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The preliminary conceptual design study of prismatic-type Very High Temperature Reactor (VHTR) has been performed with 950 o C outlet coolant temperature for higher efficient hydrogen and electricity production. First, the core internals that enable higher outlet temperature are considered in the viewpoint of reduction of core bypass flow. Three-dimensional thermal and hydraulic analyses are carried out and show that the 950 o C outlet temperature requires approximately 90% fuel flow fraction and it can be achieved with the installation of the seals in bottom blocks, the coolant tubes in the permanent side reflectors and the core restraint devices. Next, the core and fission product (FP) release analyses are performed. The analysis methods that have been developed for the pin-in-block fuel, one type of prismatic VHTR cores, can be applied to multi-hole fuel, another type of the cores, with some adjustments of the analytical models.

The carbon fiber reinforced carbon-carbon composite (C/C composite) is one of the candidates due to its excellent thermal stability as well as high strength. A two-dimensional C/C composite has great anisotropy in those properties in with- and across- fiber directions. It is, therefore, important to consider the anisotropy for the stress evaluation and for the fracture probability of the components. In the present study, FEM analyses on deformation and stress of the component were carried out taking account of the anisotropy. In addition, the fracture probability of the components was evaluated by the statistical fracture theory. It was found that anisotropy affect the thermal stress and the risk of rupture.

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