Structural Analysis of Mechanical Modules for Modular Designed Nuclear Plants

Most nuclear power plants being designed and constructed in the world today utilize advanced light water reactors with improved economics and safety; they are referred to by the US Department of Energy as Generation III+ Nuclear Plants. Overall, the Generation III+ power plants are expected to be safer and more affordable than those presently in operation. The offsite construction of structural and mechanical modules is a key element of the Generation III+ plant design; this feature significantly reduces the amount of onsite laborers and compresses the construction schedule. Each mechanical module must be structurally qualified to support its attached components for transportation, lifting, and operation scenarios. Qualification of the modules is very complicated because of the applicable codes and criteria as well as the diversity of components that may be attached to them. The purpose of this paper is to provide analysis instruction and to recommend special modeling techniques for structurally analyzing mechanical modules; the recommendations provided in this paper should not be taken as absolute rules but rather as guidelines to be altered, as needed, in order to more accurately simulate specific plant requirements. A mechanical module may be classified as a large gang hanger which supports many system components; a module may have dozens of pipe supports attached to it as well as tanks, piping, valves, pumps, conduit, ductwork, and cable trays. Mechanical modules are a fundamental aspect of the Generation III+ plant, and therefore must be properly analyzed and qualified. Due to the practically infinite possible arrangements of structural members and components, special modeling techniques are often required for considering all the possible loadings that may exist for the transportation, lifting, and operation scenarios. A structural analysis computer program such as GTStrudl or StaadPro must be used to build an analytical model of the module; the module frame and its attached components must be simulated in the model. Loadings such as dead weight, live load, thermal expansion, earthquake, wind, pressure, and flow transients are commonly applied to modules and their components. The analysis and qualification of the mechanical module frame must address structural member stresses, weld stresses, connection local stresses, and module support design. Results of the module qualification must be documented and verified according to plant procedures and criteria.