To cope with the"5wt(percent) enrichment barrier", the development project on Erbia bearing super high-burnup fuel with high uranium enrichment, such as critical experiments with fully Erbia loaded core in KUCA, criticality safety analyses against simple shapes, physicochemical properties measurement and so on, is under going. Following the three experiments of fully-Er-loaded core, the result of the fourth core of higher content of Erbia is presented. As a second topic, fuel design using isotopically modified Erbia, such as a) Er-166 removed from Erbium content, b) Only Er-167 used, is shown. In use of isotopically modified erbia, the ECOS diagram, which shows minimum Erbia contents that should be mixed into above 5wt(percent) enriched fuel, so that the criticality safety will be equivalent to that with the conventional below 5wt(percent) enriched fuel, should be modified in order to correspond to various erbia isotope vectors.
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