THE CENTRAL VOID REACTIVITY IN THE OAK-RIDGE-NATIONAL-LABORATORY ENRICHED URANIUM(93.2) METAL SPHERE

The reactivity worth of a central void region in the Oak Ridge Nationa l Laboratory (ORNL) unmoderated and unreflected uranium (93.20 wt% U-2 35) metal sphere was obtained by replacement measurements in a small ( 0.460-cm-diam) central spherical region in this 3.4420-in.-radius sphe re. The measured central void region worth was 9.165 +/- 0.023 c using the delayed neutron parameters of Keepin, Wimett, and Zeigler to obta in the reactivity from the measured stable reactor periods. This value is slightly larger than measurements for GODIVA I with larger cylindr ical samples of uranium (93.70 wt% U-235) in, the center: 135.50 +/- 0 .12 c/mol for GODIVA I and 138.05 +/- 0.34 c/mol for the ORNL sphere m easurements. The difference could be due to sample size effect. The ce ntral worth was also calculated by neutron transport theory methods to be 6.02 +/- 0.01 x 10(-4) Delta k. The measured and calculated values are related by the effective delayed neutron fraction. The value of t he effective delayed neutron fraction obtained in this way from the OR NL sphere is 0.00657 +/- 0.00002, which is in excellent agreement with that obtained from GODIVA I measurements, where the effective delayed neutron fraction was determined as the increment between delayed and prompt criticality and was 0.0066. From these ORNL measurements, using the delayed neutron parameters of ENDF-B/VI to obtain the reactivity from the stable reactor period measurements, the central void worth is 7.984 +/- 0.021 c, and the inferred effective delayed neutron fractio n is 0.00754. These values are 14.2% higher than those obtained from u se of the Keepin, Wimett, and Zeigler delayed neutron data and produce a value of effective delayed neutron fraction in disagreement with GO DIVA I measurements, thus questioning the usefulness of the six-group delayed neutron parameters (fast fission) of uranium from ENDF-B/VI fo r obtaining the reactivity from the measured reactor period using the Inhour equation.