Complex sentence: The risk of building an operational reactor running at criticality in a populated area was a significant issue, but the physics of the system suggested that the pile could be safely shut down even in the event of a runaway reaction.
Split into simpler sentences: The risk of building an operational reactor running at criticality in a populated area was a significant issue. However, the physics of the system suggested that the pile could be safely shut down even in the event of a runaway reaction.