Swelling of safety rod guide tubes in nonuniform fields of temperature and irradiation

Safety rod guide tubes are important components of fast reactor cores for ensuring safe reactor operation. Their failure or considerable changes of their size may lead to safety rod wedging that is unacceptable. Two guide tubes, one each from BN-350 and BOR-60, were examined post-irradiation to determine the reasons for their deformation and loss of functionality. These tubes were constructed from high-nickel alloy EP-150 and austenitic 18Cr9Ni, respectively.It is found that various forms of deformation of safety rod guide tubes occur due to non-uniform swelling along the tube height, perimeter and across-wall thickness. The swelling gradients can lead to bowing and ovality, and can be accompanied by significant internal stresses within the tube material. The latter can lead to size reduction of guide tube dimension in some directions due to irradiation creep. High levels of swelling-induced residual stresses, in combination with a swelling-induced embrittlement of the tube material, can lead to the tube failure even in the absence of any external loading.