Half-life isn't half the lifespan of a radioactive thing. (If so, then why would we bother dividing it in half? Just say what the lifespan is.) Rather, it's the length of time that it takes half of that radioactive thing to decay.

So if you have, for example, 1 g of material with a half-life of 1 year, then after 1 year, you'd have half of it left, so 1/2 g.

Then, you have 1/2 g of that material, so after another year (2 years total), you'd have half of that 1/2 g left, so 1/4 g.

Then, you have 1/4 g of material, so after another year (3 years total), you'd have half of that 1/4 g left, so 1/8 g.

And after 4 years, you'd have half of 1/8 g left, so 1/16 g.

And after 5 years, you'd have half of 1/16 g left, so 1/32 g.

And so on.

The "lifetime" of the material is effectively infinite, in a mathematical sense. Leaving aside the physical atomic limit, you'll never get rid of all of it. But you'll have an ever-decreasing amount of it.

BTW, Shenzhen is a city with a population of 18 million.