Dear Sirs:
Of all the actual and potential nuclear powers, China is not yet in a position to drastically increase its nuclear weapons stockpile, at least not without significant investment in its nuclear weapons materials infrastructure.
Of all the countries with potential to do so, Japan must take first place and also, to a certain extent, South Korea. Both these countries have much more potential to develop and greatly expand their nuclear weaponry than suspected nuclear powers like Israel or declared nuclear powers like India and Pakistan.
Why so? Mainly due to the fact that Japan and South Korea have many nuclear power reactors operating.
One of the by-products of operation of nuclear reactors is the production of plutonium, hence if you have many operating reactors, then you have potentially a lot of plutonium available.
Only recently has China been operating large nuclear power reactors (11 at last count, I believe) hence only recently has it had those potential plutonium resources available.
Of course this is a simplistic assessment, mainly due to the following:
1) the plutonium produced in a power reactor is not ideal for nuclear weapons, because it contains significant amounts of Plutonium-240,
2) power reactors are to a certain extent designed to use the plutonium formed as part of their fuel, thus they burn-up a large part of your potential weapons material,
3) extracting the plutonium will require frequent stopping and restarting of the reactor, as well as frequent removal and shuffling of the fuel elements which renders it's operation as a power reactor uneconomical, even in natural to low-enriched uranium designs with low neutron-capture cross-section moderators (i.e. the British Magnox or the Canadian CANDU designs).
Also it predicates that all this can be done despite IAEA safeguards against it.
Here India serves as a good example of materials diversion. Its power reactors are all derivatives of the donated Canadian CANDU prototype using natural uranium as a fuel and heavy water (deuterium oxide) as the moderator.
In power plant terminology we have a figure we call the plant's Availability - that is, the amount of time (including maintenance periods) it is available to produce electric power over one year.
In France the figure is over 80%, for Japan the US and South Korea its around 70%-80%, but for India, it is between 10% and 20%.
Clearly the Indian reactors are being started and stopped frequently, for whatever reasons - I leave it to you to draw the necessary conclusions.
Alternatively the main nuclear materials source for those countries who operate light-water, (pressurized water or boiling-water reactors) like Japan and South Korea are the spent fuel rods extracted from the core - this is typically done during maintenance and refueling - once every 3 or so years.
The spent rods are usually stored in a cooling pond or pool next to the reactor building, where they are 'temporarily' stored until a viable form of disposal is developed. This is necessary as the spent rods are still 'hot' - radioactive.
It would not be difficult for either country to take out a portion of the spent rods and reprocess them to extract the plutonium and uranium in them, and then to put them back in the pool as if nothing happened.
This way they could build up significant amounts of weapons material.
For China to undertake a large build-up of nuclear weaponry will require all this and more.
Still there is no real strategic need for more than a few hundred to a thousand warheads.
China's greater need is for more reliable, longer-ranged, MIRVed ballistic missiles.
Best Regards,
Dusky Lim