What is this on the output of my heater?

I got my pump running after the freeze, and then a plug of some sort push out of the heater, at the outlet side, and water gushes out of a hole about 1-1/2". So I looked at it, and it's a plug of sorts with 2 o-rings, with a threaded nut holding it in. And the nut broke with the freeze. I'm not sure what this is. I may be ok if I get a new nut, but I don't know what to look for. Here are picturesThe two round piceces are the parts of the nut that broke. It's formed like a cap, but with that hole in the middle,

Steve T