Measurement of the Effective Delayed-Neutron Fraction in Three Different Cores of the Fast Assembly FRO

The effective delayed-neutron fraction, β_eff, has been measured in three cores of the fast zero power reactor FRO. The variance-to-mean method, in which the statistical fluctuations of the neutron density in the reactor are studied, was used. The experimental results were compared with theoretical values of β_eff calculated with perturbation theory. The difference was at most 10%, which should be com-pared to the accuracy in the experimental values of ≈3%.