Load Combination Reduction Methodology for the US EPR™ Standard Nuclear Power Plant

The Safety-Related structures of the U.S. EPR™ Standard Nuclear Power Plant (NPP) predominantly consist of reinforced concrete shear walls and slabs; thus they are typically modeled using shell finite elements and analyzed and designed for a large number of applicable load combinations. This paper presents a load combination reduction methodology that has been specifically developed for and applied to these types of structural elements in order to methodically reduce the full set of applicable load combinations to a manageable sub-set of load combinations, termed “controlling load combinations” for structural design purposes. Load combination reduction criteria involve code-specified section capacities (i.e., allowables), structural demands (i.e., forces and moments), and demand-to-capacity ratios (DCR) as complemented by reinforcing ratios. For a particular Safety-Related structure or portions thereof, the controlling load combination produces the most demanding forces and moments relative to design allowables in accordance with applicable codes and standards for reinforced concrete design, resulting in the highest DCR among all applicable load combinations. To facilitate the load combination reduction process, portions or segments of a particular Safety-Related structure that are in close proximity and thereby most likely to be designed for a common reinforcement pattern are identified and grouped as a single design component and termed an “evaluation level component.” It is demonstrated that the load combination reduction methodology developed herein is instrumental in narrowing down numerous applicable load combinations to a sub-set of controlling load combinations for the U.S. EPR™ Nuclear Island Safety-Related structures.