Facebook Instagram Youtube Twitter

Loss of Tightness of Fuel Cladding – Fuel Reliability

Nuclear Fuel - TemperaturesFuel cladding is the outer layer of the fuel rods, standing between the reactor coolant and the nuclear fuel (i.e., fuel pellets). It is a corrosion-resistant material with a low absorption cross section for thermal neutrons (~ 0.18 × 10–24 cm2), usually zirconium alloy. Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. Cladding constitutes one of the barriers in the ‘defense-in-depth ‘ approach, and therefore its coolability is one of the key safety aspects.

Loss of Tightness of Fuel Cladding – Fuel Reliability

Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. The emergence of a leak in that cladding results in:

  • the transport of specific chemical elements (fission products) that are stable and radioactive (iodine, xenon, krypton…) into the reactor’s primary circuit
  • deposits of long-lived isotopes (cesium, strontium, technetium…), or even, in exceptional circumstances, of alpha emitters onto the piping of the primary circuit or of ancillary circuits
  • an increase in the overall level of irradiation for that circuit from the level already due to activation products (corrosion products, e.g., cobalt, chromium, iron in particular)

A leak thus poses a major challenge in operational terms for a power plant operator since it directly affects the radiological exposure workers are subjected to when running the plant or carrying out maintenance. Although fuel failures have rarely been a safety-related issue, their impact on plant operational costs due to:

  • premature fuel discharge,
  • following cycle shortening,
  • possible unscheduled outages,
  • increased spent fuel volume

One of the necessary steps to reach the zero defect goal is to understand the root causes of the failures and their mechanisms so that some corrective actions can be implemented, either through improvements in fuel design and fabrication by fuel suppliers or operational changes, such as reduced power maneuvering.

Special Reference: CEA, Nuclear Energy Division. Nuclear Fuels, ISBN 978-2-281-11345-7

Fuel Failure Mechanisms

Various fuel failure root causes have been identified in the past, and these causes were predominantly fabrication defects or fretting in the early days of PWR and BWR operations. The following list is not complete, and there are also failure mechanisms that are typical for certain reactor and fuel designs. It must also be noted that many fuel failure causes were never identified and remain unknown.

  • Fretting. Fretting was one of the main failure mechanisms in the early dates of PWR and BWR operations, and it typically has two variants.
    • Debris fretting. Debris fretting can be caused by any debris (foreign material – usually metallic) that can enter the fuel bundle and potentially become lodged between the spacer grid and a fuel rod. Fretting wear of fuel cladding can result in cladding penetration.
    • Grid-to-rod fretting. Grid-to-rod fretting arises from the vibration of the fuel element generated by the high coolant
      velocity through the spacing grid. Spacing grids are welded onto the guide tubes and ensured, using springs and dimples, fuel rod support, and spacing. High coolant velocity can cause the rod to rub against the part of the spacer grid
      that holds it. This type of cladding wear can be minimized by properly designing the spacing grid. Baffle-jetting is usually grouped under grid-to-rod fretting.
  • Pellet-cladding interaction (PCI). Failures due to PCI are typical for power changes, rod movement, and plant startup. They usually occur within a few hours or days following a power ramp or control rod movement, which results in startup ramp rate restrictions.
  • Dryout. In BWRs, when the heat flux exceeds a critical value (CHF – critical heat flux), the flow pattern may reach the dry-out conditions (a thin film of liquid disappears). The heat transfer from the fuel surface into the coolant deteriorates, resulting in a drastically increased fuel surface temperature. This phenomenon can cause the failure of an affected fuel rod.
  • Fabrication defects
    • End-plug weld defects.
    • Cladding creeps collapse. Cladding collapse can be caused by densifying the fuel pellets forming axial gaps in the pellet column, resulting in collapse from outer pressure. Since creep is time-dependent, full collapse typically occurs at higher burnup. This type of failure can be eliminated using pellets with moderate densification and pre-pressurization of rods.
    • Missing pellet surface
  • Internal Hydriding. Inadvertent inclusion of hydrogen-containing materials inside a fuel rod can result in hydriding and thus embrittlement fuel cladding. Hydrogen sources were mainly residual moisture or organic contamination in fuel pellets/rods. This cause of failure has been practically eliminated through improved manufacturing.
  • Crud-induced corrosion. Crud-induced corrosion failures are either due to abnormally high heat flux exceeding heat flux or burnup corrosion limits or to water chemistry problems leading to excessive crud deposits. In BWRs, crud-induced corrosion was one of the major causes of fuel failure in the 1980s.
  • Delayed hydride cracking (DHC). Delayed hydride cracking is time-dependent crack initiation and propagation through fracture of hydrides that can form ahead of the crack tip. This type of failure can be initiated by long cracks at the outer surface of the cladding, which can propagate in an axial/radial direction. This failure mechanism may potentially limit high burnup
  • Fuel handling damages

See also: IAEA, Review of fuel failures in water-cooled reactors. No. NF-T-2.1. ISBN 978–92–0–102610–1, Vienna, 2010.

See also: Stress-corrosion Cracking

See also: Hydrogen Embrittlement

Nuclear and Reactor Physics:
      1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
      2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
      3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
      4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
      5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
      6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
      7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
      8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

      1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
      2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
      3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
      4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Fuel Cladding