Two ideas: 1) don't try to achieve a chain reaction, but do single atom reactions and fully cycle the energy. Simpler to think about, and raw energies add up

Two ideas: 1) don't try to achieve a chain reaction, but do single atom reactions and fully cycle the energy. Simpler to think about, and raw energies add up (see link). 2) if chain reaction needed, orient the reaction to aim the outputs towards their secondary targets e.g. with a crystal lattice fuel package.