Western sources claim that China had provided Pakistan with fissile material in exchange for centrifuge technology assistance. Zia-ul-Haq hoped to exploit the close relationship with the Chinese further in order to protect Pakistan from potential preventative attacks… [T]he impact of Israeli attack on Osirak and the crash of the centrifuges in 1981 forced Zia-ul- Haq to realize that the nuclear program was vulnerable not just to preventive strikes but also to natural calamities. Zia-ul- Haq then dispatched Lieutenant-General Syed Zamin Naqvi and A.Q. Khan to request bomb-grade fissile materials and bomb designs. Their visit bore fruit as Pakistan then received the Chinese CHIC-4 weapon design along with 50 kilograms of HEU in 1981, material sufficient for two bombs. A.Q. Khan confirmed in a purported 2004 letter to his wife, “The Chinese gave us drawings of the nuclear weapon, gave us 50 kg of enriched uranium, gave us 10 tons of UF6 (natural) and 5 tons of UF6(3%).”
According to A.Q. Khan’s accounts, the Chinese nuclear material was kept in storage until 1985. When Pakistan acquired its own uranium enrichment capability and wanted to return the fissile material, China responded that “the HEU loaned earlier was now considered as a gift … in gratitude” for Pakistan’s help with Chinese centrifuges. It was then that KRL “promptly fabricated hemispheres for two weapons and added them to Pakistan’s arsenal.”
Khan, Feroz Hassan. Eating Grass: The Making of the Pakistani Bomb. Stanford University Press; Stanford. 2012. p. 188 (typographical inconsistencies in original)