Coolant void reactivity in the core with BC control-rods

not registered; Fukumura, Nobuo*; not registered; not registered; not registered

Coolant void reactivities due to 0100% change in void fraction have been measured in two-region core involving BC control-rods fully inserted in order to evaluate the control-rods effect for the void reactivity and an accuracy of reactor calculation codes. The 28-rod PuO-UO clusters were placed in a central region of the core and the surrounding region was occupied with 28-rod 1.2wt% enriched UO clusters. Fuel clusters are arranged in a square lattice at a 25.0-cm pitch. The PuO enrichments in the mixed oxides are 0.54 and 0.87wt%. The fissile content of plutonium in both mixed oxides is 91%. The void reactivity was measured by changing the number and/or position of the control-rods inserted, and loading ratio of mixed-oxide clusters to UO clusters. The void reactivities were obtained by integrating a function of the level reactivity coefficient of moderator (/) for the 100%-voided core between the critical levels at the non-voided and the 100%-voided cores. Values of (/) were measured by converting the doubling time of neutron flux change due to a small rise of moderator level (H) to small reactivity (). Corrections are given to the void reactivities measured for the reactivity effects of a leakage due to the difference of axial extrapolation distances between 0% and 100% void fractions and of a temperature change during the measurement. Thermal flux distributions were also measured by -counting of irradiated copper wire or samples, in order to evaluate the change of flux distributions in the core due to coolant voiding. Experimental void reactivities were compared with calculated ones by WIMS-D4 and CITATION codes. Inserted control-rod is able to deal as a corner rod in unit cell in WIMS-D4 calculation which is applied the square boundary condition. The following are concluded from the present study. (1)Insertion ...