Formic acid and oxalic acid degradation by gamma rays under high-alkalinity and low-oxygen-concentration conditions

Abstract

Radiocarbon (C-14), which is released from irradiated stainless steels, is one of the key radionuclides in radioactive waste disposed at an intermediate depth. It is important to understand the chemical species of C-14 under an intermediate-depth disposal condition (high alkalinity, low oxygen concentration, gamma-ray irradiation) for safety assessment. In this study, the stability and degradation behavior of formic acid and oxalic acid that leached from irradiated stainless steels were investigated. 1.0 × 10-4 mol/L of formic acid and oxalic acid were irradiated with 50–10,000 Gy of gamma-ray under high-alkalinity (pH 12) and low-oxygen-concentration (<0.1 ppm of O2) conditions at 25 °C. In the gamma-ray irradiation of oxalic acid, the concentration of CO2 produced by degradation of oxalic acid increased as the dose increased, and about half of oxalic acid were degraded to CO2 at 10,000 Gy. There is only one pathway in the degradation of oxalic acid, that is, oxalic acid is directly degraded to CO2. In the gamma-ray irradiation of formic acid, the concentration of oxalic acid and CO2 increased at doses <1000 Gy, but oxalic acid decreased and only CO2 increased at doses >1000 Gy, the concentration of CO2 became around 7.1 × 10-5 mol/L at 10,000 Gy. There are two possible pathways in the degradation of formic acid, one in which formic acid is directly degraded to CO2, and the other in which formic acid is first changed to oxalic acid, which is then degraded to CO2.