In regards to nuclear terrorism, are improbable events with great consequences greater risks than more likely events with lesser consequences?
For example, the probability of terrorists groups obtaining highly enriched uranium and attaining a nuclear-weapons capability is low, but the consequences of detonating such a device would have dire consequences.
On the other hand, terrorists releasing radiation via a "dirty bomb" is more probable, but the consequences are not so dire when compared to the detonation of a nuclear weapon.
Yes, these sorts of events are much greater risks simply because their consequences (both physical and psychological) are so much greater than the smaller but more likely events.
Part of the point of terrorism is to shock people. This is one reason why the 9/11 attacks were so effective as terrorism. They were the sort of thing that few people had ever imagined really happening. Because of this, they had a much greater impact on our psyches than a number of smaller, more conventional attacks would have.
The same is true of nuclear terrorism. A true nuclear weapon might destroy an entire city. At the same time, it would be so unexpected that it would have a much greater shock value. It would devastate us to have something like that (which we did not expect) happen.
Therefore, even those these types of attacks are less likely, they are a greater danger because they would have greater consequences.