Coherent scanning interferometry for copper canister authentication and applied forensics

One of the nuclear safeguards challenges to store spent nuclear fuel in long-term geological repositories consists of the continuity of knowledge of the fuel from the encapsulation plant to the final repository, demanding for containment and surveillance measures. Following the proposal of the Swedish Nuclear Fuel and Waste Management Company (SKB), spent fuel will be encapsulated in copper canisters with an insert of cast iron in an encapsulation plant, transported via ship some 300 nautical miles before being placed in the underground bedrock. These canisters will be marked with an engraved string identifier on the lid, stating the country of origin, the serial number and a control digit. In an ideal case, this identifier should be unique and impossible to counterfeit. In this work, we propose an all optical measurement technique able to identify the natural variations in the fabrication scratches of the engraved characters. Coherent scanning interferometry (CSI) is a non-contact measurement technique, exploiting the interference of a broadband light source to achieve a three dimensional reconstruction of surface morphologies. As a complement to classical laser triangulation techniques for surface reconstruction, CSI is well suited for highly reflecting objects, inherent to unoxidised copper surfaces and has the capacity to achieve sub nm height resolution. Using the CSI technique, we investigate if the naturally occurring manufacturing artefacts of the engraving process at the bottom of the characters can serve as a fingerprint for unique identification of a copper canister, independent of its identification string. This would have the advantage that simple traditional tags could be sufficient to discriminate between different canisters and provide evidence of cask substitutions. In our work, we examine different copper samples with the same identification tag and show, that we can identify them according to their engraving traces. Furthermore, we demonstrate that the CSI technique can be a suitable detection instrument, based on a high accuracy, non-invasive measurement principle. Our work substantiates a possible route for nuclear safeguards of spent fuels and has implications for cross-disciplinary fields, such as generic surface authentication or forensics analysis.