^ "closed", you mean like near the coast of Novaja Semlja

And you cannot keep it enclosed, unless you build a big container like in Chernobyl, and stuff it with the toxic waste that cannot be recycled. Any metal that was exposed to radiation, from cooling pipes to cranes, to all kind of equipment, will keep radiating for a some hundred-thousand years.
We do not hear much about it, but Fukushima has become a real problem, they see no way of being able containing the stuff; the radiation is rising and they do not find an explanation. And Chernobyl will be there for millenia to come.. not to speak about all those atmospheric nuclear tests.