Combined ToF-SIMS and XPS characterization of 304L surface after interaction with caesium iodide under PWR severe accident conditions

Highlights: • Multitechnique characterization of oxidized 304L SS. • Chromium and manganese oxides formed in the outer layer. • High-temperature interaction between caesium iodide and oxidized 304L SS. • Formation of mixed caesium-chromium oxides. In case of a severe accident (SA) occurring in a pressurized water reactor (PWR) fission products (Cs, I) are released from the degraded fuel and transported through the reactor coolant system (RCS). A part is deposited onto surfaces of the RCS and can be subsequently chemically re-mobilized, in case the atmosphere composition changes. In order to improve our understanding on the re-mobilization processes, it is important to describe the interactions between deposited fission products and surfaces representative of RCS in SA conditions (mainly oxidized SS 304L) and to identify the species formed after remobilization. In this study CsI aerosols were deposited on the surface of 304L coupons, which have been previously oxidized. The deposits were afterwards re-heated (up to 750 °C) in either air or steam. At every step, the 304L coupons were analysed by a combination of surface analysis techniques (XPS, ToF-SIMS), Raman spectroscopy and SEM. It has been established that the initial surface state of the 304L coupons (i.e. oxidation in air or steam) has no effect on the release of caesium or iodine. However, the composition of the carrier gas during the re-heating phase influences significantly the release of caesium. Specifically, in air a significant amount of Cs remains on the coupon and forms mixed oxides with chromium, such as Cs₂CrO₄ and Cs₂Cr₂O₇. The results are then discussed and compared with literature data.