Anna,
If a nuclear event ever occurred that would warrant the practical use of such an underground shelter, then the world as you know it would basically be gone, and within a year or three (after considerable starvation and suffering) you would die from a cancer from the radio-active particles carried by the wind, settling on the top of the soil, in the groundwater, and in your lungs.
As for a shtf scenario, a structure like this could never ever be kept secret. All of your neighbors, indeed the whole town, would know about it. It would become a magnet for attack and siege, because starving folks would rightly or wrongly conclude that you have years and years of food and supplies buried within. Ever see the WWII films of the German grenadiers attacking the "indestructable" buried concrete fortresses on the French Maginot line? It was pretty easy to open them up. And in a shtf scenario, you can rest assured that the thousands of tons of commercial tnt stored at commercial facilities will not just disappear into space. People who work there will take it, trade it, use it.
Nope, the only fortresses that ever lasted for a really long time against constant attack were the walls and defenses erected by the Romans and the Eastern Holy Roman Empire around the City of Constantinople. (This city was originally named Istanbul, and was taken by them from the Muslims.) I believe that the defenses were built around the year 200AD, and held back the constant attacks by the Turks and Muslims for about 800 years, until it finally fell around 1,000 AD. They promptly renamed it Instanbul and it remains so named to this day.
Just my thoughts.
Mannyrock