The Lunar Evolutionary Growth-Optimized (LEGO) §Reactor Reactor is a modular, fast-fission, heatpipe-§cooled, clustered-reactor system for lunar-surface §power generation. The reactor is divided into §subcritical units that can be safely launched within §lunar shipments from the Earth, and then emplaced §directly into holes drilled into the lunar regolith §to form a critical reactor assembly. The regolith §not only would just provide radiation shielding, but §also would serve as neutron-reflector material. The §reactor subunits are to be manufactured using proven §and tested materials for use in radiation §environments, such as uranium-dioxide fuel, §stainless-steel cladding and structural support, and §liquid-sodium heatpipes. The LEGO Reactor system §promotes reliability, safety, and ease of §manufacture and testing at the cost of an increase §in launch mass per overall rated power level and a §reduction in neutron economy when compared to a §single-reactor system. Additional subunits may be §launched with future missions to increase the §cluster size and electrical power generation §according to desired lunar base power demand and §lifetime.