Sooooooo, I've been building a ReactorCraft fission reactor in my Minecraft LP, using a design I took straight from the author of the mod himself, and the reactor blew right the fuck up, flooded my whole house with radiation, and forced me to restore from backup. According to him, fission hasn't changed at all between the time he posted the design and today, so the only possible culprit I can find would be the neutron irradiation chambers not having deuterium in them somehow leading to the reactor overheating and causing a meltdown.
That said, I'm surprised how little physical damage there actually was. IC2 reactors destroy, like, 4 chunks. This thing put a bit of a dent in my floor...though, it killed almost all of my crops. Also, HA, I just realized I'd inadvertently made a Chernobyl. Radiation is a bitch.