To use the magma for energy, workers wouldn’t drill directly into it. Instead, they could either tap into superhot water in nearby magma-heated rock and use its steam to turn turbines, or make artificial steam by injecting water from the surface.
Volcano power? Scientists drill a well into magma
— By Janet Fang on February 9, 2014, 10:15 PM PST
Heck, they've been getting superheated steam for years at The Geysers, near where I once lived. For some time, however they have had to inject treated wastewater to run the turbines.
It's a bit sloppy to call the Earth's core a nuclear "reactor". It is indeed powered by nuclear energy, but the term "reactor" should be reserved for stimulated reactions, either fission or fusion.
The Earth has stayed molten for more than a hundred times as long as the greatest thermodynamicist of the 19th century could account for. It has done so because of the decay energy of very slightly unstable and therefore long lasting isotopes.
The difference is worth noting, inasmuch as people far less informed and careful than Janet Fang refer to the mere fission products of the Fukushima-Daichi reactors as "going critical".
When a neutron splits a uranium nucleus into two approximately equal pieces plus a few neutrons, the energy given to these various bits greatly exceeds the energy of the mere alpha particles that are the side product of the isotope's gradual decay.
A mass of fissile atoms arranged so that exactly one neutron from each fission event is likely to cause another fission. That's what a reactor is design is considered "critical". That's what a reactor is designed to create and maintain. A bomb is designed to go "supercritical", to cause a rapid multiplicative chain of these events faster than the explosion can blast the whole mass into a large sphere 1000 times as hot as the surface of the Sun.
There exist good nuclear reactor designs that without human or mechanical intervention respond to excessive temperature (loss of coolant pumping, for example) by going "sub-critical". The Fukushima reactors, even, did in fact shut down. What failed, and the reason the fission products melted the reactors, was the auxiliary diesel electric supply designed to circulate the coolant very high pressure water.
Even before Chernobyl, there was a breeder reactor that was proven by actual test to shut itself down and cool itself safely without auxiliary power, it it got hotter than it should.
The primary coolant was molten, unpressurized sodium metal, in a steel vessel, under an atmosphere of argon.
The electrical circuit that used the reactor's own electrical production to drive the coolant loop was interrupted.
As designed, the rising temperature caused the reactor core to expand just enough to lower by a fraction each neutron's chances of hitting a nucleus. So it went sub-critical. The heat of radioactivity of the fission products was easily dealt with by convection in the liquid sodium.
What a concept! Thanks for the 411 Janet!
Perhaps residents of Naples should read this, and with careful study and due diligence, utilize the potential energy of Vesuvius, and at the same time, in no doubt just a tiny way, relieve some of the pressure building up there! Just make sure any bore holes are aimed in directions where blow-outs can be contained.
Or maybe we should not mess with Mother Nature .