The coolant depressurizes to atmospheric pressure due to a pipe break in the case of a primary coolant depressurization accident of HTGRs, In such case, it is difficult to remove the residual heat by using the primary cooling system because the air entering from pipe break may oxide the graphite in the reactor. Therefore, the residual heat is conducted to the reactor vessel outer surface and is removed by the passive reactor vessel cooling system. This method has high inherent safety due to no use of active systems. However, the available reactor thermal power for removing the residual heat is about 200 MW. This is not large enough and not economical. From that background, the annular core in which fuel blocks at the core center region are replaced by graphite blocks was proposed for reducing the maximum fuel temperature which appeared in the core center as shown in Fig. 4.43 [55]. By taking advantage of its capability to enlarge the thermal power, originally proposed by K. Yamashita [55], the annular core with the thermal power of 600 MW was designed while keeping the inherent safety [56].

The calculated results of a primary coolant depressurization accident for that design are shown in Fig. 4.44 [57]. The maximum fuel temperature appearing 70 h after the initiation of the accident was calculated as 1,595 °C which is below the limit of 1,600 °C. Thus, it was ensured that the additional failure of the coated particle fuels does not occur.

To calculate the fuel temperature of the annular core during accidents, thermal analysis codes such as TAC-NC are used. The TAC-NC code considers radiation heat transfer in the cooling holes and the gap between fuel blocks, natural convec­tion in those spaces, etc.