Complex sentence: The demon core was a, subcritical mass of plutonium which went briefly super-critical in two separate accidents at the Los Alamos laboratory in 1945 and 1946, and resulted in the acute radiation poisoning and subsequent deaths of scientists Harry K. Daghlian, Jr., and Louis Slotin.
Split into simpler sentences: The demon core was a, subcritical mass of plutonium which was involved in two criticality accidents. It went briefly super-critical in two separate accidents at the Los Alamos laboratory in 1945 and 1946, and resulted in the acute radiation poisoning and subsequent deaths of scientists Harry K. Daghlian, Jr., and Louis Slotin.