Complex sentence: The French planned a nuclear propelled submarine in the late 1950s and laid down a hull (no Q244) but the inability of French industry at the time to produce a workable marine reactor led to the project being canceled in 1959.
Split into simpler sentences: The French planned a nuclear propelled submarine in the late 1950s and laid down a hull (no Q244). But French engineers were unable to produce a reactor small enough to fit into the submarine, which led to the project being canceled in 1959.