Hmm...
Edit: To be clear (sorry if I wasn't before) I meant from KSP version 1.3.1 to 1.4.5 something has changed, I was working off of an Interstellar version someone was using as part of a modpack (from Janurary 2nd of this year) and now am work off of version 1.19.4
well it was a beam core reactor in 1.3.1 as well so I have both, to be on the safe side I added some liquid hydrogen but still nothing, here's a screen shot, center reactor window is the small anti-matter reactor dedicated to electrical power and the two on the right are dedicated to the engines.