Make sure the control rods are all the way up. When I built it it would explode, but I haven't used or tested it since I built it and I don't know if the devs have changed anything with reactors. It should be noted that when it has a "meltdown" it will explode and radiation will spike but that's about all. The reactor itself will still be intact.

Do they eventually actually melt? Radiation got up to about 85 then started going down on its own, I'd hoped for an explosion or something haha

I know. I was just telling people that and the alarm stayed in the whole time untill it dipped below 1 and then it turned off

When that is the case either the radiation has returned to safe levels or the battery has died. Either of which I consider to be fine. The purpose of this is for people who don't want to build their own reactor but just want to see what the reactor having a meltdown is like. It wasn't intended to be a long term solution to playing with the reactor, just a way to watch it blow up.

You do realize that if you disengage the fuel rods the radiation peaks at 88-ish and the slowly goes back down and the alams go off.