Measurement of the Cadmium Ratio in Esfahan Heavy Water Zero Power Reactor and Comparison of the Values Obtained in Two Different Pitches

Measurement of cadmium ratio is one of the basic parameters in nuclear reactors. In this paper, the amount of cadmium ratio for the Esfahan Heavy Water Zero Power Reactor (EHWZPR) in two pitches of 18 and 20cm have been measured. For the cadmium ratio measurements Mn-Ni foils were used. The activity of the Mn-Ni foils were obtained by plastic scintillator and NaI(Tl) detectors for beta and gamma rays. Also, changes of the cadmium ratio in the axial direction of the reactor, asymptotic region or stable spectrum were obtained. The length of stable spectrum of the reactor increased to approximately 19cm, in pitch of 20 to 18, due to the reduced number of fuel rods and critical height of heavy water. To verify the measured values, the Esfahan Heavy Water Zero Power Reactor was simulated with the MCNPX-2.6.0 code. The  results of simulation were in good agreement with those of the experiments.