Development of Alternative Reductant Application in PWR Primary Systems

In primary coolant of pressurized water reactors (PWRs), high concentration dissolved hydrogen (DH) has been added, to prevent generation of oxidizing species through radiolysis of coolant. Recently, number of ageing plants is increasing and utilities are concerned about primary water stress corrosion cracking (PWSCC). Although mechanism of PWSCC is not fully clarified, some researchers consider that occurrence of PWSCC and crack propagation rate are affected by DH concentration. The authors consider that one of possible mitigation methods toward PWSCC is use of alternative reductant for hydrogen. From the radiation chemical aspect, aliphatic alcohols are typical scavengers of oxidizing radical generated through the radiolysis of water. The aliphatic alcohols are promising candidates of the alternative reductant. In the present work, possible alternatives of hydrogen were screened, and methanol was selected as the best candidate. Corrosion tests of type 304 stainless steels were carried out at 320°C in two conditions: (1) DH 1.5 ppm (part per million) and (2) methanol 2.9 ppm. Under two conditions, electrochemical corrosion potential was measured during the immersion tests. After the immersion tests, surface morphology of the stainless steel specimens was observed by scanning probe microscope. Major component of oxide film was analyzed by X-ray diffraction. From comparison of the test results, it is concluded that addition of methanol 2.9 ppm has almost the same effect as addition of DH 1.5 ppm.