1. We know from science class that a "nucleus" is so tiny we can't even see it. So what is so big about the nuclear problem?
A nucleus is tiny, but very heavy for its size, because the material that makes up a nucleus is extremely compressed. When a nucleus is split - a process called "fission" - it gives off energy in a form that can cause nearby nuclei to also split. If the right type and amount of material is involved, this fission can cause an incredibly powerful and fast chain reaction.
It is this power that scientists have only managed to use in modern times. Properly controlled, the energy of the atomic nucleus can be used to generate electricity, to assist in medicine, and to make special radioactive materials with benefits in agriculture, industry and water management.
But the explosive energy of the nucleus has also been used to make bombs - bombs far more powerful than humans had ever invented before. It is this potential for massive destruction that makes nuclear issues so important to international security.
2. How different are nuclear weapons from other types of weapon?Nuclear weapons are different in two main ways. First, they are many times more powerful than any other type of gun, cannon, bomb or explosive. The first time a nuclear weapon was used in a war situation, over the city of Hiroshima, Japan, more than 70 000 people were killed in a matter of seconds - and many more died from the intense radiation effects.
The second difference is that nuclear weapons are much harder to make than other types of weapons. Each weapon requires a significant amount of "fissionable" nuclear material: either uranium that has been "enriched" through a special process; or plutonium, a metal that does not occur in nature, and can only be produced artificially using a nuclear reactor. The process of creating these materials - plutonium or high enriched uranium - has been mastered by relatively few countries, and it is one of the hardest steps in making a nuclear weapon.
Even with the needed amount of nuclear weapon material on hand, making that material into a nuclear weapon is not easy. It requires sophisticated engineering designs and skills.
3. How many nuclear bombs are there in the world today?There are five countries with nuclear weapons that are part of the Nuclear Non-Proliferation Treaty - China, France, Russia, the United Kingdom and the United States. Together, they possess about 27 000 nuclear warheads.
The United States and Russia control about 95 percent of these warheads.
Three countries - India, Pakistan and Israel - have not joined the Treaty and have or believed to have nuclear weapons. North Korea also says it has produced nuclear weapons, but no other country or international organization has been able to verify this.
4. What challenges do you face in trying to stop the spread of nuclear weapons?
The International Atomic Energy Agency works to make sure that nuclear material is only used for peaceful purposes, and not used for making nuclear weapons. We monitor 900 nuclear facilities in more than 70 countries - but in order to succeed, we need the help of every single country.
One challenge we face is that modern technology spreads more easily than in earlier times. The use of the Internet, modern communications, and global trade and travel has made it much harder to keep sensitive nuclear technology from spreading. This, in turn, makes it harder to ensure that no one is working to develop nuclear expertise in secret.
Another even bigger challenge is that, unfortunately, the ownership of nuclear weapons has become more attractive. Because some countries have these weapons, other countries who feel insecure or want to project their power may also want to have them. And as long as nations continue to feel insecure, the process of "nuclear disarmament" goes very slowly.
5. It sounds like a big job. How much help do you get from world leaders, and what else do they need to do?
I get a lot of help. World leaders from nearly every nation take nuclear issues very seriously, and are eager to find solutions to the nuclear threats we face. The problem, of course, is that not everyone has the same view about how to fix these problems.
One step I have proposed is to change the way we control the processes to produce sensitive nuclear material. Processes like uranium "enrichment" are needed for legitimate uses of nuclear energy - such as making nuclear fuel to generate electricity. But we could reduce the risk of "proliferation" (in which new countries would use nuclear material to make bombs) if these processing centres were put under multinational control, rather than being under the control of a single country.
Another very basic need is for countries to get more involved in developing peaceful solutions in regions like the Middle East and South Asia. These regions have a lot of insecurities - due to conflicts, distrust and fear that go back for many decades. And these insecurities, over time, can lead the countries involved to seek nuclear or other powerful weapons. By working harder on peaceful solutions to these problems, we could reduce the motivation for seeking such weapons.
6. Do you think scientists made a huge mistake when they tinkered with atoms and invented nuclear bombs?
Of course I would wish that nuclear bombs had never been invented.
But I would characterize the problem differently. I would say that our "technical intelligence" - the brainpower that we focus on technological advancement - has grown much faster than our "social intelligence" - the intellectual effort we make to deal with social issues, including global insecurity.
We cannot change history, and "un-invent" nuclear weapons. But we can work harder on our social intelligence, to become smarter about how we can live together as one human family.
