Why a 1930s naval reactor was possible but unrealistic

How early could a nuclear reactor for ship or submarine propulsion have been built?

A rudimentary reactor is theoretically imaginable in the 1930s, but inadequate control knowledge makes that timing unrealistic without extraordinary luck.

Theoretical possibility is different from a realistic development programme. Enough underlying science and machinery might, with unusual luck, produce some form of ship-powering reactor in the 1930s. The greater barrier would be understanding how to control and manage the reaction safely and reliably in naval service. The scenario is technically imaginable but far earlier than institutions and operational knowledge were ready to sustain.