Going to check the status of my reactor with a tinker's construct hammer in my hand. I right clicked the reactor controller and the modpack crashed causing my save to become corrupted.
[Yogscast Complete Modpack on the ATLauncher]

Until the bug isn't gone, open the world with mcedit and try removing the controler. You can save your world.

Which version of Big Reactors are you using, and can you post the crashdump?

If it's pre-0.3.4, please upgrade. This sounds similar or identical to #191 which was fixed in 0.3.4A.

I am running version 0.3.3A so that would be the problem. I did manage to recover my world by pasting the region files into a new world.