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I. Core reaction process: Oxidation reaction of the uranium matrix. In a high-temperature water environment, the uranium matrix of highly spent nuclear fuel first undergoes a reaction to form a UO₂ protective layer； When hydrogen diffuses into the matrix, UH₃ hydrides are formed, causing volume expansion that damages the existing protective layer. Further reactions then result in the formation of non-stoichiometric UO₂.06, which prevents the creation of an effective protective layer, allowing corrosion to progress uniformly. Oxidation process of zirconium cladding: In high-temperature coolant, a dense monoclinic ZrO₂ protective film forms on the zirconium alloy cladding. Once the thickness of this film exceeds a critical value, it changes from a black, dense state to a grayish-white, porous state; at this point, its protective function is lost, and corrosion proceeds uniformly. Eventually, the film peels off, resulting in uniform thinning of the entire cladding. II. Acceleration mechanism under high burnup conditions: At high burnup levels, the residence time of fuel in the reactor increases significantly; the oxide layer and scale thicken simultaneously, the temperature on the component walls rises, and the diffusion rate of oxygen accelerates markedly. This creates a vicious cycle of \"thickening of the oxide layer → rising temperature → accelerated corrosion\", which greatly speeds up the rate of uniform corrosion.