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Among the activities led by the Generation IV International Forum (GIF) relative  to the future nuclear systems, the improvement of recycling of fuel elements and  their components is a major issue. One of the studied systems by the GIF is the  graphite-moderated high-temperature gas cooled reactor (HTGR). The fuel elements  are composed of fuel roads half-inch in diameter named compacts. The compacts  contain spherical particles made of actinide kernels about 500 m in diameter  coated with three layers of carbon and silicon carbide, each about 50 m thick,  dispersed in a graphite matrix. Recycling of compacts requires first a  separation of triso-particles from the graphite matrix and secondly, the  separation of the triso-coating from the kernels. This aim may be achieved by  using pulsed currents: the compacts are placed within a cell filled by water and  exposed to high voltage between 200 – 500 kV and discharge currents from 10 to  20 kA during short laps of time (about 2 µs) [1-2]. This repeated treatment leads  to a progressive fragmentation of the graphite matrix and a disassembly of the  compacts. In order to improve understanding of the fragmentation properties of  compacts a series of quasi-static and dynamic experiments have been conducted  with similar cylindrical samples containing 10% (volume fraction) of SiC  particles coated in a graphite matrix.  First, quasi-static compression tests have been performed to identify the  mechanical behaviour of the material at low strain-rates (Fig.1). The  experiments reveal a complex elasto-visco-plastic behaviour before a brittle  failure. The mechanical response is characterised by a low yield stress (about 1  MPa), a strong strain-hardening in the loading phase and marked hysteresis-loops  during unloading-reloading stages. Brittle failure is observed for axial stress  about 13 MPa. In parallel, a series of flexural tests have been performed with  the aim to characterise the quasi-static tensile strength of the  particulate-graphite and the corresponding standard deviation. The behaviour  being non linear before failure, a numerical simulation has been conducted to  build the relation between the applied load and the maximum tensile stress. A  statistical approach applied to experimental data allows deducing the mean  tensile strength (about 2.5 MPa) and the scatter of failure stresses (Weibull  modulus m = 12)

Experimental approach and modelling of the mechanical behaviour of graphite fuel elements subjected to compression pulses