Refining U-235 out of natural U-238 can be done directly instead of using a reactor, but it is quite hard. Those centrifuge stories you may see are about that other route - that is equipment needed to pick U-235 out of natural uranium.
It now appears the uranium was less than originally stated, hundreds of grams not kilograms. 15 kilograms is probably the weight of the whole thing they took out of the car, shielding and all.
On breeder reactors, North Korea and Iran have both tried to go that route. The NKs had a reactor built, and "promised" to shut it down in return for payoffs from us. The Russians are helping Iran build a reactor as we speak. Notice that these are the three countries that made the "axis of evil" speach - the three countries that support terrorism abroad and also have active nuclear research programs...
The 15.7 kg of Ur never quite made sense anyway. That's alot of radioactivity to contain the transportation device would need alot of lead to protect the courier (making the package even more massive). I was more concerned with the story that this material could have come from the former Soviet Union and that it was consistent with material lost by FSU in the construction of low-yield weapons. I'm no expert in nuclear weapons, but the finding of Ur (any quantity) substantiates the stories published here about traffic in these materials in East Europe.