alt Hacker NewsIf they can spend 5 millions just for mast removal, I am pretty sure they could build some rough reinforced concrete dome / something similar all around it (15m depth ain't something impossible to build on) that could absorb most of the blast.
Say 50 million budget. Of course red tape, bureaucracy and just laziness/incompetence of state bureaucrats would make such budget balloon and drag for a decade or two, but it should be doable.
Water is a great blast-absorbent material.
If 15m of water can't contain the blast, adding concrete or similar is more likely to add solid projectiles to the explosion than diminish the force of the blast.