In a nuclear reactor a spontaneous nuclear process results in a net decrease in the net mass of all the particles involved. The "missing mass'' appears as the kinetic energy of the reaction products, which is dissipated by what amounts to friction and generates heat that boils water; the steam is used to spin turbines that run generators that send electrical power down the wires.

This leads to an obvious question: can we do the opposite ? Can we take electrical power out of the wires, use it to raise the kinetic energy of some particles to enormous values, smack the particles together and generate some extra mass? Yes! This is what a particle accelerator does, like TRIUMF, the one at the University of British Columbia. Every such accelerator is a sort of "reactor in reverse,'' taking electrical power out of the grid and turning it into mass. Find out more at Jess's Skeptic's Guide to Physics.