Dick question, but is that actually specified anywhere?Yes, an unloaded reactor will melt down.
That will not work forever.
I don't know how items are nulled in minecraft, but I half-expected a destructor method on the waste item to check its location prior to voiding and punish me for my cleverness. I haven't touched java in 15 years; do destructor methods even exist? (this would also punish for dropping in lava)
I'll make an effort to do the proper waste-disposal method when I need to.