The Pebble Bed Modular Reactor (PBMR) plant is a promising concept for inherently safe nuclear power generation. This paper presents two dynamic models for the core of a High Temperature Reactor (HTR) power plant with a helium gas turbine. Both the PBMR and its power conversion unit (PCU) based on a three-shaft, closed cycle, recuperative, inter-cooled Brayton cycle have been modeled with the network simulation code Flownex. One model utilizes a core simulation already incorporated in the Flownex software package, and the other a core simulation based on multi-dimensional neutronics and thermal-hydraulics. The reactor core modeled in Flownex is a simplified model, based on a zero-dimensional point-kinetics approach, whereas the other model represents a state-of-the-art approach for the solution of the neutron diffusion equations coupled to a thermal-hydraulic part describing realistic fuel temperatures during fast transients. Both reactor models were integrated into a complete cycle, which includes a PCU modeled in Flownex. Flownex is a thermal-hydraulic network analysis code that can calculate both steady-state and transient flows. An interesting feature of the code is its ability to allow the integration of an external program into Flownex by means of a memory map file. The total plant models are compared with each other by calculating representative transient cases demonstrating that the coupling with external models works sufficiently. To demonstrate the features of the external program a hypothetical fast increase of reactivity was simulated.

Complex thermal-fluid systems may consist of many interacting components such as pipes, heat exchangers, turbines and boilers. The first of two major design challenges is to predict the performance of all the thermalfluid components on the system level and the second is to predict the performance of the integrated plant consisting of all its sub-systems. The solution to both is an integrated Systems CFD (Computational Fluid Dynamics) approach that deals with various levels of complexity between individual models. To account for the interaction between components on system level a progressive approach can be followed by first using lumped models for all components and then refining individual models where necessary.

To illustrate the application of a progressive analysis, this paper presents the practical example of a coal-fired boiler at a power station. A one-dimensional pipe network was used to determine the quality of the steam mixture and the heat transfer in the boiler riser tubes and these were linked to the detailed three-dimensional CFD model of the furnace. Results from the CFD model showed gas flow patterns and heat distributions inside the furnace. From the network model the temperatures and steam quality inside the riser tubes were obtained and it illustrated the process of steam generation inside the riser tubes.

This paper presents the observations from the analyses of small, medium and large pipe breaks that are presumed to occur in the Pebble Bed Modular Reactor’s (PBMR) Main Power System. The PBMR is a multibillion-rand grossly ambitious project that is tasked with the design, commissioning and marketing of a first-of-a-kind pebble-bed, high-temperature and gas-cooled nuclear technology. The main loop of the envisaged PBMR nuclear power plant comprises of a single Brayton thermodynamic cycle which integrates the pebble-packed reactor to a system of heat exchangers, and to a single-shaft turbine-generator-compressor system. The coolant for the reactor is helium which under 9000kPa and exiting the reactor at close to 900 °C, drives the downstream turbine in a direct action setup, thereby supplying roughly 163MW of electricity into the grid. The PBMR’s Main Power System and its support systems have since progressed through to advanced design phases and thermal-hydraulics analysis has played an integral role in providing various design and systems engineering functions with the necessary input data. There are certain critical positions in the Main Power System (MPS) of the PBMR’s Demonstration Power Plant (DPP) that warrant analysis in order to determine the effect of pipe breaks on certain critical components. Twenty-eight positions around the MPS have been identified for the capture of the helium pressures, temperatures and velocities which are then incorporated into the shear ratio calculation. The calculated shear ratios are thereafter employed in estimating possible dust lift-off fractions that could result from the pipe break scenarios and also in the subsequent prediction of nuclear doses that are likely to affect the reactor’s adjacent compartments and the immediate civil environment. This work provides the observations captured from the pipe break analyses and a brief insight into the implications of the evaluated shear ratio values to dust lift off and dust deposition within the main power system.

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.