Impact of power density profile on passive decay heat removal in prismatic HTGR

Reactor designs with passive safety features have been actively developed ever since the Fukushima Daiichi Nuclear Power Plant accident in 2011. In our previous works, we investigated the design of a small, prismatic HTGR for passive decay heat removal and the relations of parameters for successful decay heat removal. For the power density distribution of the HTGR, the Bessel function was used in the radial direction and the cosine function in the vertical direction, assuming a barrel-type cylindrical core. In practice, the power density distribution is usually flattened by introducing burnable poisons or control rods during operation to prevent non-uniform power production throughout the core. The power density profile might have an effect on the passive safety features of the HTGR. Therefore, the influence of a flat power distribution on the passive safety features of a prismatic HTGR was investigated in the present study. Results showed that flattening the power density profile allowed greater thermal power with the same safety performance without changing the reactor dimensions or calculation conditions. On the other hand, a same-power reactor could be designed with a smaller size without worsening its passive safety features, which is highly advantageous from an economical point of view. Hence, it was confirmed that reactor safety characteristics could be improved by implementing uniform power density in the design.