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Today in History: Harry Daghlian Dies in First Manhattan Criticality Accident

8/21/2026

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Summary

Story summary

On August 21 1945, physicist Haroutune “Harry” Daghlian Jr. dropped a tungsten-carbide brick onto a plutonium core during a criticality experiment at Los Alamos’ Omega Site. The accident released an uncontrolled neutron burst, delivering a lethal dose that killed him 25 days later, marking the first Manhattan Project fatality.

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Story summary

On August 21 1945, physicist Haroutune “Harry” Daghlian Jr. dropped a tungsten-carbide brick onto a plutonium core during a criticality experiment at Los Alamos’ Omega Site. The accident released an uncontrolled neutron burst, delivering a lethal dose that killed him 25 days later, marking the first Manhattan Project fatality.