Material Degradation

Neutron fluence causes several material degradation mechanisms in reactor structures, systems and components (SSCs). Each degradation effect is slightly different in scope and scale and in highly dependent on the environmental conditions impacting the location of interest. A list of possible types of material degradation is provided below.

1. Radiation Embrittlement

Radiation embrittlement is the loss of ductility and fracture toughness in reactor pressure vessel (RPV) steels caused by long-term exposure to fast neutrons (E > 1 MeV) during reactor operation. This phenomenon is critical because the RPV is a non-replaceable component and must maintain structural integrity throughout the plant’s life.

2. Loss of Fracture Toughness

Fracture toughness is a material’s ability to resist crack propagation under stress. High fracture toughness means the material can absorb energy and deform plastically before fracturing. Low fracture toughness means cracks can grow suddenly and catastrophically. When RPV steels and internals are exposed to fast neutron irradiation over time the steel can experience:

• Atomic displacement - neutrons displace atoms and create material defects
• Microstructural changes - increases in hardness and decreased ductility
• Effects on toughness - increased susceptibility to brittle fracture

These effects can make components more brittle at lower temperatures, increase risk during cold shutdowns or pressurized thermal shock (PTS) events, and are critically evaluated under ASME Section XI and other NRC guidelines.

3. Swelling and Dimensional Changes

Swelling refers to the increase in volume of a material caused by radiation-induced damage during long-term exposure to fast neutrons. Dimensional changes include distortion, bowing or warping, or elongation of reactor internals and structural components. Core internals (baffle plates, bolts, fuel assembly components, core shrouds) experience more effects than RPV steels.

Swelling and dimensional changes occur when neutron fluence causes atomic displacement and void formation in reactor internals, leading to volumetric expansion and distortion that must be managed through aging management programs during extended operation.

4. Irradiation-Assisted Stress Corrosion Cracking

IASCC is a radiation-induced form of stress corrosion cracking in stainless steel reactor internals caused by neutron fluence altering material chemistry and microstructure, requiring aging management programs for detection and mitigation during extended operation.

Neutron irradiation is not the only mechanism to consider for IASCC. Considerations to environmental factors (water chemistry and temperature) and stress loads of components also contribute to increased risk of SCC resulting in cracking of structural components, a loss of pre-load in bolts, and long-term integrity concerns.

5. Hardening of Welds

Welds are of particular concern due to the higher impurities found in the weld materials (Cu, Ni, P) compared to base metals. These impurities can accelerate radiation-induced degradation over long-term neutron exposure.  Additional factors of increased stress at weld locations and load limits make welds locations a considerable concern during inspections.