Analysis of the experimental neutron noise from the PHENIX reactor

Journal article, 2013

This paper deals with the interpretation of the neutron noise measurements in sodium-cooled fast neutron reactors and the problem of the core vibration monitoring. Given that the compaction of a SFR core results in a positive reactivity coefficient, monitoring any core movement is mandatory. Although several sodium-cooled reactors have been operated throughout the world, only the French PHENIX reactor matched the needs in terms of instrumentation and available data. This paper presents an analysis oriented towards the core compaction monitoring, of the measurements performed on the PHENIX reactor recorded in the SARA system. The main result is the observation of the neutron noise spectra as a function of power: as already proposed in the early years of PHENIX and SUPERPHENIX (SPX), the spectrum reflects the mechanical vibration of the fuel assembly lattice. The cross correlation with measurements such as vibration, sonar and temperature do not provide significant additional information to confirm or disprove this interpretation of the neutron noise spectrum: the temperature fluctuations lie in a distinct frequency domain, the sonar and vibration measurements on the control rods suffer from high detection noises. This paper also highlights that the interpretation of the noise measurements depends on the recording of the raw data, allowing data post analysis, development of new interpretative techniques, and feedback in terms of design of the instrumentation. As such, the SARA system is an example to reproduce, as far as the sustainability of the knowledge is concerned.