France has relied on nuclear power more than any other nation for many years. The Messmer plan, initiated after the 1973 oil crisis, saw nuclear become France’s main source of power generation by the mid 1980’s. While the sheer scale of nuclear generation in the USA and China surpasses France’s, these much bigger nations remain far less reliant on nuclear power. To this day, no other nation has such a high share of nuclear power. Although several smaller countries now rival France by their share of power generation, none of these come close to the scale of France’s nuclear industry. When considering a future energy system that uses more nuclear, France is often the most obvious example. It was therefore a dramatic turn of events when 15 reactors were shut down due to concerns over stress corrosion cracking.
A report by the French civil nuclear regulator said: “In 2022, the number and duration of reactor outages were unprecedented. It was made worse by the detection, during periodic inspections, of a phenomenon of stress corrosion on the welds of the safety injection system, something never before encountered on the international pressurised water reactor (PWR) fleet.”
To put the shutdown in context, it affects more than a quarter of France’s 56 in-service reactors. It is also not the first time there have been multiple reactor shutdowns due to safety concerns. In 2016, 18 reactors were shut down as carbon content irregularities were found in some large steel forged components of the primary cooling circuit. Increased carbon content in the hardened shell of these components reduced their impact or fracture toughness, making them more vulnerable to fast and catastrophic failure. France is particularly vulnerable to fleetwide issues, as all its reactors are standardised on a single PWR design, in order to reduce construction costs.
The nuclear industry in France is dominated by two organisations. Électricité de France, usually referred to as EDF, is a fully state-owned company which owns and runs all of the reactors in France. In addition to the French fleet, it also runs the UK’s reactors. Autorité de Sûreté Nucléaire (ASN), referred to in English as the French Nuclear Safety Authority, is the independent authority that regulates nuclear safety and radiation protection on behalf of the French government.
STRESS CORROSION CRACKING
This metal failure mechanism is the combined effect of tensile stress and corrosion, leading to cracks which propagate. Although rare, it can lead to rapid degradation of a structure, often with no visual indication of corrosion, culminating in sudden and catastrophic failure. It is particularly dangerous since the corrosion naturally occurs where tensile stress is highest, and that is where the structure is most susceptible to brittle failure.
Tensile stress initially creates chemically active zones in the material which are much more susceptible to corrosion. Tensile stress promotes corrosion since the energy stored in the chemical bonds creates a more energetic environment. Cracks start when the protective oxide layer is ruptured, corrosion pits form, and cracks initiate at the base of the pits. As a crack starts to appear, a local stress concentration forms at the root of the crack. This therefore accelerates corrosion in this region. Tensile stress may be present in a component with no external loads due to residual stress from metalforming operations. It is therefore possible for significant stress corrosion cracks to form in an unloaded component. Cracks grow slowly until a critical length is reached, at which point brittle failure will occur. Although the final brittle failure could also occur without any external load, it is more likely this will happen when a load is applied.
Stress corrosion cracking requires a very specific chemical environment for a given material, as well as elevated temperature. While certain chemicals are required to create an environment for stress corrosion cracking, they may only need to be present at very low concentrations. The corrosive environment that causes stress corrosion cracking often doesn’t cause any significant general corrosion. The conditions may be quite different to those normally associated with corrosion in a particular material. This means that a metal component may appear bright and shinny, despite having many microscopic cracks which massively reduce its strength and ductility.
Mild steels typically experience stress corrosion cracking in the presence of alkali solutions (caustic stress corrosion cracking or boiler cracking) and nitrates. Stainless steels and aluminium alloys experience stress corrosion cracking in the presence of chlorides.
Periodic inspection of reactors in 2021 identified stress corrosion cracking in welds on stainless steel components of the safety injection system. This system is designed to flood the reactor with water to initiate an emergency shutdown. This would be required in the event that normal control methods have failed and the reactor is going supercritical. Failure of this system could therefore form part of a chain of events leading to a major nuclear disaster. The presence of stress corrosion cracking in this system is particularly worrying, since it could lead to a catastrophic failure of the system as it is activated, and therefore at the time when it is most desperately needed.
While stress corrosion has been previously seen in other components of French reactors, this was the first time it had been detected in stainless steel components, which are considered much more resistant to this type of damage. France’s investigation found that two types of reactor, its largest, were most at risk: N4 (such as at Chooz in the Ardennes) and P4 (such as Normandy’s Paluel).
EDF decided to opt for the systematic replacement of the lines considered to be more sensitive to the phenomenon on the 16 reactors likely to be the most severely affected, by the end of 2023.
Ultrasonic inspection is used to monitor cracks in the inner diameter of the pipes making up the safety systems. Periodic inspection was originally carried out due to a known risk of thermal fatigue cracking (stress corrosion cracking was not expected in these components).
STATE OF PLAY
Following the detection of stress corrosion cracking, EDF shut down the most at-risk reactors in May 2022 and started to replace the affected safety injection lines. Thirteen of the 16 originally identified reactors have now been repaired. However, additional stress corrosion cracking of components previously not thought to be at risk, as well as thermal fatigue, were identified in March 2023, and ASN asked EDF to revise its strategy for repairing the reactors. In April 2023 ASN approved a plan for all welds identified as a priority to be checked within a year. These additional checks are ongoing.
Stress corrosion cracking is a pervasive risk in boiler-based and hot water systems. The combination of difficulty of detection and the potential for sudden brittle failure makes SCC very dangerous.