1.0 Background
CANDU reactor pressure tubes carry nuclear fuel and belong to the primary heat transport system. They are fabricated from cold-worked Zr-2.5Nb alloy prone to hydrogen ingress under normal operating conditions. Increasing hydrogen concentration and thermal transients during reactor operation develop a brittle phase, hydride, changing material behavior. The hydride downgrades fracture toughness properties towards brittle and uncontrolled response at the macro scale, and reduces material ductility.
CNSC seeks to investigate the parameters governing material fracture toughness properties and the load carrying capacity of Zr-2.5Nb pressure tubes with high hydrogen concentration in brittle and ductile-to-brittle transition regions. These properties are used to demonstrate leak-before-break and defense-in-depth if cracking would have initiated in a CANDU reactor core. Increasing hydrogen concentration in pressure tubes, with time, reduces fracture properties and increases the transition temperature for ductile behavior leading to an end of their life. In practical terms, normal operation of a nuclear reactor in the brittle or brittle-to-ductile transition region is undesirable. Operation in the brittle or brittle-to-ductile transition region is assessed to be undesirable as crack initiation in the pressure tube can lead to uncontrolled rupture.
Fracture toughness material properties affect CANDU reactor operating procedures by imposing requirements on the pressure-temperature operating envelope. In order to withstand a postulated or a service induced through-wall crack, and to demonstrate leak-before-break, water pressure during reactor cool down must be decreased to reduce a crack driving force. If the crack driving force is higher than the fracture toughness an uncontrolled pressure tube rupture may occur.
Investigating the fracture behavior of Zr-2.5Nb pressure tube material with high hydrogen concentration is necessary to assess the pressure tube life.
2.0 Objectives
The objective of this project is to model fracture behavior of Zr-2.5Nb pressure tube material with high hydrogen concentrations (50 – 120 ppm [H]eq) at CANDU reactor operating temperatures by analytical and/or numerical modeling.
This independent investigation will assist CNSC with implementing and improving new fracture toughness curves for pressure tube material with high hydrogen concentrations.
3.0 Scope of Work
The scope of work includes:
- Modeling the fracture process in Zr-2.5Nb pressure tube material with high hydrogen concentrations (50 – 120 ppm [H]eq) and determining governing parameters.
- Developing a model that predicts fracture toughness at different dissolved hydrogen concentrations, material microstructures and thermal histories.
- Interacting with investigators on a parallel experimental program to simulate the experimental setups.
- Validating the aforementioned models with experimental results.
4.0 Tasks to be Performed
1. Develop a detailed work plan that is subject to CNSC review and acceptance.
2. Perform parametric studies on the fracture behavior of a curved compact tension specimen (CCTS) [1] and a representative specimen used in pressure tube burst tests. These studies involve comparing the stress and deformation state, fracture toughness parameters such as
J-integral, crack tip opening displacement, crack mouth opening displacement and others found appropriate. The material properties applied are of homogeneous isotropic and anisotropic materials.
3. Identify differences, if any, between a CCTS and a pressure tube section used in burst tests affecting measured fracture toughness at the maximum load. Propose a methodology for the adjustment of fracture toughness measured from CCTS to a burst test specimen.
4. Model the mechanical behavior of pressure tube materials with hydrides (heterogeneous material) under a crack propagation process until uncontrolled fracture occurs. This may involve development of a new material model for numerical simulations. The models must have the capability to predict the fracture toughness of a pressure tube section with an axial through-wall crack in a burst test. Different hydrogen concentrations and material microstructures shall be considered.
5. Determine parameters describing the fracture toughness of pressure tube material with high hydrogen concentrations. Develop a methodology or a model to predict fracture toughness of pressure tube material with elevated hydrogen concentrations (50 ppm and 120 ppm [H]eq). Validate the predictions against experimental results, derived from a parallel experimental program team commissioned by the CNSC.
6. Prepare and give a presentation summarizing findings, conclusions and recommendations.
