It's an extra section for a tunneling machine that Mantic's not-skaven army use. I bought two, because they were cheap and had an interesting shape. However, I thought that they looked more convincing as terrain than vehicles.
I put one on top of the other to make a cylindrical tower. Then I started adding extra bits from old toys and other terrain kits to fill out the model and give it additional detail:
The original kits provided a lot of interesting shape. A lot of my Necromunda terrain is quite low - small houses, market stalls etc - and it's good to have something that towers over them to break up the monotony, rather like the reactor in the middle of Diamond City in Fallout 4. I was able to extend the tower to the side, using leftover plates from the tunneling machines. Then it got some extra details in the form of control panels and what might be fuel rods, and some paint.
As you can see, it's quite large and bulky. As with the favela-type houses that I made a while ago, I tried to paint some panels in bright (but dirty) colours to break up the monotony of grey. The base was an MDF frame from the local art shop. The reactor doesn't actually lean to one side - that's just my dodgy photography!
And here's a picture of the top.
Thinking about it, I wonder if I should add a lift to the side to explain how people get to the top. I've also got a strange mounted device from Warlord that could be used as a communications array or some kind of gun, which could sit on the helipad. But at the moment, I'm happy with how it's turned out.