Two loss-of-coolant accident (LOCA) simulation tests were conducted to investigate the effects of temperature uniformity and radial restraint boundary conditions on Zircaloy cladding deformation with a very low flow of superheated steam. In one test (B-5), boundary conditions typical of a large array were imposed on an inner 4-by-4 square array by two concentric rings of interacting guard fuel pin simulators. In the other test (B-3), the boundary conditions were imposed on a 4-by-4 square array by a noninteracting heated shroud. Test parameters conductive to large deformation were selected to favor rod-to-rod interactions.

The tests showed that rod-to-rod interactions play an important role in the deformation process. While burst temperatures and burst strains were not affected appreciably, the interactions in the large array significantly influenced the deformation patterns and caused greater volumetric expansion of the interior simulators. Volumetric expansion of the simulators in the small array was significantly less than that for the central array of the large test; burst strains were approximately the same. Burst temperature-pressure data for the small array and for the outer ring of simulators in the large array agreed with predictions based on single-rod heated shroud tests. All the B-5 simulators burst at approximately the same temperature; however, the interior simulators burst generally at much lower pressures than the exterior ones, and their performance did not agree as well with the prediction.

It was concluded that, for conditions conductive to large deformation, at least two concentric rings of deforming guard simulators are necessary to model radial temperature and mechanical boundary effects of large arrays in LOCA simulation tests and that flow area restriction may be underestimated by small unconstrained bundle tests.