Inert medium (helium) irradiation testing of pressure tube samples

Irradiation tests currently performed in C-5 capsule aim at obtaining data and information concerning behavior to irradiation of pressure tubes of CANDU type fuel channel, to evidence the factors limiting operation life span. A calculation code for analysis and prediction of pressure tube behavior should be based upon periodical inspection results, post irradiation examination of the removed from reactor pressure tubes as well as on the experimental results obtained with materials subjected to irradiation conditions identical with the operational ones. Mechanical behavior analysis should focus both complex thermal-mechanical type stresses and mechanical properties alteration under irradiation. The experimental results should be applied: - to evaluate the irradiation effects upon mechanical properties of Zr-2.5% Nb exposed to fluences up to 1021 n·cm-2; - to gather data concerning the real stress / real deformation characteristic from which characteristic quantities can be deduced as, for instance, elasticity modulus, plasticity modulus, exponent of stress term in the Tsu-Berteles relation, to be used within the CANTUP simulation code describing pressure tube behavior, currently developed at INR Pitesti; - to develop prediction methods of pressure tube behavior and merging with in-service inspection procedure in order to forecast the life span and the proper timing for replacement before major failures occur. The samples irradiated in C-5 capsule were extracted from the ends of Zr-2.5% Nb pressure tubes resulting from Cernavoda NPP Unit 1. The samples for tensile tests were extracted on longitudinal and transversal directions of the pressure tube. The tests were carried out under following conditions: - test environment temperature, 260 - 280 deg.C; - testing medium, helium at 1 - 6 b pressure; - neutron flux (En > 1 MeV), 1 - 2 · 1013 ncm-2s-1; - neutron fluence (En > 1 MeV), 4 · 1020 ncm-2. The following characteristics were obtained from tensile test: - real stress/real deformation characteristic; - flowing limit; - rupture strength; - rupture elongation; - elasticity modulus; - elasticity strength; - Tsu-Berteles strength exponent. Procedures for samples' temperature, thermal neutron flux and fast neutron fluence measurements are presented. After cooling the samples were analyzed by microstructure metallographic methods to obtain information about grain size on both longitudinal and transversal directions in the oxide layers and, possibly, hydrides