Hydrogen ingress through EDM surfaces of Zr-2.5Nb pressure-tube material

Mechanical test specimens can be easily manufactured from Zr-2.5Nb pressure tubes using electrical discharge machining (EDM). We discovered that zirconium specimens manufactured by EDM using a brass electrode and paraffin oil as dielectric fluid picked up a large amount of hydrogen during irradiation in 250 C water. This paper describes the investigation of the hydrogen ingress route. Our results showed that EDM in paraffin oil using a brass electrode modified the surface such that it became permeable for hydrogen when the specimen was later exposed to water. Out-reactor experiments on unirradiated material showed that irradiation was not necessary for the hydrogen ingress but might influence the ingress rate. The hydrogen ingress rate for out-reactor tests was estimated to be in the range 2 x 1017-8 x 1017 atoms/m2 s at 300 C water. There was a large scatter in the results indicating that the surface conditions, water chemistry and radiolysis of the water may influence the hydrogen ingress rate. (orig.)