There is no such thing as perfect security but 80% lapse is far fetched also.
Chances of getting caught with radioactive material (i.e. nuke) into U.S. are much higher than anything else. Biological and chemical agents can be of serious concern.
Making RDD or dirty bomb from radioactive materials is quite risky. Radioactive materials like plutonium or cobalt-60 will kill the handler if he is no expert. But if he is an expert chances are he'll get caught since he'll need a proper infrastructure to build workable weapon (be it RDD) - the resources and time he'll require are enough to trigger off intelligence. Plus, this will have to be done on U.S soil to keep the risk of getting caught at borders and ports minimum to zero. Blame would be on U.S alone then.
I totally agree with you, thats why i said in my previous post that its very difficult to make a RDD, however there are some "rebel" group out there who have alot of support.
So lets say for example that a "rebel" group gets enough support and aid from a goverment to carry out a covered ops then even if the risks are high it does not guarantee that it fails.
I mean the security of most western nations are for most nations a serious prio.
I think that there is still alot that can be done to ensure security but most intel organisations like, FBI, CIA, NSA, Interpol, SAS and Mosad (Not to mention russian and chinees agency's) to name a few will probably have their stuff well sorted.
But is it enough?
If there is money (if enough) then anything is possible.
Personally i think that ruling out the possibilities of a serious RDD (Or any kind of such terror attacks) would be a massive mistake as history does show us that with the right amount of effort, money and planning even less organised groups can be very very deadly and even with all the security networks around the world they still where able to carry out their missions.
Having that said, IMO a normal group would not be able to pull such a stunt off at least not at a grand scale.
But if a (rogue/evil) agency or goverment has special intrest and thus supporting a rebel group then i believe that any nation can be hit as they have enough time, planning, money and acces to the proper infrastructure and people.
And thats exactly the scary part as you never know how when and where...there is only IF.
Btw you said that the plutonium or cobalt-60 will kill the handler if he is not an expert.
Well personally i think that the guy who is going to create a RDD based upon these components does not care if he dies or not as it is only a matter of time.
If he fails he dies probably and if he has succes he will be dead to cover up the tracks.
And if he surives the previous 2 options then he probably will get shot on sight if they ever find out who he is and what he has done, as others said before, the targetted nation will probably shake heaven and earth upside down to find who ever is responsible for such a attack.
And as i said before if you got money and connections then the proper infrastructure and people are not that big of a problem to obtain.
Iam not sure but there are alot of arms dealers and corrupted highranking peepz out there who can get their hands on the needed stuff and if not they might have at least the right connections.
just my 2 cents