Erosion corrosion of CuCrZr specimens exposed for simulated ITER operational conditions

Abstract

Corrosion is one of the main factors that determine the operational lifetime of any type of thermal power station. The ITER In-Vessel design includes coolant water interfaces with the copper alloy CuCrZr that are exposed to high flow velocities. These interfaces are near the plasma and thus subject to oxidants such as H2O2 generated through irradiation-induced radiolysis of the coolant water when the plasma is active.The erosion corrosion and release rates of CuCrZr and CuCrZr/316L(N)-IG joints were derived from weight change data and metallographic examinations of specimens after autoclave exposures at 110°C, 150°C and 250°C at reducing-, oxidizing- and cyclic redox water chemistry conditions. Reducing water chemistry represents periods considered as the nominal off-plasma operational conditions while the oxidizing environment simulates situations when the plasma is active. The cyclic redox conditions represent periods with shorter cycles simulating plasma activations with subsequent periodical variation of the water chemistry from reducing to oxidizing. The erosion corrosion rates for CuCrZr at active plasma conditions were 20 and 40µm/year at 110 and 150°C. At 250°C the corresponding rate was much higher. This result gives important information on what may happen if, for example, a First Wall panel is exposed to an unexpectedly high heat flux. Under reducing conditions the erosion corrosion rates were 3µm/year and 20µm/year at 110°C and 250°C respectively. The results at 250°C under off-plasma conditions reveal that the effect of erosion corrosion also has to be taken into account during baking. Cyclic conditions with respect to oxidant content turned out to be the most demanding environment (more demanding than pure oxidizing conditions) for the CuCrZr-alloy. Erosion corrosion rates of 90µm/year and 370µm/year at 110 and 150°C were recorded respectively. The highest temperature 250°C was not tested. This raise the question whether measures should be introduced that renders the system either oxidizing or reducing. In summary, the erosion corrosion rates recorded for CuCrZr under simulated ITER coolant water conditions are high, especially during plasma operation. For comparison it can be mentioned that corrosion rates of structural materials in the primary loop of light water reactors generally are considerably lower than 1µm/year. The estimated rates for CuCrZr are then 1–3 orders of magnitude higher, however the short total duration of plasma operation during ITER life time must be considered in this context. Erosion corrosion of CuCrZr can potentially cause problems for the ITER coolant systems and the chemical control systems.