What counts as the first working nuclear reactor?

What is the earliest date at which a working nuclear reactor could be built?

Chicago Pile-1 achieved the first self-sustaining chain reaction in 1942; the much earlier date proposed for a different kind of device remains unsupported here.

Chicago Pile-1 is identified as the first self-sustaining nuclear chain reaction, achieved in 1942 under Enrico Fermi. That milestone follows disagreement over what counts as a working reactor. One participant claims an earlier controlled-fission device that could not produce enough heat for steam, suggesting the early 1900s and then dates around 1910 or 1920. The device is not identified, so those early dates remain an unsubstantiated claim within the exchange.