Recording, evaluation and interpretation of process noise in asea-atom BWRs

Abstract

This paper describes ASEA-ATOMS activities in the noise field, and presents some practical experience on how results from process noise analyses may be interpreted. Process noise has been studied in all the operating ASEA-ATOM BWRs, chiefly during reactor commissioning, but also after commencement of commercial operation. During commissioning, process noise is studied in various modes of the plant control systems, and at different power levels, normally on - or near - the flow control line. Noise is also recorded in connection with commissioning tests of the plant control systems. During commercial operation, process noise is studied in response to customer requests, and in connection with special reactor dynamic tests; these are required if the plant is uprated, or some other operating limit is extended. Such tests may also be performed, if it is descrable to investigate reactor operational characteristics outside the operating limits, for example: at natural circulation. All noise recordings since 1977 have been made utilising a computerised data collecting system, developed specially to record data in nuclear power plants, during both steady-state operation and transient situations. The process signals are obtained from the plant instrumentation. The recorded noise data can be analysed on either the data-collecting system, or on other computers. The analysis is normally performed in two stages: routine evaluations (establishing noise levels, amplitude distributions, auto correlations, etc.), and more detailed evaluations, utilising spectral analysis techniques (calculating auto spectra, cross spectra, coherences, “transfer functions”, etc.). The interpretation of the noise analysis results has been made in the light of process knowledge, gained by other means: reactor dynamic tests, model studies. With this support, it has been possible to identify BWR process noise directions, for example: that neutron flux noise is caused by noise in both coolant flow and steam flow (to the turbine). In this context, noise analysis results have been found to contain valuable information on core dynamic characteristics, for qualifying reactor dynamic models. Experience also shows that when core decay ratios rise above “quarter damping”, core stability parameters may be identified from the neutron flux noise, merely by auto correlation.