To easily and accurately calculate an addition amount required for obtaining the degree of reaction equivalent to fuel of U235 concentration degree of 5% with the usage of concentrated erbia, with respect to fuel with the U235 concentration degree exceeding 5%.

A method for calculating the addition amount by an erbia credit with the usage of the concentrated erbia has an abundance ratio of Er167 which is equal to or more than that of natural erbia, with respect to a nuclear fuel having a given U235 concentration degree, and the addition amount ε by the erbia credit is calculated according to formula 3, wherein ε is the addition amount of the concentrated erbia, αiis the reactivity coefficient for the erbia isotopes (i represents Er164, Er165, Er166, Er167, Er168 or Er170), νiis an isotopic composition of the concentrated erbia (i represents Er164, Er165, Er166, Er167, Er168 or Er170), βjis a reactivity coefficient of uranium isotope (j is U234, U235, U236 and U238), and μjis an isotopic composition of uranium (j represents U234, U235, U236 or U238).

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