With regard to the cracking (disbonding) which occurs at the fusion boundary of austenitic overlays and the base metal of high-temperature, high-pressure hydrogen reactors, the accumulation of hydrogen at the fusion boundary subsequent to cooling from a high to a normal temperature, which is considered to be the largest causative factor of such cracking, has been evaluated by numerically solving diffusion equations using differential calculus. It has been shown that the amount of accumulated hydrogen can be expressed by applying a simple, approximate formula which incorporates the service pressure of the hydrogen, service temperature, thickness of overlay and base metal, cooling rate, hydrogen solubility, and hydrogen diffusion coefficients.