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

Abstract

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 2 study CsI aerosols were deposited on the surface of 304L coupons, which have been previously oxidized. The deposits were afterwards reheated (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 reheating 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 Cs 2 CrO 4 and Cs 2 Cr 2 O 7. The results are then discussed and compared with literature data.

Revaporization Behavior of Cesium and Iodine Compounds from Their Deposits in the Steam–Boron Atmosphere

“…The experimental results of TeRRa-RBS-2 have signified that the cesium chromate Cs 2 CrO 4 was the condensate on the surface of stainless steel. Chemical interaction between the formed chromium oxide layer and cesium iodide deposits could not readily proceed to the formation of cesium chromate at a certain reaction temperature, but instead, it involves the carrier gas …”

Chemical stability of caesium iodide deposits in air/steam atmosphere

“…The CsI aerosols deposits obtained by impaction at room temperature have been described by Obada et al [33]. SEM analysis has revealed the presence of particles with irregular shapes and particle agglomerates with sizes from 5 to 20 µm.…”

“…As previously reported [31] [33], the carrier gas composition during the re-vaporization phase is one of the key parameters regarding the nature of re-vaporized and residual species. As such, in an argon/steam carrier gas, caesium and iodine are released integrally from the surface of stainless steel coupons (condition 5).…”

“…The CsI impacted aerosols were obtained following the procedure described by Obada et al [33] which consists in nebulizing a concentrated CsI aqueous solution. The obtained wet aerosols are then dried in a strong argon flow (10 L/min) and afterwards impacted on the pre -oxidized 304L coupons placed in an exposition chamber (8 coupons per batch).…”

“…A detailed surface characterization by XPS and ToF-SIMS of 304L samples after re-vaporization in air or argon/steam has been discussed by Obada et al [33]. The same process has been applied to the 304L samples obtained after re-vaporization in air/steam atmosphere and the results show evidence of the formation of mixed Cs-Cr oxides, specifically Cs 2 CrO 4 and Cs 2 Cr 2 O 7 .…”