Burnable poison for reactivity management in a very high temperature reactor

A physics study has been performed to find the optimal application of burnable poison (BP) for an excess reactivity management in a 600 MWth block-type very high temperature reactor (VHTR). Six potential BP materials (B, Gd, Er, Eu, Sm, Dy) were evaluated for an equilibrium cycle in terms of the major core performance parameters such as the burnup reactivity swing, discharge burnup, fuel and moderator temperature coefficients, and fuel temperature. In addition to the conventional 6-hole BP loading in a fuel block, a 12-hole BP application was also considered in order to accelerate the depletion rate of the BPs. The self-shielding effect of the BP was optimized for a successful management of the excess reactivity by changing the effective diameter of the BP region. Additionally, a mixed BP application of Gd and Er was proposed to make better use of the two BP materials in terms of the reactivity swing and the core temperature coefficients. It has been demonstrated that the burnup reactivity swing over a long cycle period can be reduced from ∼15,000 pcm to 3000–5000 pcm with only a small reduction of the discharge fuel burnup.