Last console line is "Saving worlds"
No relevant logs

Mods:
Create 0.5.1h
Destroy 0.1.0-snapshot-8
e4mc 5.2.1
JEI 15.20.0.101
Petrolpark's Library 1.0.6

What's in the world? I'm presuming its due to a Vat, in which case what's in the Vat?

there were 2 vats, one with mercury, sodium, air, and open vent, and one with air, chlorine solution, and closed vent. will test them separately later if neccesary.

It appears that the bug is caused whenever there exists an open vent vat in the world, irregardless of contents.