Swelling of guide tubes for safety rods in nonuniform fields of temperature and irradiation
The purpose of the present work is to investigate shape changes and void swelling in safety rod guide tubes of the reactors BN-350 and BOR 60 after operation for the scheduled life time. Post-irradiation measurements of diameters of the reactor BN-350 temperature compensator were carried out for two mutually perpendicular positions after 50-100 mm along the core height with accuracy of 0.1 mm. Sizes of the reactor BOR-60 safety tube were measured for various cross sections along the core height at edges and face middles. The swelling of the tube material was measured by an Archimedes technique.
As a result of examination it was established, that neutron irradiation of the guide tube of the temperature compensator rod in BN-350 reactor core to the maximum dose of 65 dpa has led to its significant curvature (the maximal bending deflection is equal to 15 mm) and to diameter changes. Near the core center the tube diameter has increased in one of the directions to 89 mm from the initial value of 84 mm. In the perpendicular direction the tube diameter has decreased up to 82 mm, i.e. below initial value.
The operation of the safety rod guide tube in the BOR-60 reactor core to the maximum dose of 120 dpa has led to a significant increase of the hexagonal tube size between opposite edges and to a reduction of the flat-to-flat size at the tube face middle.
The change of safety rod guide tube sizes occurs due to a non-uniform swelling of the material (austenitic stainless steels EP-150 and 18Cr-9Ni) along tube wall height, perimeter and thickness. The swelling gradient leads to occurrence of significant internal stresses, under action of which a further deformation of the guide tube occurs in the form of initial size reduction due to irradiation creep. The high level of residual stresses in a combination with a complete brittleness of the tube material can lead to the tube failure even in the absence of any external loadings.
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