The prismatic gas-cooled very high temperature reactor (VHTR) is one possible option for the generation IV nuclear power plant. The prismatic VHTR basically involves stacks of hexagonal graphite blocks that are drilled to accept cylindrical fuel compacts and provide coolant channels for the helium coolant. Between the hexagonal blocks, there are gaps, which allow the coolant flow to bypass the coolant channels. The gaps are not intentionally designed to occur in the core, but are present because of tolerances in machining the blocks, imperfect installation and expansion and shrinkage from heating and irradiation. Based on previous studies of a loss of flow accident (LOFA), the cooling provided by flow in the bypass gaps has a significant effect on the nature and strength of the attendant natural circulation. One of the mechanisms that occurs after a LOFA for the transport of heat out of the core is by the natural convection of the coolant. It is of interest to know if there are problems for the core associated with the natural circulation and what is the role played by the bypass flow in such an event.

The distribution of heat generation and the separation of the partial columns included in the CFD model of the heated core have a strong effect on the natural circulation. In the present paper, a 1/12 symmetric section of the active core is considered for the CFD model. Two regions of the 1/12 section are employed to perform the LOFA transient calculations. Several scenarios are investigated including with and without the bypass gap in the model. The present study also reports the effects of bypass flow on the natural circulation with time for these cases.

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