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

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

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