Reactivity worths of plutonium--uranium clusters in a heavy-water-moderated assembly by the reactor oscillation method

The reactivity worths of synthetic plutonium-uranium clustered fuel elements in a heavy-water-moderated assembly have been experimentally determined using the reactor oscillation method.Several test-fuel compositions have been investigated, representing varying degrees of fucl burnup and burnup distributions; two uranium samples with different U enrichment have been used as standard.The technique selected was aimed to establish "clean" experimental conditions, in order to effectively simplify the analysis of the results. Basically, the technique involved oscillating, according to a square-wave pattern, a 6-m-long fuel element containing a 50-cm-high test section with the fuel composition to be investigated the corresponding neutron density modulation was interpreted in terms of a Fourier analysis.The results of the experiment form a consistent set of data that can be used as test values for refined reactor burnup calculation codes. The overall experimental error, typically ±0.015 pcm (1 pcm = 10-5 Δkeff/keff). is considered remarkably low in view of the massive experimental setup required.A method for the theoretical analysis of the measured reactivity worths is presented. A multigroup perturbation transport calculation in one dimension (S4 approximation) has been developed to account for the radial environmental conditions. The axial effects have been evaluated with a two-dimensional transport calculation. The group cross-section data used in the analysis were basically taken from the GAM-II and GATHER-II libraries.Using the same basic one-dimensional code with an appropriately adjusted input parameter, infinite lattice multiplication factors have also been calculated from he experimental reactivity results. These results are compared to the values of k∞ obtained from null-reactivity measurements of identical clusters which were performed in association with the Comitato Nazionale Energia Nucleare, Italy, in RB-1 Reactor in Bologna. The agreement between the two sets of results is satisfactory.