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The fuel handling operating rules exclude any accidental risk. However in the framework of the PRECCI R&D project, the impact on a spent fuel rod is taken into account. This study was carried out in CEA laboratories with the funding and the technical support of EDF. Since fuel rods are slender structures, it is of particular interest to model them with beams in finite elements calculations. While the zircaloy behavioural law can be directly used for the plugs and plenum regions, an equivalent elasto-plastic law is necessary in the fuel height, especially in the case of high burnups since the contact between the pellets and the clad reinforce the contribution of the fuel to the rod mechanical behaviour. The equivalent law as well as the correlation between the clad and the equivalent plastic strains were established from CAST3M [1] finite element modelling of four point bending tests of fuel rods segments. This model, meshed with three dimensional elements, takes into account elasto-plastic behavior for the clad and cracking of the UO2 fuel pellets. Bending experiments on spent fuel rods performed at LCMI laboratory allowed to validate the equivalent law. The axial dynamic load impact test perform by Hirose et al [2] on PWR fuel rods specimens using 3 kg weight and impact speed in the range of 8 - 11 m/s were modelled with CAST3M. The experimental and calculated maximum loads are in excellent agreement. The deformed shapes as well as the observed order of bending (buckling) are also in good agreement. The maximum strains calculated in in the clad reaches the ultimate strain of zircaloy in agreement with the failures obtained experimentally. This study reinforce the confidence in the equivalent behavioural law. It moreover indicates that zircaloy behaviour for dynamics loadings can be modelled with the laws established at lower strain rates, as suggested by the moderate variation of the mechanical properties as a function of strain rates determined by dynamic tensile tests and reported in [3].
References:
[1] Verpeaux P, Millard A. et Charras T., CASTEM 2000 : Une approche moderne du calcul des structures, IN : J. M. Fouet, P. Ladeveze and R. Ohayon, Calcul des structures et Intelligence artificielle, pp. 261 -271. Pluralis, Paris, France, 1989. http://www-cast3m.cea.fr/
[3] T. Hirose, M. Ozawa et A. Yamauchi, ‘Fuel rod mechanical behavior under dynamic load condition on high burnup spent fuel of BWR and PWR'.
http://www-pub.iaea.org/iaeameetings/cn226p/Session7/ID75HIROSE.pdf
[3] Tsutomu Hirose, Masaaki Ozawa, Hiromichi Miura, Toshikazu Baba, Katsuichiro Kamimura Japan Nuclear Energy Safety Organization (JNES). RESEARCH ON INTEGRITY OF HIGH BURNUP SPENT FUEL UNDER LONG TERM DRY STORAGE AND TRANSPORT. NEA/CSNI/R(2013)10.
