Thermal Hydraulic and Safety Analysis of Heat Transfer and Distribution in the Ghana Research Reactor-1 (Gharr-1) Core Using Star-Ccm+ Cfd Code

Abstract

Operational power of a nuclear reactor core is limited by thermal considerations. In principle, the allowable core power is restricted by the rate at which the heat can be transferred from the fuel to the coolant. Unlike in conventional combustion facilities where the heat is released in gas-flows, heat in the nuclear reactor core is mainly released in the structural material. If not properly removed the structural materials inevitably melt causing release of radioactive material. STAR-CCM+ CFD code was used to perform thermal hydraulic analysis of the Miniature Neutron Source reactor also known as the Ghana Research Reactor-1 (GHARR-1). The reactor has a Highly Enriched Uranium (HEU) core consisting 344 fuel pins arranged in 10 multi-concentric circle layers at a pitch distance of 10.95 mm. The present study however considered only the first two concentric rings with varying power of 15 to 30kW. The choice of the first two concentric rings was informed by the results of the computation of local power peaking factors based on power densities. The result indicates that the maximum radial peaking factor of 2.02 occurred at the mid-plane of the core while the maximum axial peaking factor of 1.57 was at the region of the second ring. The extent of the domain considered in this work was also limited by the computational resources available. Analysis of the temperature, pressure, mass flow rate and turbulent intensity in the first two concentric rings was performed to assess the steady state Thermal hydraulic behaviour of GHARR-1 core under natural convection cooling. Computational simulation was performed at 15 and 30 kW at a fixed mass flow rate of 0.11 kg/s. The mass flow rate was varied from 0.11 to 0.15 kg/s in steps of 0.02 kg/s at a fixed power of 30 KW. Effective heat transfer and cooling of the reactor was achieved at the maximum coolant mass flow rate. Surface Average temperature increased along the flow channel from the inlet to the outlet in conformity with operating trends reported in the GHARR-1 Safety Analysis Report. The highest surface average temperature in the normal operating power range (15-30kW) was observed to be 296.21K at 30 kW and coolant mass flow rate of 0.11kg/s. The hottest channel is located between the inner and outer ring on the triangular pitch distance bounded by rods 4, 5 and 14. The hottest segment has a temperature of 301.85K and is located at an axial distance of 0.2020cm. This is below the maximum cladding Temperature of 333.15K hence the reactor is operating safely. Computed Distribution of the mass flow rate generated in the domain varies proportionally with the assigned inlet mass flow rate for the normal operating powers (15-30kW) A turbulence surge at the inlet due to instantaneous injection of fluid coupled with a drop as a result of development of the flow was observed. A general turbulence buildup was also observed as the flow develops and the fluid stabilises. Turbulent intensity was observed to peak at the upper part of the fuel rods consistent with the predictions of the GHARR-1 Safety Analysis Report. All computed accident situation temperatures were below the melting point of the U-Al alloy satisfying regulatory requirements for safe operations.

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Thesis (MPhil) - University of Ghana, 2016

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