South African Research Chairs Initiative from the Department of Science and Technology and the National Research Foundation of South Africa, funding. Most of the research done on various aspects of the PMR200 Prismatic Block Reactor and its associated RCCS (Reactor Cavity Cooling System) has been conducted using 1D (one-dimensional) and 3D (three-dimensional) numerical simulation packages. The integrated model will be used to evaluate the heat transfer and coolant flow performance of the PMR200 reactor and the RCCS system for normal and selected anomalous conditions.
Therefore, it was concluded that the PMR200, the RCCS alone, and the integrated PMR200-RCCS model are valid representative models that can be used to evaluate the thermal-hydraulic performance of the reactor and associated RCCS. The performance of the models has been shown to be helpful in evaluating the thermal-hydraulic performance of reactor systems, as demonstrated by selected simulated scenarios. An initiative of the South African Research Chairs of the Department of Science and Technology and the National Research Foundation of South Africa (Grant No. 61059).
This work is based on research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation (Grant No. 61059).
INTRODUCTION
- Background
- Problem Statement
- Methodology
- Contribution of the Study
- Chapter Outline
The CFD part takes into account the conduction heat transfer in the solids of the system and the one-dimensional approximation takes into account the fluid flow in the system. Many studies have been done on parts of the PMR200 reactor and the RCCS, using 3D and 1D simulation packages. However, no research has been done in which a Full Core model of a PMR200 prismatic block reactor has been built in Flownex, as well as a Full Integrated model of the PMR200 reactor and the associated RCCS.
The purpose of this study is to construct a complete numerical core model of the PMR200 and the associated RCCS. The study aims to generate a network model of PMR200 and RCCS using Flownex, which uses less computational resources than other conventional CFD simulation models. A 1D system network model of an SCFM is modeled and evaluated to establish the validity of the SCFM model in Flownex and demonstrate that Flownex can be used for modeling SCFM.
2014) ‘1D AND 3D NUMERICAL SIMULATION OF THE REACTOR HEAT COOLING SYSTEM OF A VERY HIGH TEMPERATURE REACTOR’, IHTC 15, Japan, August 2014.
1D AND 3D CFD MODELS OF THE HEAT TRANSFER IN A PRISMATIC
The simulations take into account the conductive heat transfer through the fuel compact and moderator graphite, the convection heat transfer from the coolant channel wall to the coolant channel fluid, and the flow of the coolant in the coolant channel. The heat transfer through the SCFM, which is responsible for the temperature distribution in the center and wall of the fuel compact, will be investigated. An extruded mesh was implemented for 1/6th of the SCFM in the full 3D model.
The external surfaces of the fuel compact and moderator graphite were specified to be adiabatic and the external surfaces of the coolant channel as planes of symmetry. The graphs also compare the results of the temperature distribution results obtained by Flownex and STAR CCM+ with each other. With the results obtained by full 1D and full 3D simulations, the temperature distribution results in both the center and the wall of the fuel compact differed by less than 1%.
With the results obtained, it was found that the values predicted by Flownex and STAR CCM+ for the temperature distributions in the center of the fuel compact and the wall of the fuel compact agree well with each other.
NUMERICAL SIMULATION OF A REPRESENTATIVE PMR200
The heat transfer through the core of a very high temperature reactor (VHTR) is important in the normal operation of the reactor. The CFD part accounts for the conduction part of the system and the one-dimensional approach accounts for the fluid flow in the system. For the construction of the 1/6 part of a fuel block, Khoza (2019) made use of data from the unit cell for heat transfer between the fuel rods and coolant channels developed by Sambureni (2015).
For the modeling of the fuel block/s, the plugs of the block were not modeled in Flownex, and the bypass and cross flows were included in the models. One of the triangles connected to a solid node represents the conduction, convection and pipe components for heat transfer from the graphite to the coolant shown in Fig. Similarly, the fuel rod or hole is representative of the fuel rods or holes contained in the CV.
To the right of the graphite nodes and conduction components are the conduction and convection components for heat transfer from the graphite to the coolant and the pipe components to model the representative coolant channel. The case considered is similar to a fuel block located in the upper layer of the active core of the PMR200 reactor. These results confirm the ability of the 1D fuel model to simulate the heat transfer in a fuel block.
Graphite plugs at the top and bottom of the fuel rods (Tak et al., 2014) were therefore not considered. It is important to remember that the coolant flows from the top to the bottom of the fuel assembly. Heat transfer between adjacent layers was modeled by implementing axial conduction components composed of helium in the transverse gaps between the layers.
In the case of the reflector it blocks the fuel rod components, radiation and conduction components in the gap between the fuel. 3(5)) calculate the convection heat transfer from the tip of the top reflector and the radiation component for the radiation heat transfer between the top reflector and the core barrel. The radiation components are used to calculate the radiation heat transfer between the external surface of the CB and the internal surface of the RPV.
However, the VCS mass flow rate has a negligible effect on the maximum fuel center temperatures and coolant outlet temperature.
NUMERICAL SIMULATION OF AN INTEGRATED RCCS WITH A FULL
In these accidents or normal operating conditions, the RCCS (Reactive Cavity Cooling System) is absolutely necessary for the heat removal process of the reactor system. Radiant heat transfer was also the main source of the heat transferred from the Reactor Pressure Vessel (RPV) surface. When a fracture reached a critical size, the direction of fluid flow in the affected half of the manifold reversed.
With the reversal of the fluid flow direction, they observed a 10% decrease in heat dissipation capacity. The RCCS is integrated with the fully representative reactor model by coupling it to the outer wall of the RPV. The core of the reactor is enclosed in the core cylinder which in turn is enclosed in the RPV (Reactor Pressure Vessel).
As expected, the RCCS mass flow in the case of the FNX 2021b model is greater than the corresponding value for the FNX 2021a model. Five positions were selected for the postulated fractures between the InHead and OutHead of the RCCS. In the FNX 2021a and FNX 2021b models, the fracture mass flow rate approaches the asymptotic value from below.
13 the mass flow rates for break position 2 through the Break, OutDuct12, OutMani12, OutMani34 and the Riser are shown as a function of the break size. At the same time, the mass flow rate through the risers decreases due to the mass flow shorting through the Breek. So before the critical break size is reached, an increasing part of the mass flow is diverted through the risers to OutMani34.
14 the mass flow rates for break position 4 through the Break, OutDuct12, OutMani12, OutMani34 and the Riser are shown as a function of the break size. As the break size grows, the mass flow rate through the break increases, as does the density of the air above the break. In any case where a break occurs, the heat removal performance of the RCCS deteriorates as the mass flow through the risers decreases.
In this model, the RCCS extracts heat generated by the active fuel core of the reactor released by the RPV.
CONCLUSIONS AND RECOMMENDATIONS
It was assumed that breaks would occur in the RCCS system between the OutHead and the InHead of the manifold. A-2 shows more details of the Top Head Plenum and Top Reflector, including the intake plenum. A-3 shows more details of the inner reflector and the first fuel block ring in the top three layers of the active core.
A-4 shows more detail of the second and third Fuel Block rings in the top three layers of the active core. A-6 shows more details of the lower reflector, exit plenum and lower support section. APPENDIX B: COMBINED 3D AND 1D HEAT TRANSFER MODELS IN PRISMATIC BLOCK VHTR (CO-AUTHOR).
They then proposed an improved convection heat transfer correlation and implemented it in a coupled 3D/1D simulation of SCFM. A 1D model of the fuel compact, graphite moderator and coolant channel using the system code FLOWNEX was constructed by Nel & Du Toit [15]. An explicit 3D model of the fuel compacts, graphite moderator and coolant channel at 1/6 of the SCFM was created using the CFD code STAR-CCM+.
A basic base size of 6 mm, based on the radius of the fuel compact, was chosen for the mesh. This ensured that the temperature gradients at the interfaces were sufficiently resolved and contributed to the stability of the solution. The various stages of the fuel compact and graphite moderator were categorized and assigned to reflect their nature, e.g.
In the ANSYS FLUENT model, prism layers were defined only on the moderator side of the coolant channel wall. This is also reflected in the values obtained by the 3D/1D model for the variation in temperatures along the centerline of the fuel compact. The radial distributions are from the center of the coolant channel to the center of the fuel compact.
The coupled 3D/1D model of the 1/6th SCFM was generated using ANSYS FLUENT for the explicit 3D representation of the fuel compactor and graphite moderator and the system CFD FLOWNEX for the 1D representation of the refrigerant channel.
