This paper presents a liquid NaK-78 cooled, fast spectrum nuclear reactor for lunar surface power. The reactor nominally operates at 500 kW th-1000 kW th and same inlet and exit temperatures of 850 and 900 K, but different coolant circulation rates. Depending on the type of energy conversion technology employed, the power system could generate 30-270 kW e for operation lives of 20-40 years. The reactor core is comprised of 6 hydraulically independent, but thermally and neutronically coupled sectors loaded with highly enriched UN fuel pins in a triangle lattice. Each sector has a separate pair of primary loop with energy conversion unit(s) and secondary loop with heat rejection radiator panels. The liquid metal coolant is circulated through the primary and secondary loops using separate electromagnetic pumps powered with separate thermoelectric conversion assemblies. The pumps continue to operate after reactor shutdown, effectively and safely removing the decay heat. Presented are the results of a multi-physics neutronics and 3-D thermal-hydraulic design optimization analysis performed to satisfy launch safety subcriticality requirement, determine beginning-of-life excess reactivity and the reactor full power operation lives, and ensure proper coolant distribution in the reactor core sectors and mixing in the upper plenums. © 2011 Published by Elsevier Ltd.