Khushab-1 is a 40-50MW plutonium production reactor, moderated by heavy water and utilizing natural uranium fuel. [1] Construction of Khushab-1—reportedly with Chinese assistance—began in 1987, and the reactor commenced operations in 1998. [2] Pakistan officially announced the operations at Khushab-1 prior to its nuclear weapons tests. [3]

Spent fuel from Khushab-1 is reprocessed at the New Labs Facility near Rawalpindi, and Pakistan has likely been accumulating weapons-grade plutonium since 2000. Depending on its capacity, Khushab-1 had produced approximately 60 to 130 kilograms of weapons-grade plutonium as of 2010. [4] Assuming that 8kg of plutonium are needed to produce one nuclear weapon, Pakistan could have had sufficient plutonium for between 7 and 16 nuclear weapons from Khushab-1 production alone as of 2010. The facility is not under IAEA safeguards.

Sources:
[1] Global Fissile Material Report 2010, Chapter 10, International Panel on Fissile Materials, December 2010, www.fissilematerials.org.
[2] Mark Hibbs, "Bhutto May Finish Plutonium Reactor Without Agreement on Fissile Stocks," Nucleonics Week, 6 October 1994.
[3] "Analysis of IKONOS Imagery of the Plutonium Production Reactor at Khushab, Pakistan," Institute for Science and International Security, 16 March 2000, www.isis-online.org.
[4] Global Fissile Material Report 2010, Chapter 10, International Panel on Fissile Materials, December 2010, www.fissilematerials.org.