A Washington-based science think tank has reported that Pakistan is increasing its capacity to produce plutonium at its Khushab nuclear facility. The sixth nuclear test (codename: Chagai-II) on 30 May 1998, at Kharan was quite a successful test of a sophisticated, compact, but "powerful plutonium bomb" designed to be carried by aircraft, vessels, and missiles. These are believed to be tritium-boosted weapons. Only a few grams of tritium can result in an increase of the explosive yield by 300% to 400%." Citing new satellite images of the facility, the Institute for Science and International Security (ISIS) said the imagery suggests construction of the second Khushab reactor is "likely finished and that the roof beams are being placed on top of the third Khushab reactor hall". A third and a fourth reactor and ancillary buildings are observed to be under construction at the Khushab site.