[Flyndaran]

Fission reactors seem like an odd thing to power it with.
I'd assume such facilities would be hooked up to the planetary grid with large batteries/capacitors to handle the short term enormous power required.

And also it would be easier to set up numerous small laser launchers to launch small ships into LEO.
Greater safety, back ups, easier to manage power requirements, and less usability as single ginormous weapons.