A prototyping failure on a fission reactor is a little different than a rocket exploding on the launchpad...
Edit: where did I get the idea it’s 100MW from? Title talks of 1MW. That makes it a lot easier.
100MW would be a lot trickier to test due to infrastructure limitations, but you could probably figure something out with natural gas or propane burners.
But you also want to run it at reactor temperatures. I don't know what that is, but finding resistors that won't melt may be anywhere from difficult to impossible.