Evaluation of Tendon Anchorage Zone Stresses in Prestressed Concrete Nuclear Containment Using Detailed Finite Element Analysis

Abstract

Nuclear containments serve the critical function of providing a leak proof boundary for containment of radiation in nuclear power plants. The containments are, generally, steel, reinforced concrete or prestressed concrete depending upon the diameter and internal design pressure. Prestressed concrete containments are used in large nuclear containments with significant design internal pressure. In these situations, the externally applied prestressing serves to counter internal design pressure due to LOCA (loss of coolant accident) and other accident loads thus reducing the required thickness and reinforcement demand. The prestressing tendons are placed in sheathing within the concrete. After the concrete achieves its required strength, the tendons are stretched and locked off against the ends of the concrete called anchorage zones. These anchorage zones are thus subjected to substantial compressive and splitting stresses and need to be properly designed and detailed. Since anchorage zones are the primary location of the prestressing force transfer to concrete, they experience very large and localized bearing and splitting stresses which can have significant safety and structural consequences for the containment integrity. Simple analysis based on strut-and-tie model is generally used for design of prestressed concrete anchorage zones. But because of the stress concentrations and potential impact to structural integrity, it is prudent to utilize detailed finite element method to verify and/or substantiate the results from simple analysis. The finite element (FE) analysis of tendon anchorage zone requires a refined mesh in order to capture the geometry of details surrounding tendons. This paper presents a detailed and practical finite element model used to perform a comprehensive stress analysis of an anchorage zone of a large post-tensioned containment. Both local and general anchorage zones are evaluated. A fictitious case of tendon anchorage zone is established as an example case based on typical parameters of nuclear plants. A 3D finite element model is then developed using ANSYS Version 13.0, in which the effect of tendon sleeve / sheathing into concrete is modeled explicitly. This paper also discusses anchorage zone analysis approaches in various state-of-the-practice codes and standards using hand calculations. The result of finite element analysis are compared with analyses using various hand calculation approaches. In particular, importance of adequate reinforcement design and detailing in anchorage regions is discussed based on the stress profiles from FE analysis and compared with hand calculation methods. It is concluded that a detailed finite element evaluation of anchorage regions is necessary to develop a level of confidence required for ensuring safety and integrity of nuclear containments. The FE modeling also serves as verification for results from simple hand calculation methods.