Surrogate measurement of chlorine concentration on steel surfaces by alkali element detection via laser-induced breakdown spectroscopy

Chlorine can play an important role in the process of stress corrosion cracking of dry cask storage canisters for used nuclear fuel, which are frequently located in marine environments. It is of significant interest to determine the surface concentration of chlorine on the stainless steel canister surface, but measurements are often limited by difficult access and challenging conditions, such as high temperature and high radiation fields. Laser-induced breakdown spectroscopy (LIBS) could enable chlorine concentration measurements while meeting the other constraints of this application, but suffers from high excitation energy of chlorine and the interference of the atomic emission lines of iron, thus limiting the sensitivity of detection, especially when LIBS has to be delivered over an optical fiber. We demonstrate that chlorine surface concentrations in the range of 0.5–100 mg/m² can be inferred by the detection and quantification of sodium contained in chlorine salts if the speciation and neutralization of salts are not of major concern, whereas minor components of sea salt such as magnesium and potassium are less attractive as surrogates for chlorine due to the lower sensitivity of LIBS for their detection and quantification. The limit of detection, measurement accuracy, and other features and limitations of this surrogate measurement approach are discussed.