Examination of the Plenum and Deposition Coupon of the Phebus FPT4 Test by Scanning Electron Microscopy and Photoemission Spectroscopy

Samples of the thoria thermal shroud from the plenum and a deposition coupon from a filter cartridge of the FPT4 test of the Phebus PF project were examined using electron-optical analysis and by photoemission spectroscopy (XPS). The aim was to determine the nature of the fission product deposits formed under conditions of severe nuclear accidents. The plenum had considerable deposits containing Zr, Mo, Ba, Cs, and locally U was found by energy dispersive analysis. The XPS analysis showed that the deposits are mostly in oxidic form, but no U was found in the deposit, although chemical analysis showed considerable amounts present. A possible explanation is that there is layering of the deposit and that U is not released in the early stages. The coupon had very sparse deposits with only occasional U and Re particles; however large, pure Cs particles were observed on the surface that may have come from the filter deposits upstream. Thus an estimate of the deposition behaviour of fission product and fuel dispersed under extreme nuclear accident conditions is built up.