Complex sentence: The plutonium "demon core" (the same as in the Daghlian incident) was inside at the time of the accident and would not be visible, but its dimensions are comparable with the two small half-spheres shown resting nearby.
Split into simpler sentences: The plutonium "demon core" (the same as in the Daghlian incident) was inside at the time of the accident, and would not be visible. However, its dimensions are comparable with the two small half-spheres shown resting nearby.