Nuclear subterrenes do they exist?

A number of patents were filed by scientists at Los Alamos, a few federal technical documents were written -- and then the whole thing just sort of faded away.
Or did it?
Nuclear subterrenes work by melting their way through the rock and soil, actually vitrifying it as they go, and leaving a neat, solidly glass-lined tunnel behind them. The heat is supplied by a
compact nuclear reactor that circulates liquid lithium from the reactor core to the tunnel face, where it melts the rock. In the process of melting the rock the lithium loses some of its heat.
It is then circulated back along the exterior of the tunneling machine to help cool the vitrified rock as the tunneling machine forces its way forward.

From Grace Powers Earth Talk Radio

US Patents #3,693,731