Against creepers and anything else normal NPCs can throw at it; construction foam is 100% effective once hardened.
Against a nuclear explosion, it's better than stone, but no where near enough; and for chunk loading issues, you won't be able to sufficiently contain a reactor within a single chunk, so it's out.
You /need/ to use re-enforced stone. 3 layers should be enough for any normal reactor explosion. I've done stupid tests with a 6 chamber reactor filled with uranium cells; remove the redstone torch beneath and boom; 2 layers of Re-stone are abraded. The reactor actually pops it's outer chambers first, followed by the core with that explosion. It's more intense than a single nuke, but quad nukes make a bigger boom.