Investigation on degradation mechanism of ion exchange membrane immersed in highly concentrated tritiated water under the Broader Approach Activities

The ion exchange membrane is a key material for electrolysis cells of the water detritiation system. Durability of ion exchange membrane has been demonstrated under the Broader Approach Activities. Long-term exposure of Nafion® ion exchange membrane in 1.38 × 1012 Bq/kg of tritiated water was conducted at room temperature for up to 2 years. The ionic conductivity of Nafion® membrane after immersed in tritiated water was changed. The change in color of membrane from colorless to yellowish was caused by reactions of radicals on the polymer and oxygen molecules in air. Infrared Fourier transform spectrum of a yellowish membrane revealed a small peak for bending vibration of CH situated at 1437 cm−1, demonstrating the formation of hydrophobic functional group in the membrane. The hydrophilic network in Nafion® membrane was partially obstructed by the hydrophobic free products. This caused the decrease in ionic conductivity. The peak for bending vibration was clearly eliminated in the spectrum of the membrane after treatment by acid for removal of free products. The high-resolution solid state 19F NMR spectrum of a membrane after immersed in tritiated water was similar to that of a membrane irradiated with gamma-rays. From the 19F NMR spectrum, no distinctive degradation in the membrane structure by interaction with tritium was measured.