Lithium hydride
Before the Castle Bravo nuclear weapons test in 1954, it was thought that only the less common isotope lithium-6 would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful lithium-7 also does so under extreme conditions, albeit by an endothermic reaction.
Duck and cover
A notable comparison to underline this is found when one compares the 50 megaton air-burst Tsar Bomba, which produced no concentrated local/early fallout, and thus no known deaths from radiation, with the surface burst of the 15 megaton Castle Bravo, which in comparison, due to the local fallout produced, was implicated in the death of 1 of 23 crew on the Lucky Dragon and made the entire Bikini Atoll unfit for further nuclear testing until enough time elapsed and the intensity of the radiation field had decayed to acceptable levels.
Furthermore, regardless of if a nuclear attack on a city is of the surface or air-burst variety or a mixture of both, the advice to shelter in place, in the interior of well-built homes, or if available, fallout shelters, as suggested in the film "Duck and Cover", will drastically reduce one's chance of absorbing a hazardous dose of radiation. A real-world example of this occurred after the Castle Bravo test where, in contrast to the crew of the Lucky Dragon, the firing crew that triggered the explosion safely sheltered in their firing station until after a number of hours had passed and the radiation levels outside fell to dose rate levels safe enough for an evacuation to be considered. The comparative safety experienced by the Castle Bravo firing crew served as a proof of concept to civil defense personnel that shelter in place (or "buttoning up" as it was known then) is an effective strategy in mitigating the potentially serious health effects of local fallout.