SAFE: Safe LTO in the context of environmental effects on fracture, fatigue & EAC
The SAFE-I (2012 – 2014) and SAFE-II (2015 – 2017) projects are supported by the Swiss Nuclear Safety Inspectorate ENSI and aim to fill selected important knowledge gaps in the field of environmentally-assisted cracking (EAC) and environmental effects on fatigue and rapid fracture in safety-relevant pressure boundary components in the primary coolant circuit of light water reactors.
Pressure boundary components in the primary coolant circuit of LWRs are made of low-alloy and stainless steels and are very critical components with regard to safety and lifetime. During service, toughness and ductility of these materials can decrease with time, due to irradiation induced embrittlement (RPV only), thermal ageing or potential environmental effects. Under simultaneous effect of the reactor coolant, thermo-mechanical operational loads and irradiation, cracks can initiate and grow by EAC and thermo-mechanical fatigue (TMF), which finally could lead to a large leak or component failure. Several EAC and TMF cracking incidents occurred in LWRs in the last four decades. Critical components are thus periodically inspected by non-destructive examination to detect defects before they reach a critical size necessary for rapid fracture.
An accurate knowledge on the degradation of the toughness and fracture properties of these materials during service and of the system conditions which may lead to EAC initiation and growth is thus evidently indispensable to ensure the safe and economic long-term operation in this context. Reliable quantitative experimental data on these phenomena and a basic knowledge on the underlying mechanisms are essential to evaluate their possible effects on structural integrity/safety and lifetime of components, to identify critical component locations/operating conditions and to define and qualify possible mitigation, repair and maintenance actions.
Within the various sub-projects of SAFE-I & SAFE-II the following unexplored aspects and concerns are currently evaluated in this field:
Pressure boundary components in the primary coolant circuit of LWRs are made of low-alloy and stainless steels and are very critical components with regard to safety and lifetime. During service, toughness and ductility of these materials can decrease with time, due to irradiation induced embrittlement (RPV only), thermal ageing or potential environmental effects. Under simultaneous effect of the reactor coolant, thermo-mechanical operational loads and irradiation, cracks can initiate and grow by EAC and thermo-mechanical fatigue (TMF), which finally could lead to a large leak or component failure. Several EAC and TMF cracking incidents occurred in LWRs in the last four decades. Critical components are thus periodically inspected by non-destructive examination to detect defects before they reach a critical size necessary for rapid fracture.
An accurate knowledge on the degradation of the toughness and fracture properties of these materials during service and of the system conditions which may lead to EAC initiation and growth is thus evidently indispensable to ensure the safe and economic long-term operation in this context. Reliable quantitative experimental data on these phenomena and a basic knowledge on the underlying mechanisms are essential to evaluate their possible effects on structural integrity/safety and lifetime of components, to identify critical component locations/operating conditions and to define and qualify possible mitigation, repair and maintenance actions.
Within the various sub-projects of SAFE-I & SAFE-II the following unexplored aspects and concerns are currently evaluated in this field:
- Environmental effects on fracture toughness and tearing resistance of RPV steels
- Stress corrosion cracking in Alloy 182 dissimilar metal welds
- Stress corrosion cracking initiation in Alloy 182 weld metal
- Environmental effects on fatigue in stainless steels