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.
You’re correct about the skin penetration, but the risk with tritium is in inhalation. Experiments with mice demonstrate carcinogenesis. The increase in cancer risk from inhaled tritium in humans is unknown, but the substance is considered dangerous enough that it’s a pain in the ass to get the certifications to use it in your laboratory. It definitely should not be considered harmless.
I worked with Stephen Bodner on the fusion program mentioned in the fine article. We did our experiments with deuterium. The reason we did not use DT, which would have been better for the experiments, is because nobody wanted to (1) go through the hassle of getting the lab certified to handle tritium' (2) get anywhere near the stuff. It is considered very hazardous.
Tritium is a pain in the ass to get certifications for because it is so difficult to contain. Even in an airtight setup, it will leak. If you spend a substantial amount of time in lab with a tritium leak, and are inhaling the stuff over months or years, yes it's dangerous, but hydrogen doesn't remain in your body long enough for a single brief exposure to do anything.
Mercury vapor is very hazardous, that doesn't mean a mercury bomb is an effective weapon.
Your points are solid; I accept that this material would not be very effective as a weapon, except maybe in a confined space. But it’s still something whose possession we want to control, so handling and transporting it adds to the cost and complexity of operating a commercial fusion facility.
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.