Assessing Prestress Losses in a Nuclear Containment Structure for License Renewal

Nuclear power plants around the world are nearing the end of their designed service life. Sufficient structural capacity must be demonstrated to extend each plant’s operating license when accounting for concrete creep, shrinkage, and tendon relaxation past the original design life. This may take the form of in-situ values which meet the design allowable or, as outlined in this paper, analysis models which demonstrate capacity. This paper presents an analysis methodology for a concrete containment structure utilizing grouted post-tensioned tendons representative of a non-US design. The methodology is intended to demonstrate that a structure can still meet established design requirements while accounting for creep, shrinkage, and tendon relaxation. The analysis effort is performed in multiple stages. First, design parameters feeding into post-tensioning loss calculations are identified and assigned statistical distributions. Probabilistic estimates of the post-tensioning losses are developed using both a variational and Monte Carlo approach. Second, a finite element model of a representative containment structure is developed with tendons and reinforcement explicitly modeled. Lastly, the finite element model is used in example analyses to demonstrate future performance and pressure capacity accounting for projected tendon losses.