After eight productive years at Oxford, Halban was invited back to France in 1954 by the Prime Minister, Pierre Mendès-France, to direct the building of a nuclear research laboratory at Saclay, outside Paris, which greatly expanded the French Commissariat à l'énergie atomique (Atomic Energy Commission). He took up the appointment in 1955, following his divorce from his wife Aline, who shortly afterwards married Isaiah Berlin. The CEA Saclay laboratory developed the independent French nuclear bomb and oversaw the development of French civil nuclear energy.