Corrosion Inhibition of Carbon Steel By Inert Gas Ultrafine Bubbles
Authors: Yano Miri , Tsutsumi Yusuke , Katayama Hideki , Akashi Takaya 
Published: December 2024

The Fukushima Daiichi Nuclear Power Plant experienced power loss in Units 1-4 due to the Great East Japan Earthquake on March 11, 2011, causing the shutdown of the fuel cooling systems for the reactor cores and spent fuel pools. Consequently, the reactor buildings suffered damage from core meltdowns and hydrogen explosions, releasing radioactive materials. During this incident, fuel debris accumulated within the reactor. Currently, efforts are made to minimize corrosion inside the reactor by maintaining low dissolved oxygen concentration underwater through degassing treatment of circulating water for fuel debris cooling and nitrogen injection into the Primary Containment Vessel (PCV) to prevent hydrogen explosions. However, future fuel debris retrieval operations may expose the PCV to the atmosphere temporarily, raising concerns about increased dissolved oxygen concentration, inducing acceleration of corrosion reaction. Hence, it is necessary to establish corrosion inhibition techniques resilient to radiolysis, as an alternative to nitrogen injection. Utilization of inert gas ultrafine bubbles (UFB) are anticipated to effectively reduce dissolved oxygen in water. Therefore, in this study, effects of the argon gas UFB introduction treatment on corrosion reaction of carbon steel in a simulated reactor environment (without radioactive material) were evaluated to explore methods for effectively inhibiting corrosion reactions during fuel debris retrieval.

The round-shaped carbon steel (SM490A) specimen mounted in epoxy resin was used as corrosion-monitoring sensor electrode. Two types of electrodes were prepared: immediately after polishing and rust-formed. The latter electrode was prepared by accelerating corrosion reaction using a saltwater spray testing machine (STP-30, Suga Testing Machine, 50 gL-1 NaCl solution at 303K) for three days to induce the thick rust layer formation on the surface.

A solution of artificial seawater (Aquamarine, Yashima Pure Chemicals, Japan) diluted 200-fold was employed for the simulated reactor environment. UFB introduction treatment was conducted for 10 liters of the solution using an ultrafine bubble generation machine (BUVITAS, NEXCO Engineering Kansai, Japan) with Ar gas flowing at a rate of 650 mL/min for 30 minutes. The aerated (open air condition) test solution without UFB introduction treatment was also prepared for comparison. To prevent overheating the solution during the treatment, an immersion cooler (BE201F, Yamato Scientific, Japan) was used. It was confirmed that the solution temperature could be kept below 25°C even after 30 minutes of the UFB-introduction treatment by cooling the solution to 5°C in advance.

A glass water vessel with a cubic shape (15 cm × 15 cm × 15 cm) was used as electrochemical cell, and it was completely sealed inside a larger 20 L plastic container directly connected to the UFB-generation machine. A drainage trap was installed to prevent the backflow of oxygen from the atmosphere during the solution discharging process. Pairs of the electrodes were placed facing each other with distance of 1 cm. These steel electrodes for corrosion monitoring were fixed at the bottom of the cell. The measurements were conducted in the following manner. Before pouring the solution inside the cell, Ar gas purging for 30 minutes was performed in advance to eliminate the effect of residual oxygen in the container. The corrosion rate was evaluated by simplified electrochemical impedance spectroscopy using corrosion monitor (CT-7, Riken Densi, Japan).

The dissolved oxygen (DO) concentration during UFB introduction treatment decreased rapidly from the beginning of the treatment and reached nearly 0 ppm within a few minutes.

The corrosion rate of carbon steel without rust layer in with UFB condition remained stable and was kept lower value than that without UFB condition during testing period of 6 d. Thus, it was considered that the initial corrosion reaction of carbon steel in diluted seawater environment was effectively inhibited by the presence of UFB with lower DO concentration.

In the case of the rust-formed specimen, the corrosion rates were nearly an order of magnitude higher than those of the specimens without rust layer. The corrosion rate tended to decrease in the UFB-introduced solution. On the other hand, it exhibited a clear increasing tendency under the UFB-free condition.

These results clearly demonstrate that the introduction of Ar gas UFB effectively suppresses corrosion of carbon steel in diluted seawater, regardless of the presence of rust layer.

It has been demonstrated that the introduction of ultrafine bubbles (UFB) using inert gas effectively reduces the dissolved oxygen (DO) concentration in the solution for longer period. In addition, , corrosion inhibition effect due to low DO condition was found to be available regardless of the presence of rust layer formed on carbon steel surface in advance.