4 ways for terrorists to get / use radioactive material in attack: To Acquire a nuclear weapon, to Build a nuclear weapon, to build a crude nculear weapon or to attack a nuclear power plant or a transport of fissile material.
Nuclear weapons provide no protection against nuclear terrorism. The mere existence of the weapons pose a threat of them being used in a terrorist attack. The only way to prevent nuclear terrorism is to disarm nuclear weapons on a global scale and to safeguard fissile material and nuclear power plants sufficiently. A number of other steps have been proposed and can be found on the websites of IPPNW and similar organisations.
First strike is the decision to use nuclear weapons against another nuclear weapon state if you believe they are attacking your country and by using nuclear weapons you assure that your own nuclear weapons will be used and not be destroyed.
According to the World Court Advisory Opinion from 1996 the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict. However the court cannot conclude definitively whether the threat or use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defence, in which the very survival of a state would be at stake.
There might have been situations during the Cold War where a reasonable doubt existed of whether the survival of the Soviet Union or the US was at stake due to the threat of Mutual Assured Destruction. Today, however, it seems doubtful that such a situation will occur. This does not mean, though, that nuclear states will not use the argument of national security as an excuse should they wish to use their nuclear weapons. It is possible to imagine that the ongoing crisis in South Asia might lead to an escalation of the conflict to an extent where the use of nuclear weapons would be considered. As late as in 2002 the Kashmir Crisis deteriorated dramatically leading to the cut-off of all diplomatic relations between the two countries. Should one country have resolved to the threat of use of nuclear weapons this might have lead to a full-scale nuclear war in South Asia.
According to the NPT non-nuclear weapon states are protected from nuclear attack from nuclear weapon states by negative security assurances and during the Cold War most nuclear weapon states respected that. Today non-nuclear weapon states are targets for nuclear attack. This has been exemplified by the US Nuclear Posture Review, in which a number of Rogue States have been defined, which nuclear weapons might be used against. One of these states was Iraq and another North Korea.
Currently many of the Nuclear Weapon States are developing new weapons. Most notably the US have been working for years on developing low-yield nuclear weapons called Mini-nukes and Bunker Busters. In all administrations there are hawks who will push for the use of nuclear weapons based on the argument that "now we have them, we might as well use them". This can be compared to the most frequently expressed reason for the US use of nuclear weapons over Hiroshima and Nagasaki in 1945. It was calculated that the no. of American casualties if conventional weapons would have been used would have been much larger and therefore the US decided to use nuclear weapons instead despite the devastating effects on the civilian population.
A recent example of this policy was the US strategic plans for Iraq in which an attack on Baghdad was planned with a nuclear weapon instead of repeated conventional bombings. The US also had plans of using nuclear earth-penetrating weapons, known as Bunker Busters to bomb underground shelters and weapon arsenals. None of these plans were put into action, but that is no guarantee that similar plans will not be used in future wars.
Some of the major implications of such policies are that the threshold for the use of nuclear weapons will be lowered in the future, which might make it more "plausible" to continue to not only rely on nuclear policies, but also on the use of the weapons. This might create a new arms race and it might also lead to proliferation of weapons to new states.
I. Helfand, L. Forrow & J. Tiwari: Nuclear Terrorism, BMJ, Vol 324, p. 356-358
Collatral Damage, Medact
Jonathan Schell. The Unfinished Twentieth Century
4 Ways in which Terrorists Could use Nuclear Material in an attack:
Soviet suitcase bombs are 1 KT mini-nukes and there is conflicting information of whether all of them are accounted for. The Pakistani nuclear weapons in the range of a 15 TK bomb are small enough to fit inside a shipping container or a truck. According to the IPPNW study, Bombing Bombay a weapon of that size exploded over a larger city would result in 160,000 866,000 deaths. There are a lot of supporters of the Taliban regime and Al Qaeda in Pakistan and should political unrest result in the current Musharraf regime being overthrown by fundamentalists the Pakistani weapons could fall into the hands of terrorists.
The arrest of Pakistani scientists from the countries nuclear weapons program has raised concerns of the transfer of nuclear know-how and fissile material to terrorist networks like the Al Qaeda. Only 10 kgs of radioactive material would be required to build a nuclear weapon. It is estimated that there may be 20 tons of surplus plutonium and 500 tons of surplus highly enriched uranium in the former Soviet Union.
A so called "Dirty Bomb" consists of radioactive material and a conventional explosive device. Low-level radioactive material from medical and nuclear energy waste could be used as well as fissile material used in nuclear weapons. Even a weapon with low-level radioactive material would cause wide-spread injury, long-term cancer deaths, panic and contamination. It is estimated that a casket-sized radiological dispersion weapon loaded with spent fuel from a nuclear power plant and detonated in New York City at mid-day would cause 2000 immediate deaths and injure thousands more.4. Attack Nuclear Power Plant
According to the Time of London the 4th plane that crashed in a field in Pennsylvania on September 11th for the Three Mile Island Nuclear Facility in Pennsylvania. The security of all nuclear power facilities is insufficient and the effects of flying a plane into a power plant would be on the scale of the Chernobyl accident due to core meltdown. Other targets could be transports of nuclear waste, which is regularly transported through major population centres.