Thermal energy stored for decades without losses

I suspect it's a giant version of those hand warmers which do enthalpy when you snap a thing inside the bag. Phase change stuff.

Not sure what's novel here. Maybe its some small specific increment of efficiency.

Heat storage in "salt" is a common pathway. Salt being a generic word not sodium chloride specifically.

Almost anything in the "other phase" state is by definition stable for years without loss unless it sublimated. Ok, explosives aside..

(Not a chemist or a doctor or lawyer or engineer so there's that. Corrections welcomed)