What type of CBRN (Chemical, Biological, Radiological, Nuclear) attack would be more likely to occur in the US and why?
I believe that the most likely type of terrorist attack in the United States (of the types you mention) is a chemical attack. The next most likely is a radiological attack. The main reason for this is that these are the types of weapons that a terrorist group would be most likely to be able to obtain.
I would expect that a WMD attack on the United States would be more likely to come from a non-state group such as Al Qaeda than from an actual state. This is because a state would have much more to lose from an attack. If the US could identify the state which had attacked, it could use its massive military capabilities to retaliate. It is much harder (as we have seen since 9/11) for the US to destroy a terrorist group through military means. Therefore, we need to think about what sorts of weapons terrorists would be most likely to obtain.
My view is that a chemical weapon would be the easiest to obtain and deliver. I say this in large part because we have already seen a terrorist attack using chemical weapons. This was the Aum Shinrikyo attack that used sarin gas to kill 12 and injure thousands in the Tokyo subway. This attack occurred in 1995. If a relatively small group like Aum Shinrikyo can obtain sarin and carry out an attack, it seems likely that a large terrorist group could do the same.
For the same reason, I think that a radiological attack would be the next most likely scenario. A dirty bomb would be much easier to make than an actual nuclear weapon. This means that it would be much more possible for a terrorist group to carry out a radiological attack than a nuclear attack.
It is likely that a terrorist group would prefer to do a nuclear attack since an actual nuclear weapon would be much more destructive than either of the options I have discussed. However, chemical and radiological weapons would be much easier to obtain and deploy. Therefore, I believe that a chemical attack is the most likely, followed by a radiological attack.