[1] Core

The core of the HTTR is a layered structure of hexagonal fuel blocks, surrounded by the replaceable reflector. The replaceable reflector is further surrounded by the permanent reflector. The whole core structure is supported by the side shields and the core restraint mechanism which are provided outside

of the permanent reflector. The fuel blocks are piled up into five layers. The top and bottom replaceable reflectors are placed above and below the fuel blocks, respectively. A set of hexagonal blocks i. e. five fuel blocks as well as top and bottom replaceable reflector blocks is called a column.

As shown in Fig. 4.16, the coolant entering into the reactor pressure vessel first flows upward from the lower region of the vessel through the space between the permanent reflector block and the vessel wall to the top plenum. It turns in the downward direction at the top plenum and enters the core. The

Fuel region

Core restraint mechanism

Permanent reflector

Reactor pressure vessel

Fig. 4.17 cross sections of HTTR

coolant flows in the annular channels between fuel rods and wall of cooling holes in the fuel block. It is heated up to 950 °C. The outflows of the heated coolant merge together in the hot plenum. Then, the coolant is led to the inner tube of the double-tube main coolant outlet pipe.

As shown in Fig. 4.17, control rod guide columns are provided in the core for inserting the control rods. A control rod guide column is a set of piled-up graphite blocks. Two of three holes are for inserting the control rods. The third is used for the reserve shut down system that drops B4C pellets.