Abstract
The purpose of this work is to quantify the effects of dissolved zinc cations on corrosion and release rates from a pre-filmed Alloy 690 steam generator tubing material that was subsequently exposed to water containing zinc. The corrosion tests were performed in circulating 2 ppm Li and 1000 ppm B water without and with 60 ppb zinc at 330 °C. Gravimetric analyses and oxide characterization revealed that the corrosion rates, release rates, and oxide thicknesses decreased by subsequent exposure of the pre-filmed Alloy 690 to zinc. These benefits are attributed to the formation of a chromium-rich inner oxide layer incorporating zinc.
Highlights
The two major sources of ex-core radiation fields in nuclear power plants are radioactive cobalt isotopes, 58Co and 60Co, which originate from natural nickel (58Ni) and cobalt (59Co) dissolved from the material surfaces of the reactor coolant system. 58Co is activated from 58Ni by the fast neutron reaction, while 60Co is produced by the reaction with 59Co and thermal neutrons [1,2]
When the specimens were exposed to zinc-free water only for 3000 h (Case #1, Figure 3a,b), the surfaces of the specimens were completely coated with small, polyhedral particles approximately 100–200 nm in size and the original surface could not be observed
In Case #2 where the specimens oxidized in zinc-free water for 1500 h were subsequently exposed to 60 ppb zinc solution for 1500 h (Figure 3c,d), the surfaces of the specimens were covered with small particles, analogous to those in Case #1
Summary
The two major sources of ex-core radiation fields in nuclear power plants are radioactive cobalt isotopes, 58Co and 60Co, which originate from natural nickel (58Ni) and cobalt (59Co) dissolved from the material surfaces of the reactor coolant system. 58Co is activated from 58Ni by the fast neutron reaction, while 60Co is produced by the reaction with 59Co and thermal neutrons [1,2]. Due to excellent performance for reducing radionuclide activity, the number of plants implementing zinc addition has increased worldwide [5]. It has been reported that zinc addition increases resistance to stress corrosion cracking [6,7,8] and low-cycle fatigue life [9] of nickel-based alloys and stainless steels in simulated PWR primary water. These beneficial effects have been attributed to modification of oxide films by zinc addition. Compared to zinc-free conditions, important observations for oxides formed in a chemistry containing zinc can be summarized as follows: The size and amount of oxide particles in the outer oxide layer drastically decreases [10,11,12,13]; a significantly thinner oxide film is formed [11,13,14,15,16,17]; zinc is incorporated into the oxide films [11,12,13,15,16,17,18,19,20]; cobalt uptake in the oxide films decreases [15,21,22,23]; and the corrosion and release rates decrease significantly [11,12,14]
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