Where vs. In which

Where is used to refer to places only; to refer to events use in which

Stars like the sun can continue to shine steadily for billions of years because their light and heat are produced by nuclear fusion, in which  titanic releases of energy result in the loss of only tiny amounts of mass. - correct

Stars like the sun can continue to shine steadily for billions of years because nuclear fusion produces their light and heat, where titanic releases of energy result in the loss of only tiny amounts of mass. - incorrect

Idioglossia is a phenomenon, incompletely understood at best, where two persons develop a unique and private language with highly original vocabulary and syntax. - incorrect

Idioglossia is a phenomenon, incompletely understood at best, in which two persons develop a unique and private language with highly original vocabulary and syntax. - correct