Analytical Study of Criticality Experiments of Organic and Light Water Moderated Mixed Oxide Fuel Pin Arrays

not registered; Fukumura, Nobuo*; not registered; Koyama, Tomozo; M.J.Hai*

As part of a joint criticality data development between the power peactor and Nuclear Fuel Development Corporation (PNC) of Japan and the United State Department of Energy (USDOE), critical experiments have been conducted with organic moderated Fast Test Reactor (FTR) mixed oxide fuel pin arrays. The neutronic characteristics of an organic moderator can be examined by comparing the results of these experiments with the results of the same type of experiments performed with light water moderated system. In recent experiments performed at the Battelle Pacific Northwest Laboratories Critical Mass Laboratory, five distances of the lattice pitches were utilized which span from soft to hard neutron energy spectra. Results obtained by benchmark analyses of these experiments are discussed in this paper. The benchmark analysis was performed using the coupled WIMS-CITATION computer code whose accuracy has been demonstrated by applying them to heavy water reactor (ATR) with cluster type fuel. The scattering kernel used in the calculation for hydrogen atoms in light water was based on Nelkin's model. This same scattering kernel was used for hydrogen atoms in the organic moderator. The agreement between calculation and experiment (k-eff=1) for the light water moderated cores are fairly good (less than 0.2% k/k). On the other hand, k-eff's for the organic moderated core show a tendency to diverge from k-eff=1 gradually, as the lattice pitches become larger; the approximation used to calculate the scattering kernels for the organic moderator becomes less accurate. It is theorized that small difference in chemical bonding between light water and organic moderator account for the increasing deviation from k-eff=1 for the organic moderators, the larger the fuel pin lattice pitch. Future work would be necessary to establish a more accurate scattering kernel model of the organic moderator to precisely calculate critical conditions for organic moderated fuel pin ...