Void reactivity of the core partially loaded with plutonium fuel 0.87 wt% enriched PuO-UO fuel

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Void reactivities caused by loss of coolant have been measured in DCA core (which has a 22.5-cm lattice pitch) by changing the number of PuO (0.87 wt%)-UO fuel assemblies from 0 up to 25. The PuO-UO fuel assemblies were progressively loaded from the core center, being surrounded by the 1.2 wt% enriched UO fuel region. In order to find out the effect of the fissile content in plutonium on void reactivities, two kinds of mixed fuels were used in the experiment: one enriched with standard grade plutonium (91 wt% Pu fissile) and the other with reactor grade plutonium (74 wt%). Void reactivities were measured by means of pulsed neutron source technique. When evaluating void reactivities from measured prompt decay constants, corrections were made to the neutron generation time of a largely subcritical system, which accounted for up to 5% maximum. In the Table, measured void reactivities were compared with the values calculated by METHUSELAH-II and CITATION codes. The following are concluded from this study. (1)Void reactivities gradually shift to negative side by loading of more PuO-UO fuel. (2)The increase of fissile contents in plutonium also makes void reactivity shift to the more negative side. (3)Measured void reactivities are more negative than calculated ones. The discrepancy between calculation and experiment becomes greater with increase in fissile contents in plutonium.