INTEGRAL MEASUREMENTS OF EFFECTIVE DELAYED NEUTRON FRACTIONS IN FAST CRITICAL ASSEMBLY SNEAK

Integral measurements of delayed neutron fractions in fast reactor spectra by two different techniques were carried out. The worth of a calibrated **2**5**2Cf spontaneous fission source, together with absolute fission rates and with the normalization integral obtained from fission rate mapping, gives experimental values for the effective delayed neutron fraction of a critical assembly. These measurements were performed in three PuO//2 EN DASH UO//2 fueled assemblies and in one UO//2 fueled assembly. The pile oscillator technique was used to determine relative yields of delayed neutrons from **2**3**5U, **2**3**9Pu. The results confirm the evaluated yields by Tuttle, with a slight bias toward a higher **2**3**9Pu yield. With these data, the central worth discrepancy disappears for SNEAK measurements.