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Acceptance procedure for thermo-hydraulic simulator - an example

Computer codes and computer hardware have progressed significantly over the last number of years. This has resulted in the development of powerful tools which the engineer has at his disposal for the design and analysis of plants such as the pebble bed modular reactor. Clearly the question of code validation and verification must therefore be addressed. This paper gives an overview of the procedures which has been identified for the validation of the simulators which could be used for the thermo-hydraulic design and analysis of the pebble bed modular which is being developed by Eskom. The validation of the thermo-hydraulic network code Flownet is then used as an example. 

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Verification and validation of the HTGR systems CFD code Flownex

Regulatory requirements prescribe extensive V&V of computer codes that are used in the design and analysis of accident conditions in nuclear plants. Flownex is a dynamic systems CFD code used as the primary thermal-fluid simulation code by the PBMR. Stringent quality assurance processes have been implemented to ensure that the code conforms to the set standards. These processes include the comparison of Flownex with analytical results and experimental data. Analytical solutions are used to verify Flownex's element models so as to ensure that the basic theory is correctly implemented. Comparison with experimental and plant data is a very important feature of the V&V program to validate that the chosen theory is fit for purpose. For this, validation data from the PBMM is used. In addition to the PBMM experimental data Flownex is compared to a number of small thermal-fluid experiments in which certain specific component phenomena is validated.

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Thermal-Fluid Comparison of Three- and Single-Shaft Closed Loop Brayton Cycle Configurations for HTGR Power Conversion

With the resurgence of the high temperature gas-cooled reactor (HTGR) various closed loop Brayton cycle configurations are currently being investigated. The most notable being the PBMR, the GT-MHR and the GTHTR300. These systems are all in different phases of design and optimisation and the developers are advocating different performance parameters and arguments for and against specific cycle layouts. In order to further this debate, this paper presents a comparison of the three- and single-shaft versions of the pre- and inter-cooled, recuperative Brayton cycle configurations based on detailed steady-state and transient thermal fluid simulations. The results show that although cycle efficiency and specific power of the two configurations compare well at steady-state full power operation, the transient response shows important differences that will impact directly on the system design.

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Start-up of the Three-shaft Closed Loop Brayton Cycle Model of the PBMR Power Plant

The PBMR is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, inter-cooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the differences and similarities between the so-called Pebble Bed Micro Model (PBMM) and the actual PBMR. It provides the layout and overall specifications of the plant and describes the approach followed in the design of the start-up system. It gives the results of the actual start-up event including a comparison between the measured and simulated conditions.

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One-dimensional reactor model for the integrated simulation of the PBMR power plant

The Pebble Bed Modular Reactor (PBMR) power plant is currently being developed by PBMR (Pty) Ltd in South Africa together with ESKOM and other industrial partners. This high temperature gas cooled reactor (HTGR) plant is based on a three-shaft Brayton cycle with helium gas as coolant. The complexity associated with the thermal-hydraulic design of the cycle calls for the use of a variety of analysis techniques and simulation tools. One of the most prominent of these is the Flownet thermal-hydraulic network simulation software. Flownet allows detailed steady-state and transient thermal-hydraulic simulations of all components in the plant fully integrated with core neutronics and controller algorithms. This paper describes the theory and integration of the neutronics and thermal-hydraulic models for the reactor core and presents sample calculations to illustrate the results obtained.

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The simulation of a thermal-fluid system using an integrated systems cfd approach

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 thermal-fluid 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.

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The analysis of the pebble-bed modular reactor thermal fluid cycle using a network code

The PBMR power plant is currently being developed by PBMR (Pty) Ltd. This HTGR plant is based on a three-shaft Brayton cycle. Engineers are faced with major challenges when carrying out the thermal-fluid design of the plant. System performance predictions must be done for both steady-state and transient conditions. The complexity associated with the thermal-fluid design of the cycle requires the use of a variety of analysis techniques and simulation tools. These range from simple one-dimensional models that do not capture all the significant physical phenomena to large-scale three-dimensional CFD codes that, for practical reasons, can not simulate the entire plant as a single integrated model. An approach that has gained wide acceptance is the network approach - models of standard components are developed that can be interconnected in any arbitrary way. This paper gives an overview of one of the most prominent codes that provide a suitable compromise, namely Flownex.

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Comparison of Two Models for a Pebble Bed Modular Reactor Core Coupled to a Brayton Cycle

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 re-activity was simulated.

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Operation and simulation of a three-shaft, closed-loop, Brayton Cycle model of the PBMR power plant

The Pebble Bed Modular Reactor (PBMR) is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, intercooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the results of the various tests performed on the plant to evaluate the different control methodologies as well as preliminary comparisons between measured and simulated results.

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Sensitivity analysis of the PBMR gas cooled nuclear reactor cycle with the aid of a simplified simulation model

System simulation is a valuable tool in the design of thermo-fluid systems like the Pebble Bed Modular Reactor nuclear power plant currently being developed for ESKOM in South Africa. A simplified and easy to use cycle simulation can be employed successfully in the conceptual design phase when parametric studies are being performed to determine the influence of important design parameters on the system performance. The advantage of such a simplified simulation is that the analyst has direct control over the performance of each component as opposed to more detailed simulations where the component performance is obtained as an output. This paper introduces the EESINET simulation model developed for this purpose. It also provides the results obtained from the sensitivity analysis including the effects of overall pressure ratio, turbo machine efficiencies, maximum and minimum cycle temperatures and pressures, recuperator effectiveness as well as pressure losses and leak flows.

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