Design of high temperature reactor fuel and reactor core structure

The high temperature engineering test reactor (HTTR) which is planned by Japan Atomic Energy Research Institute produces the thermal power of 30 MW and reactor output temperature of 950 deg C, using pin in block type fuel element assemblies. A fuel element assembly is composed of a hexagonal graphite block of 360 mm width and 580 mm length, and 31 or 33 fuel rods. The outside diameter of fuel rods is 34 mm. For the fuel element blocks and fuel rod sleeves, isotropic graphite IG-110 is used. In the core graphite, high stress occurs depending on temperature distribution, deformation due to irradiation, seismic force and others, and also the weight and strength decrease due to corrosion by oxidation. These factors must be considered to optimize the fuel element design. The active reactor core of 2.9 m height and 2.3 m equivalent diameter consists of 30 fuel columns, each column composed of 5 stages of the fuel element assemblies, and 7 control rod guide columns. The specification of fuel assemblies, the design of coated fuel particles, the design of fuel rods, the guideline for core graphite structural design, the design of graphite blocks, the design of reactor core structure and so on are reported. Various concepts of high performance fuel assemblies are studied. (Kako, I.)