[en] Permanent deformation and fracture of subassembly ducts due to potential pressure pulses generated by the discharge of fission gas from fuel elements are important considerations in determining the burnup limit of fuels in LMFBR's. This paper discusses the results of a series of dynamic pressure tests that were conducted on intentionally defected irradiated EBR-II subassembly ducts. The tests were designed to simulate failure of a fuel element in a cluster of such elements. The major objective of the tests was to provide complete assurance that cladding failure of a driven-fuel element, which might conceivably cause rapid release of stored fission gas, would not deform or otherwise damage subasembly ducts in a way that would hinder movement of a control rod. The test program described provides a reliable method of assessing the failsafety of LMFBR ducts. The test procedure is particularly important since present analytical solutions cannot adequately model the subassembly and the reactor conditions. (Auth.)