Without even going into the whole backstory of the Tesseract or reciting it's lineage and origin. We have a direct, real-time, canon in universe analysis of the Tesseract's shield; performed by Jarvis after Iron Man tried to destroy it with his repulsors. A functional test if you will; Cause and effect/action reaction. The analysis concluded that the barrier/shield powered by the Tesseract itself couldn't be breached by any known force applied, including a nuclear explosion.
Considering that the Tesseract IS an energy source far beyond the technology of earth, and so powerful that even the highly advanced Asgardians put it under lock and key. It's clear that even the most powerful terrestrial explosion (simple energy release) could not have scratched the paint (shield barrier), much less destroy the Tesseract itself.
Jarvis, a top of the food chain supercomputer designed by one of the preeminent genius designers on the planet, concluded and flatly stated that the Tesseract's shield was "unbreachable". He didn't say; "It would take a nuclear blast to breach that shield", 'he' said "It's unbreachable".
Even though Jarvis has a voice interface and could pass the Turing Test, is still a supercomputer designed to analyze power sources (see Arc Reactor). His analysis of the barrier would include any and all means known to penetrate it and found none.