Preliminary Assessment on the Formation of Insoluble Caesium Compounds in the Simulated Primary Circuit of PHEBUS FPT1

The post-test analyses of the Phebus FPT1 experiment revealed the presence of caesium compounds whether water and dilute sodium hydroxide soluble or insoluble 7M HNO3. This insoluble part was then dissolved by a subsequent acid treatment 98%[HNO3(90%)/HCI (10%)] + 2%HF carried out at 180°C for 3 hours. The presence of caesium compounds insoluble in 7M HNO3 has been estimated to be due most probably to: - Formation of insoluble caesium silicates - Retention of caesium in re-solidified ceramic particles (U,Zr)O2 And furthermore, with a smaller chance, to: - Diffusion/retention of interstitial caesium (compounds) in the urania lattice. The evaluation of these hypotheses has indicated that the formation of caesium silicates should be occurred or, at least, can be seen as possible. Alternatively, the formation of metal oxides (of Cs, Sr, Zr, Mo) agglomerated by silica in a non- stoichiometric compound (as vitrified) conld be presumed.