Spectral Indices Measurements Using Miniature Fission Chambers at the MINERVE Zero-Power Reactor at CEA Using Calibration Data Obtained at the BR1 Reactor at SCK•CEN

Spectral indices measurements performed in 2004 at the CEA MINERVE facility loaded with the ${\rm R}1\hbox {-}{\rm UO}_{2}$ lattice , using calibration data acquired at the SCK•CEN BR1 facility in 2001 , resulted in ambivalent conclusions. On one hand, spectral indices involving only fissile isotopes gave consistent discrepancies between calculation and experiment. On the other hand, spectral indices involving both fissile and fertile isotopes, in particular the $^{238}{\rm U}({\rm n},{\rm f})/^{235}{\rm U}({\rm n},{\rm f})$ spectral index, showed inconsistent results depending on the type of calibration data used. For different reasons, no definitive explanation was given at that time.