Fuel claddings would be exposed to a hightemperature steam-flow after a boiled-up sequence of a severe accident of light water reactors as well as spent fuel pools. Zirconium alloys are common cladding material for light water reactors. All the zirconium alloys generate additional heat by the exothermic reaction of oxidation in high-temperature steam. Subsequent temperature rise accelerates the rate of oxidation. The countermeasure to suppress oxidation of zirconium alloy is key to mitigate severe accidents. We invent a surface modification technology of zirconium alloy, Fresh Green, to oxidize and to carbonize a zirconium-alloy surface in the same process. XRD and XPS analyses indicated that a modified layer on zirconium surface is carbon-doped zirconium dioxide, in which some of oxygen atoms in monoclinic zirconium dioxide are replaced by carbon. Experiments are conducted for three representative conditions using an autoclave at relatively high temperature: uniform corrosion at 400 degrees Celsius for 336 hours, nodular corrosion at 500 degrees Celsius for 24 hours, supercritical water corrosion (at 400 degrees Celsius, 24 hours). The Fresh Green surface modification reduces both the oxidation rate and hydrogen-pickup less than a half of that with untreated base material. This is because the Fresh Green layer is closely packed and adhered intimately to the base material. Hydrides were found in the base material without treatment, though they were scarcely observed in the Fresh Green treated specimen. Since the operating temperature of the Fresh Green process is lower than the final annealing temperature in a manufacturing process, the process does not affect the material property and crystal structure. The Fresh Green process, which can be operated at slightly higher pressure than ambient, can be introduced in the conventional manufacturing process without complicity.