(Reuters) – Pakistan’s President Asif Ali Zardari visits China from Tuesday, following mounting signs that Chinese companies are moving ahead with plans to build two reactors at the Chashma nuclear complex in Punjab province.

Here is an explanation about those plans and why some other governments are concerned.

WHAT IS THE CHASHMA COMPLEX?

Chashma in Pakistan’s Punjab province is the site of a nuclear power complex built using Chinese expertise and designs. One 300 megawatt pressurized water reactor began commercial operation in 2000, and Chinese companies are building another one likely to be finished in 2011 or 2012.

Chinese nuclear companies have also unveiled plans to build another two bigger reactors at Chashma in coming years. They have not issued detailed information about when they will start, but contracts have been signed and financing is being secured.

WHY IS CHINA HELPING BUILD MORE REACTORS THERE?

Converging foreign policy and commercial motives appear to be driving China’s decision.

Pakistan is a long-standing partner of China, and Beijing believes it is important to back Pakistan to counter Indian regional dominance. It is also wary of growing U.S. sway across South Asia.

Pakistan faces increasing power shortages, and demand is likely to keep growing quickly as the country’s population expands.

There’s also a commercial pull, said Mark Hibbs of the Carnegie Endowment for International Peace. Chinese nuclear companies want to win foreign markets, and for now Pakistan is virtually the only “springboard” they have to hone their skills abroad and nurture the expertise that they hope will later find customers in other parts of the world.

ARE THERE NUCLEAR PROLIFERATION RISKS?

In theory, Pakistan could at some later date take spent fuel from Chashma to reprocess for plutonium that could be used for nuclear weapons.

In practice, however, the International Atomic Energy Agency keeps safeguards at Chashma to prevent that happening, said Hibbs. China would keep control of the spent fuel to ensure it is not at risk of diversion to weapons programs, he said.

“There would be no connection between the fuel and reactors provided by China and Pakistan’s nuclear weapons program,” he said.

SO WHAT ARE OTHER GOVERNMENTS WORRIED ABOUT?

Some of the worry is about Pakistan, and some is about the integrity of nuclear non-proliferation rules. There are those, including many commentators in India, who say Pakistan is so dogged by instability and militant pressures that it should not receive nuclear technology, which could be the target of attacks.

Also, leading Pakistani nuclear scientist A.Q. Khan was an important illicit broker of nuclear technology to Iran, Libya and North Korea, and critics say that is another reason to worry.

The more broadly shared worry is that, however safe Chashma may be, expanding the nuclear complex there could be a fresh blow to the integrity of nuclear non-proliferation rules.

Pakistan and India have nuclear weapons, and both countries refuse to join the Nuclear Non-Proliferation Treaty, which would oblige them to scrap those weapons.

The NPT rules say that if countries not authorized to possess nuclear weapons want to receive nuclear materials from countries adhering to the Treaty, they should accept comprehensive safeguard agreements for their nuclear activities.

WHAT CAN THEY DO?

For now, the main arena for addressing this issue is the Nuclear Suppliers Group, a 46-member body that seeks to ensure nuclear exports are not diverted to non-peaceful purposes.

To receive nuclear exports, nations that are not one of the five officially recognized atomic weapons states must usually place all their nuclear activities under the safeguards of the International Atomic Energy Agency, say NSG rules.

When the United States sealed its nuclear agreement with India in 2008, it won a waiver from that rule from the NSG after contentious negotiations. Washington and other governments have said China should at least seek a similar exemption for the planned reactors in Pakistan.

But there is little likelihood of all 46 member governments of the NSG voting in favor of a waiver, and this is a group that operates by consensus, said Hibbs.

(Reporting by Chris Buckley; Editing by Sanjeev Miglani)