In order to realize the core design, the mechanical design must be realized as well as the nuclear and thermohydraulic designs. In this section, the mechanical design of the fuel rod and the fuel block is described [52].

[1] Fuel rod

The fuel rod design must ensure integrity considering production and release of FPs, thermal expansion, irradiation creep, etc. The following conditions must be satisfied at normal operation and anticipated operational occurrences.

(i) The failure fraction of the coating layers at fabrication must be made below a certain limit in order to avoid the release of FPs from the coated particle fuels. In the HTTR design, the limit is set as 0.2 % and the fraction of through-damaged particles of 2 x 10~4 % is achieved in the actual fabrication [53].

(ii) In order to avoid failure of the coating layers, corrosion of the SiC layer caused by palladium, and degradation of the coating layers caused by migration of the fuel kernel, the maximum fuel temperature is kept below 1,600 °C. As already shown in Fig. 4.19, the failure fraction of the coating layers increases when the fuel temperature exceeds 1,800 °C. Migration of the fuel kernel is caused by its encroaching upon the coating layers along the temperature gradient; this is called the amoeba effect. The cross section of a coated particle fuel exhibiting the amoeba effect is shown in Fig. 4.42 [54]. Its mechanism is based on the following chemical formula.

2CO ) CO2 + C (4.30)

At high temperature, the excess oxygen in the fuel kernel produces CO by reacting with the carbon in the low density PyC layer (first layer). At low temperature, the CO decomposes into C and CO2, so that C accumulates there. Through the products of these reactions, the fuel kernel is pushed towards cracks formed at high temperature. The coated particle fuels are designed so that the fuel kernel does not reach the SiC layer.

(iii) Cracking of the fuel rod by thermal expansion or irradiation deformation, which may threaten its structural integrity by mechanical interaction between the fuel compact and the graphite sleeve, is avoided. To do that, an adequate gap between the fuel compact and the graphite sleeve is provided in their fabrication.

[2] Fuel block

The following conditions must be satisfied for the fuel block.

(i) The integrity of the fuel block must be maintained against loads during normal operation and anticipated operational occurrences. The sum of the loads to the graphite block and the stresses caused by the temperature gradient and irradiation deformation must be below the allowable stress of graphite.

(ii) The gap between the fuel blocks needs to be as small as possible in order to reduce the coolant flow rate which is not contributing to fuel cooling. At the same time, the layout of the fuel blocks needs to be designed so that the refueling space is ensured.