Fuel 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 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 level of 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
Debris Fretting – Grid-to-rod Fretting
Various fuel failure root causes have been identified in the past. These causes were predominantly fabrication defects or fretting in the early days of PWR and BWR operations. 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.
See also: IAEA, Review of fuel failures in water-cooled reactors. No. NF-T-2.1. ISBN 978–92–0–102610–1, Vienna, 2010.