Cesium and Strontium Contamination of Nuclear Plant Stainless Steel: Implications for Decommissioning and Waste Minimization.

Adam R Lang,Dirk L Engelberg,Alex Jenkins,Hauke Bosco,Martin Weiss,Clemens Walther,Gareth T W Law,Francis R Livens

doi:10.1021/acsomega.9b01311

Abstract

Stainless steels can become contaminated with radionuclides at nuclear sites. Their disposal as radioactive waste would be costly. If the nature of steel contamination could be understood, effective decontamination strategies could be designed and implemented during nuclear site decommissioning in an effort to release the steels from regulatory control. Here, batch uptake experiments have been used to understand Sr and Cs (fission product radionuclides) uptake onto AISI Type 304 stainless steel under conditions representative of spent nuclear fuel storage (alkaline ponds) and PUREX nuclear fuel reprocessing (HNO3). Solution (ICP-MS) and surface measurements (GD-OES depth profiling, TOF-SIMS, and XPS) and kinetic modeling of Sr and Cs removal from solution were used to characterize their uptake onto the steel and define the chemical composition and structure of the passive layer formed on the steel surfaces. Under passivating conditions (when the steel was exposed to solutions representative of alkaline ponds and 3 and 6 M HNO3), Sr and Cs were maintained at the steel surface by sorption/selective incorporation into the Cr-rich passive film. In 12 M HNO3, corrosion and severe intergranular attack led to Sr diffusion into the passive layer and steel bulk. In HNO3, Sr and Cs accumulation was also commensurate with corrosion product (Fe and Cr) readsorption, and in the 12 M HNO3 system, XPS documented the presence of Sr and Cs chromates.

Highlights

The development of a low carbon-based energy economy is a global priority, and nuclear power is an efficient, low-carbon energy source
The O signal rapidly diminished after treatment in 12 M HNO3 (Figure 1C and Figure S3), which suggests that a stable surface oxide layer cannot be maintained under such strongly oxidizing conditions
The results show that the extent and character of type 304 austenitic stainless steel contamination with the high-yield fission products Sr and Cs are intimately related to the corrosion state of the steel surface

Summary

Introduction

The development of a low carbon-based energy economy is a global priority, and nuclear power is an efficient, low-carbon energy source. Reliable current estimates of the global amounts of nuclear industry radioactive stainless steels are not available, but they are a significant waste form (millions of tonnes).[5] The selection of stainless steel in the nuclear industry is based on a combination of high radiation stability and excellent corrosion resistance, the latter afforded by the spontaneous formation of a nanometer-scale passivating Cr-oxide layer at the steel surface.[6] It has been assumed that deposited radioactivity becomes bound to/impregnated within this passive layer during the accumulation process,[7,8] these phenomena have not been fully demonstrated. To make informed decisions regarding the design and optimization of effective decontamination treatments for these materials, an understanding of the principal chemical interactions driving radionuclide uptake onto or into the steel is critical

Methods

Results

Conclusion