A report by the Institute for Science and International Security (ISIS) provides evidence that Pakistan's forth plutonium production reactor in Khushab is now operational. Construction of the reactor was reported to begin in 2011. The third plutonium production reactor in Khushab began operations in 2013.
Each of the four reactors in Khushab is capable of producing of about 10 kg of weapon-grade plutonium a year. For a detailed analysis of Pakistan's fissile material production capabilities see IPFM's 2010 Global Fissile Material Report.