Determination of the effective delayed neutron fraction for training reactor VR-1

This paper describes a methodology of effective delayed neutron fraction (βeff) determination for the VR-1 training reactor located at the Czech Technical University (CTU) in Prague. The methodology contains both experimental and computational elements. The experimental determination is based on a measurement of critical, zero-power reactor response to a periodical, low-worth reactivity insertion induced by fast periodical movement of a cadmium absorber in a special pneumatic system. From the power oscillations measurement, the reactor transfer function is determined using a genetic algorithm in the numerical program MATLAB. A final value of delayed neutron fraction is then determined directly from the amplitude of the transfer function. The measurements are verified by calculations using the stochastic MCNP5 code. The results of this calculation and measurement differ only by approximately 1.76% and indicate that the currently used value of βeff on VR-1 has been underestimated by more than 8%.