Its a 50s B&W film where the aliens come to earth in small meteorites and from memory are like mushrooms/spores and when a human gets close it bursts and takes them over. One scene I remember at the end is the alien somehow forms in a nuclear reactor and get huge and people are trapped in the control room and there is blood coming out of a small access pipe as the alien absorbs humans.
Its been bugging me for days so any help appreciated
Could it be Quatermass II from 1957? It features meteorites that release gas and take over the mind of anyone who gets sprayed, and eventually it's revealed there's a giant alien life form living inside a giant pressure dome which you might have misremembered as a nuclear reactor.
The movie was an adaptation of an earlier TV serial with the same story.
Invasion of the Body Snatchers ?
The story depicts an extraterrestrial invasion that begins in the fictional California town of Santa Mira. Alien plant spores have fallen from space and grown into large seed pods, each one capable of reproducing a duplicate replacement copy of each human. As each pod reaches full development, it assimilates the physical characteristics, memories, and personalities of each sleeping person placed near it; these duplicates, however, are devoid of all human emotion. Little by little, a local doctor uncovers this "quiet" invasion and attempts to stop it.1
