My guesses: - Water would either require a tremendous sized reservoir, or infrastructure capable of withstanding the pressure of superheated steam - superheated

My guesses: - Water would either require a tremendous sized reservoir, or infrastructure capable of withstanding the pressure of superheated steam - superheated steam, and even water to an extent, has corrosive chemistry. Sand is basically inert. Those are just guesses though. I'd love an engineer who works on this to break down the real reasons.