ZEEP Reactor Reaches Criticality for the First Time

The first nuclear reactor outside the United States to achieve a self-sustaining chain reaction.

Industry, infrastructure and technology changed rapidly between 1938 and 1946, reshaping production, mobility and the material foundations of war and post-war recovery.

On 5 September 1945, in Canada, the milestone “ZEEP Reactor Reaches Criticality for the First Time” occurred. WWII CHRONOS records it as a specific industrial or technological event rather than as a vague multi-year trend.

Verified figure or fact: The first nuclear reactor outside the United States to achieve a self-sustaining chain reaction. Physical measurements in WWII CHRONOS are expressed in the metric system.

This nuclear energy / science milestone helps explain the material systems behind the era: factories, energy, transport networks, extraction, communications, new materials and production methods.

Where an industrial achievement was tied to war, coercion or forced labour, the technical accomplishment must be read together with that human and political context rather than celebrated in isolation.

Editorial status: Date and core fact verified. The source below supports the period and central claim; unsupported details have deliberately not been added.

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