(Keynote) Radiation Induced Corrosion of Copper for Spent Nuclear Fuel

Abstract

The influence of radiation on various forms of corrosion is a complex and largely unexplored issue. In principle, radiation can influence physicochemical processes in the whole interfacial region which trigger corrosion: in the gas, liquid and solid phases, and in the interfaces in-between. This presentation highlights results and conclusions from a recently completed doctoral study on radiation induced corrosion of copper (ref. 1). The storage of nuclear waste in Sweden is planned to be based on copper canisters with cast iron inserts and surrounded by bentonite which will be deposited at a depth of 500 meters in ground water saturated granitic bedrock. Gamma radiation from the fuel will penetrate the copper canisters and be absorbed by the surrounding bentonite clay. When water in the clay absorbs gamma radiation aqueous radiolysis will occur. Two oxidants from aqueous radiolysis, hydrogen peroxide (H2O2) and the hydroxyl radical (HO∙), have significantly higher standard reduction potentials than copper, and are therefore thermodynamically capable of initiating corrosion of the canisters. The aim of the present work was to explore the effect of total gamma dose on the corrosion behavior of copper under well controlled conditions. A series of experiments were performed, in which samples of copper taken from a canister received a number of different total doses. Three different dose rates of gamma radiation were used and the experiments were performed in pure, anoxic, water, in air of 60 to 100 % relative humidity, and in humidified argon. The total doses of gamma radiation used were within the range relevant for a geological deep repository for spent nuclear fuel. Qualitative characterization of the corrosion products formed on the copper surface was performed using IRAS, XPS, SEM and AFM, quantitative characterization of corrosion products by cathodic reduction, and trace elemental analysis of copper on all solutions by inductively coupled plasma atomic emission spectroscopy, ICP-AES. The overall conclusion is that gamma radiation causes significantly enhanced corrosion of copper in anoxic aqueous solution in comparison to non-irradiated samples. The main corrosion product formed on the copper surface during gamma irradiation is cuprite (Cu2O), while only a small fraction consists of Cu(II) compounds. The thickness of the oxide layer, after irradiation at dose rates of 370 and 770 Gyh-1and total doses of 35.5 and 74 kGy, was measured to be 50–100 nm (refs 2 and 3). Dissolution of copper during irradiation depends on the total absorbed dose, also on the nature of the pre-formed oxide. Complementary exposures were performed in which the kinetics and mechanisms of the stable radiolysis product H2O2 reacting with copper and copper oxides were explored. The results clearly show that aqueous radiolysis only can account for very small fraction of the radiation induced corrosion of copper (ref. 4). Further studies were performed in which the influence of pre-grown copper oxides was explored. Radiation-induced corrosion of copper is significantly more effective on pre-oxidized copper than on polished copper, and the total amount of oxidized copper exceeds the amount expected solely from radiation chemistry of water by more than three orders of magnitude. Based on the results a mechanism has been proposed in which the HO∙ radical is the main radiolytic oxidative species driving the corrosion process. When taking all results from different exposures into consideration the major mechanism found so far to explain the dramatically enhanced radiation induced corrosion of copper is proposed to be an enhanced chemical yield of HO∙ interacting with the copper oxide surface and an increased reactive copper oxide surface area (ref. 5).