Zr-2.5 Nb pressure tubes in CANDU reactors are susceptible to delayed hydride cracking (DHC), which is a sub-critical cracking process requiring hydrogen diffusion to a stress concentrator, precipitation, growth and fracture of hydrides. Service-induced and manufacturing flaws are present in some pressure tubes and these flaws may act as crack initiators. An engineering approach has been developed to assess the susceptibility of flaws to DHC. In this methodology, DHC is separated into the initiation and growth stages, and in terms of initiation, flaws are classified as blunt, sharp or crack-like. The experiments performed in this thesis are related to crack-like flaws, which are assessed in terms of the threshold stress intensity factor, K1H, below which DHC cannot occur. There is a large scatter in the overall KIH data base and a lower bound value is conservatively used for flaw assessment. Systematic studies on un irradiated Zr-2.5 Nb pressure tube material have shown that KIH increases with decreasing hydrogen in solution, increasing deviation from the radial-axial plane of the tube, and increasing temperature, while thermal cycling has no significant effect on K1H. Therefore, it may be justifiable to use higher KIH values for assessing flaws with known orientation, hydrogen concentration at the flaw location and operating thermal history. If crack initiation is postulated, as part of a defence-in-depth approach, crack growth is assessed under two scenarios. (1) When the hydrogen concentration is sufficient for cracking to continue under sustained hot conditions, a leak-before-break assessment is performed. DHC velocity is required to determine the time for a crack to grow to the critical crack length for unstable fracture. This thesis shows that crack velocities at different temperatures depend strongly on the thermal history, which affects the hydrogen concentration in solution. Crack velocity increases with increasing hydrogen in solution. In addition, hydrogen supersaturation is required for cracking to occur at the reactor operating temperatures of 2S0-31O°C. (2) When the hydrogen concentration is insufficient for cracking to occur at normal operating temperatures, cracking can only occur during reactor cool-down when hydrides can precipitate as a result of the lowering of temperature. The amount of postulated crack growth per cool-down cycle depends on the crack initiation temperature during cooling. This thesis shows that the crack initiation temperature decreases with increasing cooling rate, and by applying a load-reduction of 20% prior to cooling. Cracking during cooling can be suppressed altogether by allowing the crack tip stress to relax by creep, followed by a load reduction of 15-20%. Recommendations are made regarding reactor loading and thermal history which can reduce the propensity for DHC. From the observations on hydride morphologies and fracture surface features of the DHC cracks under different test conditions, evidence is presented which supports the hydride/stress interaction diffusion model. The observations also demonstrate the inadequacies of the critical length criterion for fracture of a hydrided region.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:629567 |
| Date | January 1998 |
| Creators | Shek, Gordon Kai-Wah |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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