Effect of Long-Term Irradiation on the Fracture Properties of Zr-2.5Nb Pressure Tubes

RR Hosbons; CE Coleman; PH Davies; M Griffiths; S Sagat

doi:10.1520/STP14298S

SYMPOSIA PAPER Published: 01 January 2000

STP14298S

Effect of Long-Term Irradiation on the Fracture Properties of Zr-2.5Nb Pressure Tubes

Source

Results from fracture toughness and tensile and delayed hydride cracking (DHC) tests on Zr-2.5Nb pressure tubes removed from CANDU power reactors in the 1970s and 80s for surveillance showed considerable scatter. At that time, the cause of the scatter was unknown and prediction of fracture toughness to the end of the design life of a CANDU reactor using the surveillance data was difficult. To eliminate the heat-to-heat variability and to determine end-of-life mechanical properties, a program was undertaken to irradiate, in a high-flux reactor, fracture toughness, DHC, and transverse tensile specimens from a single “typical” pressure tube. Two inserts were placed in the OSIRIS reactor at CEA, Saclay, in 1988. Each insert held 16 of each type of specimen. The first insert, ERABLE 1, was designed so that half the specimens could be replaced at intervals and the properties could be measured as a function of fluence. All the specimens would be removed after a total fluence of 15 × 10²⁵ n · m^-2, E > 1 MeV. The second insert, ERABLE 2, was designed to run without interruption to a fluence of 30 × 10²⁵ n · m^-2, the fluence corresponding to 30 years' operation of a CANDU reactor at 90% capacity factor. The irradiation temperature was chosen to be 250°C, the inlet temperature of early CANDU reactors. The irradiation of ERABLE 1 has been completed and sets of specimens have been removed and tested with maximum fluences of approximately 0.7, 1.7, 2.8, 12, and 17 × 10²⁵ n · m^-2, E > 1 MeV. X-ray and TEM examinations have been performed on the material from fractured specimens to characterize the irradiation damage. Results showed that there is, initially, a large change in the mechanical properties before a fluence of 0.6 × 10²⁵ n · m^-2, E > 1 MeV (corresponding to an initial rapid increase in a-type dislocation density), followed by a gradual change. As expected, the fracture toughness decreased with fluence, whereas the yield strength, UTS, and DHC crack velocities all increased. Z-ray analysis showed that, although the a-type dislocation density remained constant after the initial increase, the number of c-component dislocations showed a steady increase, agreeing with the behavior seen in the mechanical specimens. Because the flux in OSIRIS is different from that in a CANDU reactor, specimens were also irradiated in NRU, a heavy water moderated test reactor with approximately the same flux as a CANDU reactor, to fluences of 0.3, 0.6, and 1.0 × 10²⁵ n·m^-2, E > 1 MeV for comparison. These initial results show that, once past the initial transient, one can have confidence that there will be little further degradation with fluence, with the results from the NRU specimens being similar to those from OSIRIS.

Author Information

Hosbons, RR