Pakistan's Expanding Nuclear Program

Perusing AWST, I see that Pakistan is expanding its nuclear program.  The WP also has a report:

Pakistan has begun building what independent analysts say is a powerful new reactor for producing plutonium, a move that, if verified, would signal a major expansion of the country's nuclear weapons capabilities and a potential new escalation in the region's arms race.

Satellite photos of Pakistan's Khushab nuclear site show what appears to be a partially completed heavy-water reactor capable of producing enough plutonium for 40 to 50 nuclear weapons a year, a 20-fold increase from Pakistan's current capabilities, according to a technical assessment by Washington-based nuclear experts.

The construction site is adjacent to Pakistan's only plutonium production reactor, a modest, 50-megawatt unit that began operating in 1998. By contrast, the dimensions of the new reactor suggest a capacity of 1,000 megawatts or more, according to the analysis by the Institute for Science and International Security. Pakistan is believed to have 30 to 50 uranium warheads, which tend to be heavier and more difficult than plutonium warheads to mount on missiles. ....India currently has an estimated 30 to 35 nuclear warheads based on a sophisticated plutonium design. Pakistan, which uses a simpler, uranium-based warhead design, has sought for years to modernize its arsenal, and a new heavy-water reactor could allow it to do so, weapons experts say.

Construction of the larger reactor at Khushab apparently began sometime in 2000. Satellite photos taken in the spring of 2005 showed the frame of a rectangular building enclosing what appeared to be the round metal shell of a large nuclear reactor. A year later, in April 2006, the roof of the structure was still incomplete, allowing an unobstructed view of the reactor's features.

"The fact that the roof is still off strikes me as a sign that Pakistan is neither rushing nor attempting to conceal," said Albright of the institute.

The slow pace of construction could suggest difficulties in obtaining parts, or simply that other key facilities for plutonium bomb-making are not yet in place, the institute report concludes. Pakistan would probably need to expand its capacity for producing heavy water for its new reactor, as well as its ability to reprocess spent nuclear fuel to extract the plutonium, the report says.

These developments seem like a natural evolution of Pakistan's nuclear program, as explained by the source for the report, the Institute for Science and International Security:

Pakistan may have concluded that it needs the plutonium to improve the quality of its nuclear arsenal and build a new generation of lighter, more powerful weapons. Plutonium-based weapons can have more explosive yield in smaller, lighter packages than weapons based on highly enriched uranium, which is currently Pakistan’s principal nuclear explosive material and produced in abundance at its gas centrifuge plants. For example, Pakistan may want warheads small enough to fit on cruise missiles it is currently developing. It also may want larger yield (50-100 kiloton) fission weapons that can cause far more damage to Indian cities than its current relatively low-yield weapons. In addition, plutonium-based fission weapons would enable Pakistan to build deliverable thermonuclear weapons.

A reminder of how bare bones Pakistan's current nuclear capabilities are, with so far very limited testing and no test blast above 10 kt -- Pakistan claims substantially larger test blasts --, though Pakistani designs may be workable up to 100kt, and they have a Chinese design that has been tested at 25kt. 

BTW, the first nuclear blast ever, the US Trinity test, was plutonium core based, while the first weapon employment in Hiroshima was an enriched uranium design.  That's because the US was building both weapon types, and the enriched uranium design was simpler and more reliable, so less in need of testing.  Still noteworthy that an untested weapon was used in Hiroshima.  The Nagasaki weapon used three days after Hiroshima had the Trinity plutonium core. The US never again tested a Hiroshima type design -- gun type enriched uranium -- again.   Four more of these Mark I weapons were assembled, but they never entered employable stockpiles. 

The Pakistan weapon is not a gun type design, but an enriched uranium implosion device.