of shortage of the sampling rate.