Sridhar Ramamurthy

Design and development of copper coatings for long term storage of used nuclear fuel

Abstract Within ongoing work, Canada’s Nuclear Waste Management Organization (NWMO) is demonstrating that copper coatings can be developed as viable corrosion barriers for used fuel nuclear containers, emplaced in a deep geological repository. As a means of assessing container design and feasibility, coated samples, manufactured via cold spray and electrodeposition have been produced and characterised. Corrosion testing for coated samples and wrought samples reveals that cold spray samples behave similar to wrought samples in 3 mol L−1 NaCl solution at room temperature. When oxygen is absent, corrosion has not been observed.

Corrosion of Cold Spray Deposited Copper Coating on Steel Substrates

The corrosion behavior of copper cold spray coatings on a carbon steel substrate was compared to that of commercially available wrought copper under the conditions anticipated in a nuclear waste repository. Corrosion potential and linear polarization resistance measurements were conducted over 90 d to 120 d in 3.0 mol/L NaCl under anoxic (1) and oxygenated-to-anoxic (2) conditions to simulate the long (1) and short (2) term redox conditions expected in a Canadian repository. Scanning electron microscopy and x-ray diffractometry were used to observe the morphology of the corroded surface and to identify the corrosion products formed. The two specimens behaved very similarly under both sets of conditions. Negligible corrosion was observed under anoxic conditions and the formation of both Cu+1 (Cu2O) and Cu+2 (Cu2(OH)3Cl) phases occurred under oxygenated conditions. No evidence was observed to suggest the particle boundaries in the cold sprayed coating were preferential corrosion sites.

Degradation of Alloy 800 Under Steam Generator Secondary Side Crevice Conditions

Alloy 800 is the preferred steam generator (SG) tube materials for CANDU™ reactors and is also used extensively in SGs in some pressurized water reactor (PWR) systems. Degradation of Alloy 800 SG tubing has only been found in a few tubes at a limited number of stations despite the large number of SG tube operating years accumulated to date. Recently, underdeposit corrosion was detected in a few ex-service tubs removed from some CANDU SGs. Pits like wall loss of about 5% to 10% through-wall depth were found in these ex-service tubes. Evidence of intra-tubesheet cracking of Alloy 800 tubes was detected in a few European PWR SGs. There is no degradation in mechanical properties of these ex-service CANDU SG tubes. In addition, the degradation of Alloy 800 tubes observed so far is not a safety issue. However, the findings suggest that Alloy 800 tubing may have some aging degradation susceptibility after many years of service. Whether the degradation of Alloy 800 tubing is due to imperfections in its compositional or metallurgical properties inherent from manufacturing, or due to the aggressive chemistry conditions that should have been precluded by modern chemistry control strategy require clarification. Comprehensive examinations, including metallurgical examinations, orientation imaging microscopy (OIM), surface analyses and electrochemical measurements were performed on the removed ex-service CANDU SG tubes that had some underdeposit corrosion. The results were compared with a reference nuclear grade Alloy 800 tubing and with archive Alloy 800 new SG tubes from several CANDU stations. High-temperature electrochemical tests, scanning vibrating electrode Technique (SVET) measurements as well as C-ring autoclave tests were performed to determine the possible factors leading to Alloy 800 SG tubing degradation. SCC was initiated in a few C-ring specimens in the presence of artificial cold work flaws under simulated acidic SG secondary-side crevices chemistry conditions. OIM and surface analysis were also performed to characterize the degradation initiated in Alloy 800 tubing under the influence of cold work flaws. The possible factors leading to Alloy 800 SG tubing degradation under SG secondary crevices conditions are discussed.© 2009 ASME

Galvanic corrosion of copper-coated carbon steel for used nuclear fuel containers

ABSTRACT Canada is currently considering Cu-coated carbon steel containers for the long-term storage of used nuclear fuel in a deep geological repository. The Cu coating provides a corrosion-resistant barrier, protecting the underlying steel from coming into contact with groundwater. However, galvanically accelerated corrosion of steel is possible if there is a defect through the Cu coating. To investigate this scenario, the progression of steel corrosion at the base of a simulated though-coating defect was imaged using synchrotron X-ray micro-computed tomography. Results show that coatings produced using different methods (cold spray, annealed cold spray, electrodeposition) lead to different corrosion propagation geometries. These findings can be used for modelling steel corrosion at a though-coating defect under deep geological repository conditions. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

The active/passive conditions for copper corrosion under nuclear waste repository environment

ABSTRACT The focus of this research is to determine the conditions under which passivity of Cu is (with the possibility of pitting) or is not possible, in anticipated bentonite pore water compositions expected to contact a Cu-coated container in a deep geological repository. Cyclic voltammetry has been used to deduce the active/passive maps that define the environmental conditions (in terms of [Cl−], [], [], pH, and temperature) under which the Cu surface could be active or passive. A range of temperatures (up to 80°C), pH values (neutral to alkaline), and electrolyte compositions have been investigated. Such a database of maps shows that active/passive conditions change with the pH, temperature, and the type and concentration of anions. These maps will provide a basis for container corrosion models, as well as guidelines regarding the water compositions used when compacting the bentonite, the repository saturation time, and the optimum container spacing. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

The Effects of Cathodic Reagent Concentration and Small Solution Volumes on the Corrosion of Copper in Dilute Nitric Acid Solutions

The exposure conditions experienced by copper-coated high-level nuclear waste containers in a deep geologic repository will evolve with time. An early exposure period involving the gamma irradiation of aerated humid vapor could lead to the formation of nitric acid condensed in limited volumes of water on the container surface. The evolution of the corrosion processes under these conditions have been studied using pH measurements in limited volumes of water containing various concentrations of nitric acid. The extent and morphology of corrosion was examined using scanning electron microscopy on surfaces and on focused ion beam cut cross sections. The composition of corrosion products was determined by energy dispersive x-ray analyses and Raman spectroscopy. In the absence of dissolved oxygen only minor corrosion was observed with the reduction of nitric acid inhibited by the formation of either chemisorbed nitrate and nitrite species or the formation of a thin cuprite (Cu2O) layer. When the solution was ae...

Corrosion Product Analysis of Aluminum Closure Panels

Painted aluminum panels subjected to several laboratory-based accelerated corrosion tests were examined using surface analytical techniques. This paper presents some of the results from these measurements, which indicated that the nature and the extent of the corrosion attack were greatly influenced by the salt spray conditions. In general, ASTM G 85 acetic acid salt spray produced the greatest amount of corrosion, while exposure to GM 9540P and ASTM B-117 resulted in the least amount of corrosion. Moreover, filiform corrosion was the most common corrosion attack observed from exposures to many of the salt spray tests. Finally, filiform corrosion appeared to propagate by the dissolution of the phosphate conversion coating, thus lifting off the paint layer from the aluminum substrate.