In the realm of civil engineering, the assessment of concrete structures in nuclear power plants (NPPs) exposed to long-term neutron irradiation is essential for maintaining safety and reliability, especially as the operational lifespan of these facilities is extended from 40 to 80 years. Neutron irradiation leads to significant challenges, including the volumetric expansion of concrete aggregates and a decline in mechanical properties, which adversely affect the structural integrity and protective functions of concrete used in safety-critical applications, such as biological shielding.
Current research in this area faces notable limitations, with available experimental data on nuclear-irradiated concrete often being outdated, inadequately documented, or insufficient in quantity. Furthermore, many existing numerical models do not adequately represent the complex multiphase, multiscale characteristics of concrete or the intrinsic mechanisms of damage induced by irradiation at a microscopic level.
To tackle these issues, researchers Yuxiang JING from the Central Research Institute of Building and Construction Co., Ltd. MCC Group, Inspection and Certification Co., Ltd. MCC Group, and University of Colorado Boulder, along with Yunping XI from the University of Colorado Boulder, have conducted a study titled “Modeling the Mechanical Behavior and Damage Development of Concrete Materials under Neutron Irradiation.” This research introduces a comprehensive multiscale theoretical framework aimed at predicting the deformation and deterioration of concrete”s mechanical properties due to prolonged neutron irradiation.
The framework is designed with several core elements:
- Multiscale and multiphase characterization: The study categorizes concrete into four levels (concrete, mortar, cement paste, clinker) and treats it as a multiphase composite. The Generalized Self-Consistent (GSC) model is utilized to assess the mechanical properties of concrete and mortar at the mesoscale, while the Mori–Tanaka model addresses the microscale properties of cement paste and clinker. Properties derived from lower scales are utilized as inputs for composite analysis at higher scales, allowing for a comprehensive evaluation of contributions from all constituent phases.
- Incorporation of multiple degradation mechanisms: The model integrates various effects stemming from neutron irradiation, including radiation-induced volumetric expansion of aggregates and thermal strains resulting from radiation heating, along with drying shrinkage of cement paste primarily caused by dehydration and gamma-ray radiolysis.
- Composite damage mechanics for damage assessment: Using the Drucker–Prager plastic failure criterion, the model quantifies damage in cement paste due to volume discrepancies between expanding aggregates and shrinking cement paste. Damaged cement paste is modeled as a composite of “fully damaged” and “intact” phases, with the volume fraction of the damaged portion used to characterize overall damage.
The validation of this model was performed using experimental data from Con-A concrete samples irradiated at the Kjeller JEEP-II reactor, which experienced a fast neutron fluence ranging from 7.09×1018 to 9.62×1019 n/cm2 (E>0.1 MeV). The findings indicated a strong correlation between the predicted values and experimental results concerning concrete dimensional changes and variations in Young”s modulus.
Further parametric analyses uncovered several insights: an increased water–cement ratio diminishes cement paste damage under low neutron fluence but accelerates the degradation of elastic modulus under high fluence. Conversely, a higher aggregate volume fraction slightly lessens cement paste damage and slows the reduction of elastic modulus, yet it leads to increased overall concrete expansion. The rate of aggregate expansion emerged as a critical factor, with slower and smaller expansions resulting in reduced damage to cement paste and less degradation of concrete.
The full text of the open-access paper is available at this link.
