Seeing an issue on a server running unmodded Tekkit 1.2.8d whereby the reactor simply ceases to function. The reactor was not outputting any power, nor were any values in the GUI changing. If I clicked deactivate it didn't update, but if I closed the controller and re-opened it, it did reflect the change in button state. When I removed a block of the reactor housing it didn't 'disassemble'. Unloading/reloading the chunk didn't resolve the issue. Restarting the server also had no effect. I broke the controller and replaced it, and it is dark (not recognizing a reactor). Curiously, while it is in this state I am still able to eject fuel that was in the fuel rods.

I just discovered something that might be causing this. My reactor configuration has been as such:
CCFSS
CCFSS
FFFFF
CCFSS
CCFSS

C = Gelid Cryotheum
F = Fuel Rods
S = Space

It was configured this way because I didn't have enough Gelid Cryo to finish the other side. So, I thought I would dump water down that side to eke out a few more RF/tick. However, it looks like the gelid cryotheum turned some of the water flow blocks to ice and snow. Once I removed the ice/snow blocks, and replaced the casing blocks, the reactor started working. It looks like it stopped being a valid reactor when the water turned to ice/snow.

Interesting. I'll add ice/snow to the valid blocks list for 0.3.1, which should fix this.