Anatech - Linking Theory and Practice.

The recent advent of several key behavioral issues in nuclear fuel performance, resulting from the trend toward increased fuel utilization, has created the need for enhanced steady state and transient fuel rod analysis capabilities. These issues, derived primarily from burnup-induced pellet changes and increased hydrogen uptake (as a byproduct of corrosion) occurring at higher burnups, impact steady state operation and licensing issues such as postulated reactivity initiated accidents (RIA), loss-of-coolant accidents (LOCA), and spent fuel storage and transportation. In response, EPRI has undertaken the development of FALCON, an integrated steady state and transient fuel performance analysis code applicable to the full range of fuel operating regimes. The FALCON development program has emphasized the use of advanced thermo-mechanics in a fully two-dimensional finite element continuum framework. Based on the architecture of the FREY transient analysis code and including salient models from the ESCORE steady state analysis code, FALCON provides enhanced steady state and transient numerics, updated material property and behavioral models, and an expanded and relevant light water reactor (LWR) benchmarking and verification database. This paper will present an overview of the FALCON steady state analysis development program with an emphasis on the results from the benchmarking, verification, and validation activities.

The design rules for reactor containments and similar safety structures in nuclear power plants have been established decades ago based on their initial material and structural conditions, and design-based assumptions. However, as these plants begin to approach the end of their original design life and transition into the life-extension phase, analytical methods are needed to evaluate potential aging-related effects. In the case, of pre-stressed concrete containments, for example, the concrete is subject to long-term creep in the presence of time-varying loads due to tendon relaxation and possible re-tensioning, and service- and weather-dependent thermal cycling. Also, some PWR plants are facing the prospect of replacing steam generators, which require the cutting of large opening in the pre-stressed containment. Performing structural modification for such purposes should be guided by conducting detailed numerical simulations to avoid introducing new, or obscuring pre-existing, damage. This requires the use of qualified, well-validated analytical methodology with robust material-behavior representations of damage states, including the ability to model pre-existing cracks and other service-induced damage conditions. Such a methodology has been developed considering a wide range of structural behavior and service conditions. This paper describes the behavioral modeling attributes of this methodology and its application to the diagnostic evaluation of the effects of long-term service and environmental conditions on concrete infrastructures in nuclear power plants, including structural retrofitting and rehabilitation of reactor containments, and proof of performance for life-extension service.

Reinforced concrete, despite its apparent simplicity, presents one of the most difficult problems in constitutive modeling and numerical simulation. The difficulties in constitutive modeling of concrete structures stem from the fact that cracks can form at relatively low stress levels. This leads to highly complex interaction between postcracking behavioral regimes, which include: multi-directional cracking, crushing, shearing along rough crack surfaces and the consequential formation of rebar-reacted compression normal to crack surfaces. One observable manifestation of this interaction is the potential for split cracking parallel to the compressive stress field imposed by the tendon in a pre-stressed containment, especially if the tendons are located close to the external surface of the containment wall where concrete confinement is small. Such split cracking could grow with time and may lead to wall delamination, with potentially serious consequences for the containment’s design-basis function.

The ageing-dependent constitutive properties of the concrete material considered are: tensile strength, postcracking shear behavior, and compressive strength, with both, hardening effects due to the maturity of concrete compressive strength and softening effects due to environmental degradation, temperature, internal micro-cracking and past-loading-related macro cracks. A robust analytical treatment of these properties in a general three-dimensional stress-strain constitutive formulation that recognizes the interaction with the reinforcement is fundamental to the degree of fidelity of the predicted response to true physical behavior.