The metal canisters that will hold the spent fuel or other high-level nuclear waste are part of Deep Isolation’s engineered barrier system; the canisters directly protect the waste from mechanical impact, exposure to the chemical environment, and contact with fluids. One of the key decisions then, to ensure the canister’s usefulness as a barrier, is the choice of material used. Our material selection process began with an extensive analysis of the peer-reviewed literature, over the course of months, which of course included examining test results and recorded observations and measurements. I was the lead as a senior corrosion engineer, but the entire technical team helped vet the choice of best material. Our decision is to use highly corrosion-resistant nickel-chromium-molybdenum (Ni-Cr-Mo) alloys which are very stable in the deep underground environment. These alloys also have high strength and are readily fabricable by conventional methods. My paper, “Corrosion-Resistant Alloy Canisters for Nuclear Waste Disposal in Horizontal Drillholes,” summarizes the technical basis for our selection of these alloys and gives both the experimental analysis and real-world experience on performance in a wide range of highly corrosive applications.
How can we be sure that Ni-Cr-Mo alloys are the best choice for the long time periods needed? The answer lies in the fact that these alloys are passive, that is, they are protected by a self-forming and self-healing film if damaged either chemically or mechanically. This passive film is an extremely thin layer of a chromium-rich oxide, essentially a ceramic material. The general corrosion rates of the passive Ni-Cr-Mo alloy are extremely low; it would take 17,500 years for this type of corrosion to penetrate to the thickness of a quarter, and the canister’s thickness is equivalent to 5-6 quarters.
The Ni-Cr-Mo alloys also have high resistance to the localized corrosion processes of pitting, crevice corrosion and stress-corrosion cracking. Alloy 22, one of the Ni-Cr-Mo alloys, is among the most resistant to microbially-induced corrosion; its MIC resistance has been examined under a range of conditions with no evidence of surface damage. Galvanic corrosion also needs to be taken into account and will be addressed when considering the effects of the Ni-Cr-Mo alloy upon the other metals incorporated in the repository, the relative surface areas and the conductivity of filler materials, and the pore waters present in the rock.
A number of beneficial attributes of disposal in deep horizontal drillholes reduce the complexity of corrosion analysis and contribute to our conclusions regarding the high performance of Ni-Cr-Mo alloy canisters.