The results of the flow distribution calculation for 950 °C operation are illustrated in Fig. 4.39 [28]. As the coolant flow in the core, there are gap flows between the blocks and gap flows between the permanent reflectors as well as the fuel channel flow directly cooling the fuel rods. The coolant flow rate directly contributing to the fuel cooling is about 88 % of the total flow rate. It shows the minimum value at the third block from the top of the fuel region. The flow reduction is occurred at the high temperature region, because the increase in the coolant temperature leads to increases in viscous resistance and hence pressure drop.

The calculated axial fuel temperature distribution is illustrated in Fig. 4.40 [28]. The solid lines indicate the nominal temperatures and the dashed line indicates the systematic temperature at the inner surface of the fuel compact. Since the coolant flow direction is downward, the coolant temperature increases from the top to the bottom. The fuel compact inner surface temperature is

almost constant below the third layer. That is because the increase in the coolant temperature and the decrease in the power density along the axial direction cancel out each other.

The maximum fuel temperatures of each fuel column are shown in Fig. 4.41. This figure gives the maximum fuel temperature within the burnup period. The highest maximum temperature appears at the core inner region and it is 1,492 °C, which is below the limit of 1,495 °C for normal operation and does not reach the limit of 1,600 °C at anticipated abnormal occurrences. Thus, the thermal design limit is satisfied.