Understanding the Mechanism of Cs Accumulation on Stainless Steel Suspended in Nuclear High-Level Liquid Waste.

Abstract

In a nuclear facility, the surface of stainless steels (SS) was found to be contaminated during the processing of radioactive liquid waste. Their safe and secure disposal is highly challenging to the nuclear industry. If the fundamental property of steel corrosion could be evaluated, successful decontamination and effective decommissioning strategies could be planned. Although individual radionuclide contamination behavior on SS metal was studied, till date, SS contamination behavior under the exposure of high-level liquid waste (HLLW) was unexplored. In view of this, investigations were carried out to understand the nature of contamination in SS 304L alloy under the exposure of simulated HLLW (SHLLW). For understanding of radionuclide adsorption behavior on structural materials, scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy have been utilized for SS 304L. The solutions were analyzed using inductively coupled plasma optical emission spectroscopy to calculate the changes in the elemental composition of the solution and corrosion behavior of SS. The passivation of SS coupons was observed in the presence of HNO3 due to enrichment of Cr at the surface. The deposition of Cs and Mo was noticed, while SS coupons were exposed to SHLLW. At 3 M HNO3 and room temperature, the SS surface is mildly passivated by Cr enrichment by formation of a Cr oxide layer on the SS surface. However, the passive layer was not thick enough to attenuate the signal from the substrate below the passivated layer. Hence, Fe0 and Cr0 were also found along with Cr3+ and Fe3+ (in small quantity). When temperature was elevated to 70 °C, the SS surface was completely covered with the Cr oxide layer, and hence no Cr0 signal was observed. The small signal of Fe0 indicated that the signal from the substrate surface is present below the passive layer. During the passivation process, Cr diffused toward the passive layer, thereby producing a Cr-depleted layer below the passive layer (Cr0 signal was not seen). In the case of SHLLW at 70 °C, the surface was fully covered by Cr3+, Mo6+, and Cs+. Fe and Ni were not observed at all. This finding will help to design an effective corrosion inhibitor and suitable decontamination agent. In addition to that, this information was found to be useful in designing high-performance novel and modern age reactor materials with improved characteristics.