lots of stuff on the web about this. weapons tech has advanced considerably since WW2
1 kt may be "optimistic", but still extremely dangerous if true. a couple of excerpts from a google search for "suitcase nukes":
http://www.nationalterroralert.com/readyguide/suitcasenuke.htm
Suitcase Nukes
A "suitcase" bomb is a very compact and portable nuclear weapon and could have the dimensions of 60 x 40 x 20 centimeters or 24 x 16 x 8 inches. The smallest possible bomb-like object would be a single critical mass of plutonium (or U-233) at maximum density under normal conditions. The Pu-239 weighs 10.5 kg and is 10.1 cm across. It doesn't take much more than a single critical mass to cause significant explosions ranging from 10-20 tons. These types of weapons can also be as big as two footlockers. The warhead consists of a tube with two pieces of uranium, which, when rammed together, would cause a blast. Some sort of firing unit and a device that would need to be decoded to cause detonation may be included in the "suitcase."
Another portable weapon is a "backpack" bomb. The Soviet nuclear backpack system was made in the 1960s for use against NATO targets in time of war and consists of three "coffee can-sized" aluminum canisters in a bag. All three must be connected to make a single unit in order to explode. The detonator is about 6 inches long. It has a 3-to-5 kiloton yield, depending on the efficiency of the explosion. It's kept powered during storage by a battery line connected to the canisters.
http://cns.miis.edu/pubs/week/020923.htm
According to reports in late 1998, including a detailed one in the London-based, Arab daily, Al-Watan Al-Arabi, Chechens acquired approximately 20 "tactical" nuclear weapons from Russian facilities, to be subsequently transferred to bin Laden for a sum of $30 million in cash plus two tons of opium. These claims, too, were not corroborated by other sources or physical evidence
