Local fault situations in LMFBR cores may produce severe pressure pulses within one fuel element. The fact cannot be ignored that these pressures can have peaks and impulses that may expand and rupture the wrapper around the element. This will impulsively load the surrounding subassemblies and possibly the control rods due to extreme coolant pressure gradients and/or subassembly collision forces. Fast reactor safety requires this mechanical propagation process through the core to be analyzed, and therefore appropriate models and solution methods are needed to simulate the nonlinear structural dynamics of one typical hexagonal fuel element. The aim of this paper is to outline one- and two-dimensional structural models and discuss their capabilities and suitability for multirow core calculations. For this purpose static and impulsive single subassembly loading experiments are described and typical results are reported and compared with numerical predictions. A short description is given of relevant physical effects, for instance the interaction of two distinct deformation modes of the externally-loaded fuel element. The role of the fuel-pin bundle inside the wrapper is briefly mentioned, and it is shown that the strongly nonlinear transient response is due to large elastoplastic deformation of the wrapper combined with a frictional distortion of the pin bundle. Special discrete models were developed which are characterized by lumped point masses connected by elastoplastic beams or nonlinear springs. A brief discussion of an experimental program is then given designed to verify theoretical models and underlying hypotheses. A drop-weight facility is used for transverse impulsive loading of natural size SNR-type fuel element models. Honeycomb crushing material is placed between the falling mass and the specimen to limit the peak force of the loading pulse. A simplified computer model has been set up describing the elastoplastic impact between the falling mass, the crushing material and the subassembly model, which is resting on a heavy, spring-suspended table. Finally, numerical prediction and experimental response data are presented for large, elastoplastic deformations of a specific reference case under two loading conditions: (1) a constant and low rate of deformation producing a quasi-static yielding response of an empty hexcan on a rigid plate; and (2) a controlled time-dependent impact force pulse (drop mass) producing typically dynamic responses.
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May 1, 1986
H D Yun + 1
