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Zirconium Metallurgy: Management, Testing & Research, Fracture Testing, Metallography

Zirconium metallurgical analysis

Zirconium Metallurgy - Management of Pressure Tube Degradation

The pressure tubes in CANDU nuclear reactors are made from extruded and cold worked Zr-2.5%Nb.  Each core is composed of 380 or 480 individual pressure boundaries which contain the fuel and coolant.

Fuel bundle vibration and debris in the primary heat transport system can lead to flaws on the inside surface of the pressure tube. Problems arise because the hydrogen content of the pressure tube increases over time (from exposure to the coolant) and after the solubility limit is exceeded, brittle zirconium hydrides may precipitate at flaws.

To ensure safe operation, stations must demonstrate fitness for service with the presence of such flaws. An important part of this process involves material testing at bounding conditions with characterization and modeling of the resulting behavior.

Facilities for Zirconium Alloy Testing and Research

To simulate conditions during reactor heat up and cool down, samples must undergo temperature cycles under applied load. Four stepper motor driven load frames (500 lb.) are available, configured for tensile specimens.

These units are also used for material testing (KIH and overload) using custom software. Two servo motor driven load frames (10,000 lb.) are used for overload testing in bending and tensile testing of finite length surface flaws. One servo hydraulic load frame (250 kN) is available for fatigue testing and one servo motor driven load frame (100 kN) is used for overload testing of ring specimens with finite length surface flaws. All load frames are equipped with environmental chambers rated to 400°C. A miniature horizontal load frame (5 kN) designed for operation inside the SEM vacuum chamber is used for in-situ characterization of material behavior.

Fracture Testing

The precipitation of zirconium hydrides makes the alloy susceptible to a sub critical crack growth mechanism known as Delayed Hydride Cracking (DHC) and several kinds if cracking tests are performed at Kinectrics. Specimens with high precision simulated flaws are machined on site and preconditioned to form hydrides. In some tests, the hydrides are formed in-situ. Both acoustic emission and potential drop are used to monitor crack initiation, and crack velocity can also be measured.

Overload testing, similar to uniaxial tensile testing, is conducted on notched specimens with pre-formed hydrides. In this case, the desired quantity is the amount of load sustained by the sample above at which the hydride was formed.

Metallography

Examination of the sample grain structure near the crack tip as well as the morphology of the hydride precipitate can often explain the behavior of the test sample in the tests described above.

Samples are mechanically polished and chemically etched or anodized to reveal the grain structure or hydride morphology, respectively.  Samples are routinely examined using optical microscopes up to 1000x. Samples which require more detailed information about the morphology of the hydride as well as a higher magnification, commonly 5000x to 10000x, are mechanically polished and subsequently electropolished for examination in a Scanning Electron Microscope (SEM).

The SEM is also equipped with an Electron Backscatter Diffraction (EBSD) camera for microtexture determination and Energy Dispersive X-ray (EDX) in chemical analysis.

About This Solution

Industry Sector

  • Generation

Generation Type

  • Nuclear

Nature of Service

  • Inspection
  • |
  • Testing - Field Services

Client Assets

  • Pressure Tubes
  • |
  • Fuel Channels

Kinetrics Facilities

  • Materials Testing and Structural Integrity

Tags

  • Fuel Channels
  • |
  • Metallurgical Analysis
  • |
  • Zirconium Metallurgy
  • |
  • Aging