Measurement of effective delayed neutron fraction in highly enriched uranium metal critical assembly

Effective delayed neutron fraction eff is an important parameter in dynamic behaviour of nuclear reactor. Besides, eff has the role of the conversion factor between a relative unit and an absolute unit in reactivity scale, which is important for examining work of macroparameter according to reactivity. An experiment was performed to measure theeff in a fast critical assembly using Nelson number method which is based on Rossi- method. Using lead for shielding, adopting the thinner 6Li glass scintillator and pulse amplitude discrimination, the influence of is decreased. The measurements were carried out for reactivity ranging from -60 ￠ to delayed critical state. The final measured value ofeff is 0.006 66, with 7.88% uncertainty, of which the deviation is 2.15% compared with the theoretical value. Because of the good matching between the measured value and the theoretical value, the experiment method is demonstrated to be effective.