Dirty bombs are actually terribly ineffective weapons. The radiation would be unlikely to harm anyone who wasn't close enough to be killed by the conventional explosives unless they spent a considerable amount of time in the irradiated area afterwards without any sort of protection. While the cleanup operation could be expensive, the whole affair would be more an inconvenience than anything else. On the plus side though, now you can detect the terrorists' essentially conventional explosive which would otherwise be quite concealable with radiation detectors.

Also tritium is essentially harmless. It is an incredibly weak beta emitter - the electrons it emits won't make it through the upper layers of dead skin if it's outside of your body. It has an extremely short residency period in the body if it is ingested (a benefit of being chemically identical to hydrogen). It also will dissipate in an area rapidly - it rises quickly and even if it is in an enclosed space it will pass straight through the walls. Also it's worth noting that a fusion plant like ITER has less than a gram of tritium inside of it at any given time.