Loading of seedless fertile rods has been used as the central principle to maximize fertile to fissile conversion in the two thorium breeder reactor concepts, viz. ATBR and FTBR [1, 2]. At fresh state the seedless thoria rods will produce practically no fission power, or nearly thousand times less fission rate compared to the seed fuel rods. Hence it is conceived that the fuel assembly would be constituted by assembling the fresh seed rods with one fuel cycle irradiated fertile thoria rods. Even in this state there is a wide disparity between the fissile content of these rods. By judicious choice of the rod dimensions and their relative locations, a degree of balance in the fission rate is achieved in the fresh state of seeded rods. Remarkably as the burnup proceeds the initially seedless fertile rods have a continuous growth of fissile content up to an asymptotic value for a given spectrum and the fissile content in seeded rods monotonically decreases. If the discharge burnup is sufficiently large by design, it is seen that the power share of the initially seedless fertile rods can even exceed that of the seed fuel rods. The physics principles of achieving this characteristic are presented in this paper.

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