One of the most serious metallurgical problems and one that is a major concern in the nuclear industry is stress-corrosion cracking (SCC). Stress-corrosion cracking results from the combined action of an applied tensile stress and a corrosive environment, both influences are necessary. SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks that are subject to corrosion, which are then corroded further, weakening the metal by further cracking. The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching. Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile. SCC can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments.
See also: Fracture Toughness
The most effective means of preventing SCC in reactor systems are:
- designing properly
- reducing stress
- removing critical environmental species such as hydroxides, chlorides, and oxygen
- avoiding stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated.
Stress-corrosion cracking may cause, for example, a failure of nuclear fuel rod after inappropriate power changes, rod movement and plant startup. Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides and mild steel cracks in the presence of alkali (boiler cracking). Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions. Nickel-based alloys, however, are not effected by chloride or hydroxide ions. An example of a nickel-based alloy that is resistant to stress-corrosion cracking is inconel.
Special Reference: U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Stress-corrosion Cracking as Fuel Failure Mechanism
Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. There are various fuel failure root causes, that have been identified in past. In the early dates of PWR and BWR operations, these causes were predominantly fabrication defects or fretting. One of possible causes is also the pellet-cladding interaction (PCI), which may be caused by stress-corrosion cracking. Stress-corrosion cracking may cause, for example, a failure of nuclear fuel rod after inappropriate power changes, rod movement and plant startup.
In nominal operating conditions, pellet temperature stands at about 1,000° C at the center, 400–500° C at the periphery. In the event of a major increase in power, temperature at the pellet center rises steeply (> 1,500° C, or even > 2,000° C). In this case, a difference in thermal expansions between fuel cladding and fuel pellets causes an increase in stress in the fuel cladding. PCI fuel failure is caused by stress-corrosion cracking on the inside surface of the cladding, which results from the combined effects of fuel pellet expansion (especially at pellet radial cracks and the presence of an aggressive fission product environment (especially gaseous iodine). Such a failure is found to occur, experimentally, after from one to a few minutes’ holding time, at sustained high power levels.
Special Reference: CEA, Nuclear Energy Division. Nuclear Fuels, ISBN 978-2-281-11345-7