Water Chemistry Technology of Methanol Addition in PWR Primary Systems: Radiolysis of Methanol Solution at 320 °C

Primary Water Stress Corrosion Cracking (PWSCC) is one of important ageing issues in PWR (Pressurized Water Reactor) primary systems. It has been pointed out that high concentration dissolved hydrogen may lead to occurrence of PWSCC. The authors have proposed to substitute hydrogen by methanol as a fundamental countermeasure of PWSCC. So far corrosion tests of stainless steels and Zircaloy-4 in methanol solutions at 320 °C were conducted under γ-ray irradiation and without irradiation. The test results show that methanol is promising.

In the present paper, γ-ray irradiation experiments of methanol solution at 320 °C were done up to 100 kGy. A study on the radiolysis of methanol solution is important from two aspects. One concerns corrosion of structural materials. The radiolysis of methanol may result in formation of harmful compounds to the structural materials, such as carboxylic acids. It is necessary to know the yields of such compounds. The other concern is possible polymerization of methanol and formation of organic polymer deposit on fuel claddings. Large amount of the deposit on fuel claddings should be avoided to keep integrity of fuel claddings. Therefore, it should be clarified whether gaseous species are major products and whether polymerized species of methanol such as ethylene glycol is formed.

After the γ-ray irradiation of methanol solution, following species were analyzed: CO₂ and H₂, methanol, formaldehyde, formate and acetate, and ethylene glycol and glycerin.

Without γ-ray irradiation, the major process of the thermal decomposition of methanol at 320 °C is oxidation of methanol by water and generation of one CO₂ molecule and three H₂ molecules. Under γ-ray irradiation, the decomposition of methanol is accelerated; little methanol remains after 10 kGy irradiation. The major product is CO₂, and polymerization of methanol unlikely occurs. After methanol is completely decomposed, the hydrogen yield still increases. The reducing environment is maintained. Probably, transient organic species play important roles. The addition of low concentration methanol may be sufficient to maintain reducing environment of the PWR primary systems.