At this point (at least a year after the protracted nuclear exchange), radiation from most fallout would be long-gone (decayed or washed/blown away to nothing) so in most areas so there would be a noticeable effect, especially where airbursts were used. There are exceptions to the rule of course: downwind of destroyed reactors and nuclear storage facilities, in or around blast craters from groundbursts, "hot spots" left by the fragments of the bomb casing/unused fissile material for airbursts, etc.
However, it is still valid to ask what the overall effects and values of these weapons would be. "Dirty bombs" are currently mainly psychological weapons in that they incite fear and would disrupt urban areas, even if the actual area of effect is limited. If such a weapon were to be detonated nowadays, casualties would be minor, given modern medical science. In T2K:
1) People who have survived much worse would likely be knowledgeable and possibly prepared for what would be minor local fallout in comparison to an actual nuclear strike.
2) Urban areas in this phase of the Twilight War are already heavily disrupted. Probably the most one could do with a dirty bomb is maybe force an evacuation or at least make civilians and military personnel use protection and thus make life very difficult.
3) On the down side, populations are probably psychologically precarious and have little mental or physical resources to draw on in an attack. Civilians might flee because they think it's another nuclear attack, military units with poor discipline might bug out so as to not increase individual radiation doses, etc.
4) Further, surviving protective equipment is likely in poor condition, CBW decontamination protocols and equipment would be neglected, etc. Likewise effective medical care is probably not going to be available. Transportation is in a similarly dismal state, so evacuation might not be possible or even if carried out would be enormously disruptive.
5) The scarce remaining viable farmland is highly valuable and vulnerable. Detonating or threatening to detonate a dirty bomb could seriously disrupt a community's tenuous food supply. Likewise for scarce (and scarcer) water supplies.
To summarise, I think there would still be a strategic and tactical value for simple "dirty bomb" weapons and some would consider building them if they have the means.
A possible scenario might be a gang (marauders) could blackmail a community with some kind of home-made dirty bomb by, say, packing scavenged radioactive medical material into a truck-bomb and then threatening to detonate it upwind of the community and/or its fields and water supply. The community would be forced into deciding either to evacuate or ride it out, depending on what limited resources they have, or to capitulate. (Or, of course, bribe a wandering band of mercenaries/soldiers like the PCs into doing their dirty work for them!)
Or, rival political factions or adversarial military forces could conduct some kind of sneak attack for psychological or "scorched Earth" reasons. For example, the US federal government conducts a census prior to holding general elections. Concerned at the possibility of ceding further legitimacy to the civilian powers, US military personnel could attempt to seed a crucial water supply with radioactive material. This would sow confusion and disrupt enemy efforts, perhaps even drive the civilian population into the safety of the military camp. Conducted by a "rogue" unit of course, deniable and probably false-flag as well. Or maybe it really is the dastardly CIA seeking to defame the military in some way or force it out by contaminating one of its cantonments.
In my game left- and right-wing domestic terror organisations were highly active in the USA before the war. Such groups (including or especially New America) would be seeking "pocket WMDs". Left-wing terror groups could have been instructed by the KGB in preparing and using dirty bombs, even given access to stolen or smuggled radiological materials.
As to where you could get such materials, at least in North America, I think somewhere it does say most remaining US strategic weapons are secured by the US military, although bribery and corruption is possible. Medical supplies and civilian reactor waste might be a better bet once the strict controls on them start to break down. In Europe I would guess tactical nuclear weapons might be available in some form. Maybe not as deliverable weapons, but dud warheads, unexpended nuclear munitions, etc.
"TIRASPOL, Moldova -- In the ethnic conflicts that surrounded the collapse of the Soviet Union, fighters in several countries seized upon an unlikely new weapon: a small, thin rocket known as the Alazan. Originally built for weather experiments, the Alazan rockets were packed with explosives and lobbed into cities. Military records show that at least 38 Alazan warheads were modified to carry radioactive material, effectively creating the world's first surface-to-surface dirty bomb."
From "Dirty Bomb Warheads Disappear" in the Washington Post.
http://www.washingtonpost.com/ac2/wp...nguage=printer
The skill level to build such a weapon would not be that high, I would imagine maybe Average or Difficult at most. Mostly in safely dismantling the weapon or breaching containment to recover the material. Building the bomb would require Demo skill. The delivery system could be something improvised using MLRS rockets or even heavy mortars.
Tony