Power Ramp test (PRT) of a fuel element is generally conducted with a PRT irradiation rig within a research reactor, in order to study the fuel’s behavior and validate its safety under power transient. Neutronics characteristics of a new PRT irradiation rig within a typical HFETR (High Flux Engineering Test Reactor) core and its components’ heat generation rates are calculated with MCNP code in this paper. The range of the test fuel rod power is obtained with a coupled Neutronic-Thermal-Hydraulic calculation method which combines MCNP and CFX code. The results show that changing the density of 3He gas can vary the test fuel rod power effectively, and the 3He gas layer influences the neutron field intensely by reducing the thermal neutron current into the layer and decreasing the neutron flux in and near the irradiation rig. The test fuel rod power varies from 5.80kW to 15.3kW while decreasing the 3He gas pressure from 4.5MPa to 0.13MPa, along with 0.231$ reactivity addition. Power of the fuel pellet in the test rod increases monotonically along with the 3He gas pressure reducing, and its calculation results have good agreement with the curve fitting by a natural logarithm function.

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