The hypothetical notion that Pakistan’s nuclear weapons themselves or the fissile material used in them, i.e. weapons-grade highly enriched uranium or plutonium, may fall into the hands of terrorists or extremists, become a popular subject in the West. Despite repeated and elaborate briefings by the Foreign Office and the Strategic Plans Division on the command and control and safety and security of Pakistan’s nuclear weapons and facilities, such stories continue to circle in debates and publications, particularly in the United States.
All nuclear weapon states that have developed a nuclear arsenal have based their nuclear weapon designs on the implosion method, and Pakistan is no exception. Therefore, while some may deliberately choose to ignore whatever may be said in Islamabad, but the fact remains that the technical challenges in building, maintaining and using nuclear weapons that are based on the implosion method are the same throughout the world. This precludes the possibility of their falling into the wrong hands or being easily used by a rogue or terrorist group without the support of dedicated facilities and a large trained manpower.
Any such nuclear weapon typically consists of a trigger mechanism (high-explosive lenses, high-speed electronic switches), a neutron initiator or source, a fissile core of U-235 or Pu-239, a neutron reflector, and all these elements have to be integrated according to a credible theoretical design that must work.
A brief look at each of these components that make up a nuclear device reveals that it is only possible through the work of hundreds of high-technology scientists, engineers and technicians, working in dedicated facilities. This team typically pertains to at least 20 high-technology areas like, theoretical, nuclear and experimental physicists, nuclear chemists, metallurgists, nuclear, mechanical, electrical, computer and electronics engineers, and high explosives, diagnostics and laser experts etc.
Firstly, since World War II, all implosion-type nuclear weapons use “explosive lenses” made of specially manufactured chemical high explosives. These lenses form a critical component in the trigger mechanism of the atomic bomb. Each of these lenses have to be symmetrically arranged, like the panels of a football, where each panel is an identical, yet separate piece of high explosive.
When they are simultaneously detonated at approximately 32-50 points, they produce an immense single, spherical converging detonation shock wave that travels towards the inner core of the bomb that holds the fissile material. This in turn causes the fissile material to be compressed to such an extent that it becomes supercritical after reaching the state of critical mass, and the nuclear explosion is triggered.
To achieve this, the detonation of the explosive lenses has to be coordinated to the precision of less than a millisecond, which requires special electronic switches known as "krytons" to trigger the lenses’ detonation simultaneously.
Manufacturing these sophisticated high explosive lenses for the triggering mechanisms of nuclear and thermonuclear weapons is almost as difficult as producing the fissile material itself. They must be machined to tolerances "equal to less than one thousandth of an inch". Damage to any one of them will ensure that the chain reaction will not take place.
Secondly, assuming that a terrorist is able to get hold of highly enriched uranium (which is enriched UF6) , it can only be used in a bomb in metallic form. Production of enriched UF6 metal requires it to be first converted into UF4 or uranium tetra-fluoride and then into enriched uranium metal. This requires the production of fluorine compounds like hydrofluoric acid, which is highly toxic for man and machine alike. It also necessitates complete mastery of fluorine chemistry, which is one of the most challenging fields of chemistry. The production of uranium metal essentially is part of the front end of the nuclear fuel cycle, but is done in reverse order after UF6 is enriched. UF6 itself is the feedstock for uranium enrichment, and is produced after the front end of the nuclear cycle is mastered, ranging from uranium mining, refining of uranium ore, conversion of ore into uranium dioxide, UF4, and then UF6.
Reactor or weapons-grade plutonium (Pu-239) can only be used in the core of a nuclear device as fissile material in the form of plutonium metal. But plutonium, which is reprocessed from spent fuel irradiated in nuclear reactors, is also highly toxic and radioactive. It needs specialized equipment and facilities to handle, and like uranium metal, production of plutonium metal is done in dedicated facilities. Anyone who comes into contact with plutonium without adequate specialized equipment and training will end up killing himself rather than being able to do anything with it.
Thirdly, for any nuclear chain reaction to take place, a neutron source or initiator is necessary for both gun-type and implosion based nuclear weapons, that provides the initial spurt of neutrons that causes the critical mass of fissile material to undergo spontaneous fission.
Two of the most common neutron sources are a mixture of polonium-210 and beryllium metal, or a mixture of tritium-deuterium gas. Polonium-210 and tritium (also a booster in fission weapons and used in thermonuclear weapons) can only be produced in nuclear reactors. Tritium has a half-life of 12.32 years; therefore it has to be replenished frequently.
Fourthly, specialized high-precision mechanical and metallurgical facilities are required in addition to Computer Numerically Controlled (CNC) machines, to manufacture and mate the trigger mechanism with the fissile material and neutron source, and a U-238 neutron reflector/tamper material around the core.
Fifthly, foolproof safety mechanisms have been integral to the manufacturing procedures and processes of all nuclear weapon states, and Pakistan again, is no exception. Pakistan’s nuclear weapons have been designed, developed and tested by the Directorate of Technical Development (DTD), which was formed in the Pakistan Atomic Energy Commission (PAEC) in 1974. DTD carried out the first cold test of a nuclear device on March 11, 1983.
Between 1983 and 1992, it developed 4-5 implosion designs and tested them in 24 different cold tests. By 1987, PAEC was able to develop a nuclear weapon design that could be easily carried by all PAF aircraft. This was followed by a six-month long joint PAEC-PAF exercise during which dummy devices were fitted onto PAF aircraft and dropped during training exercises. During this effort, the PAEC tested the fitted safety mechanisms in each nuclear device. The long-time head of the DTD, Mr. Muhammad Hafeez Qureshi, recalled after the 1998 nuclear tests, “ these safety mechanisms were at par with the best practices of other nuclear powers.” 
In a 2004 interview, Dr. Samar Mubarakmand, former Member (Technical) PAEC and then Chairman NESCOM, said that a special code is installed in each nuclear weapon and missile at the time of its manufacture that ensures that no accidental or unauthorized use occurs. Both these scientists were intimately involved in the actual development of nuclear weapons in PAEC for almost three decades. The Director-General of SPD, Lt. General (Retd) Khalid Ahmed Kidwai recently stated, “We have well developed physical safety mechanisms and firewalls in the weapons and in the chain of command that will ensure that unauthorized use will never take place, especially by a single individual. The failsafe mechanisms even ensure that no rogue commander, pilot or soldier could launch a live nuclear weapon without coded authorization coming down the chain of command. No nuclear weapon will explode unless it is activated at the very last minute by a top secret, 12-digit alphanumeric code. If a wonky pilot dropped a bomb without the code it would be a dud.” In addition it has been said that the weapons themselves are kept in a diassembled state, further ensuring their safety.
Lastly, the safety and security of Pakistan's nuclear arsenal came under intense international scrutiny and criticism after the revelation of Dr. A.Q. Khan's involvement in an illicit private nuclear trafficking network. In addition to exporting gas-centrifuges, a Chinese atomic bomb design was also sold to Libya and offered to Iran. This design was of a bomb that China tested in 1966. But AQ Khan was never incharge of Pakistan’s nuclear weapons development program. He only headed the uranium enrichment facility at Kahuta.
Here it is pertinent to mention that Pakistan’s bomb designs were all developed by the Theoretical Physics Group of PAEC and the weapons themsevles manufactured and tested by DTD.  In addition, all nuclear fuel cycle, plutonium production and reprocessing (including plutonium metallurgy) , uranium metal production and nuclear weapon development and testing facilities were developed and run by the PAEC under Mr. Munir Ahmad Khan. And acquistion of centrifuges alone does not lead a country to nuclear weapons status, because centrifuges are one part of a long chain of enrichment technology (that may also include other enrichment methods) that again is one part of the nuclear fuel cycle that provides the fissile material to be used in a nuclear device. Thus, for any country to develop nuclear weapons, all technologies associated with the complete nuclear fuel cycle, and nuclear weapons design, development and testing have to be mastered. There is no other short-cut to the bomb.
--The author is a PhD candidate in the Department of International Relations, Quaid-I-Azam University, Islamabad.