Abstract:

The Pebble Bed Modular Reactor (PBMR) is an advanced graphite-moderated High Temperature Gas-cooled Reactor (HTGR), using helium as the coolant. Since helium is lighter than air and the graphite structure is designed to allow block movement due to effects of temperature and irradiation, it is expected that helium will distribute as follows: -main flow: helium carries the energy out of the reactor -engineered cooling flow: portion of helium is intentionally bled for cooling purposes -leakage: unintentional flow of helium within the reactor

The complex hydrodynamic phenomena of a PBMR are partly attributed to the inter-connecting flow paths. Modeling the hydrodynamics phenomena provides insight to the reactor over the range of operating conditions. A detail three-dimensional computational fluid dynamics (CFD) model is one of the modeling options; however, the hydrodynamic complexity of a PBMR would require substantial computing power and considerable modeling effort, thereby rendering CFD modeling an inappropriate tool for prompt feedback in the design-analysis cycle.

An integrated system-CFD approach treats the flow paths as a flow system, or more appropriately, a flow network. Flownex is a one-dimensional thermal fluid simulator which allows complex systems to be analyzed with relative ease. In Flownex, a basic flow path is simulated as flow traversing through a pipe element. The simpler flow paths in a PBMR are modeled using this fundamental approach. The complex flow paths that are more geometrically dependent are characterized using detailed CFD modeling to establish the dimensionless flow-pressure drop relationships. The characterization data are then defined in Flownex to model the flow paths in order to account for the effects of flow geometry and fluid properties.

The approach combines the advantage of CFD modeling in simulating the detailed flow interaction to the more robust solver applied in Flownex. Although this approach is inherently geometry-dependent, its application in a large network or a complex model has seen a reduction in the simulation time compared to the CFD modeling approach, without compromising the result integrity. The approach also improves the design process by providing faster feedback design-analysis cycle. The paper will discuss the theory of such integrated approach, and the modeling of key gaps in a PBMR within the reactor flow distribution model will be used as an example.