Why might several small units beat one giant Moon reactor?

Answer: They add redundancy and staged growth

They add redundancy and staged growth ✓ — Correct! Several smaller units can be launched and deployed in stages, giving a base redundancy if one unit fails and flexibility as demand grows. That kind of modularity is attractive in an environment where transport is expensive and repair is hard.

Large reactors cannot go critical there — Not quite. Reactor criticality is a physics and design question, not something the Moon simply forbids. The argument for multiple small units is operational resilience and stepwise deployment, not impossibility of large ones.

Small units need no control systems — Not quite. Small reactors still need control, monitoring, and protection systems. Their advantage is not zero complexity, but better fault tolerance, expansion options, and mission flexibility.