Recipe for ending nuclear meltdowns.
1 egg ....
1 good egg, so probably more like 25 eggs.
I am serious. Eggs. You need them.
Add a pinch of redstone dust. Some may say this is optional, but it's good to have. I used 3 dust, so it's ok to be afraid of it, you can still do it.
1 redstone torch. Uh oh, more redstone stuff, this is beginning to sound complicated, huh? If you want simple, you'll like the next ingredient.
Glass. and a solid block, like cobblestone, a half slab + 1 of your choice.
Stone pressure plate. Now we are back to the complicated ingredients... signal this and invert that and what's next? That's it, that's all you need to prevent nuclear apocolypse.
I suppose it would be nice to have another thing, that isn't really a thing. The ability to plan a spot for this when you build your reactor building. This one is probably the hardest for most of you who already have your buildings built.
So a picture is worth a thousand words. Here is three thousand words worth.
Very visible proof of concept during my building mock up priot to building my plant.
http://imageshack.us/photo/my-images/577/20120214151132.png/
Action shot with concealing floor removed.
http://imageshack.us/photo/my-images/254/20120216015439.png/
Shot from the walkway, what it looks like during use.
http://imageshack.us/photo/my-images/848/20120215203504.png/
A chicken will prevent nuclear meltdown. I told you you'd need eggs.
Ok, step one, take your solid block. The chicken is going to stand on it. The chicken needs to be standing in the 7x7x7 centered on your reactor, but the 3x3x3 is your external cooling, and really nasty things can happen to blocks in the 5x5x5, so i put it on the edge of the 7x7. You should too! You can go up or down, but if you put in the bottom or top edge, remember the chicken standing on it has to be in the cube, so the block is one lower than the chicken. Ok, the hard part is done. Now, we are going to put a torch on the side of the block. Not just any torch, a redstone torch! Whew, redstone is hard. The plan is to connect that torch to the reactor using redstone dust. It shouldn't take much. Make sure the redstone points into the reactor, I don't think going sideways works. If move the redstone torch to a different side of the block makes it easy, do that, but the torch needs to be on the side of the block, not the top. We have plans for the top.
If you've done what I said, a redstone torch is sending a signal to your reactor, shutting it off. This is good. Now, I had plans for the top of that block. If your thinking chicken, you are a few steps ahead, calm down pardna. First a stone pressure plate. Now hop onto it. When you do, the signal to the reactor shuts off, allowing it to start. I think you see how this is going to work now. Something on the block, reactor works. Nothing on the block, reactor is off. So we put a chicken on it, and the chicken lets the reactor work. But we gotta keep the chicken trapped. That's what the other blocks are for. Put the glass on top, above the pressure plate. Put the half slab where you want to stand when you are looking at the chicken, and the remaining block(s) of your choice whereever they need to go to make sure it is trapped on the other 3 sides. I needed only 1 extra block because it's in the corner. So you now have a 1 block space with a pressure plate, with a half slab for access and a glass top for your viewing pleasure.
It is now time... for the chicken. This part can be tricky, because the eggs like to breack on the glass, but good aim will make them hit the pressure plate. Fire away until you get a baby chick. Now your chicken prison is populated! and the chick should turn off the redstone allowing the reactor to start as you like. Why does this chicken stop meltdowns? Bad things happen as the reactor gets hot. Flammables burst into flames, in a 5x5 area. If your wire is sitting on a wood block I am going to punch you. it will burn and you can't turn the reactor off! Then water starts to evaporate. You did put your source blocks outside the 5x5 right? The worst is complete meltdown, accompanied by a massive explosion. What we just did will avoid that, but you should build a massive bunker out of reinforced stone anyway, because it looks cool. Before that happen, blocks in the 5x5 melt and turn to lava. We just prevented that too, but don't count on reinforced stone to provide shielding in this zone, it will probably melt instead. Note that when this occuring, your off switch, the redstone wire, can and will melt and become useless, so we can't let it get here either. We won't, because before that happens, at 70% hull heat, the reactor starts leaking radiation, damaging all nearby life, you included. But you probably aren't there, if you were you'd hit the off switch. The chicken is there. This brave sentinel is waiting vigilantly to respond to this event by... well, dying. He is trapped there after all. and in death he serves his purpose, by no longer stepping one the STONE pressure plate. Not even his drops would keep the redstone torch from sending the off signal... you did use stone pressure plate, not wood right? Wood pressure plate would allow his dead poultry body to keep the reactor on for 5 minutes. Don't use wood, use stone like I said,. So when the 70% threshold is reached, your helpless chicken captive dies, and the reactor gets shut off. Your reactor will not turn back on until you restock your chicken.
There are a couple unlikely ways you can still achieve meltdown. If you pack your reactor so full of uranium that it will melt down before the chicken finishes dying in 4 seconds, then you will melt down. That takes a 6 sec meltdown time, which is achievable. Manually adding buckets of lava after you start taking damage until it explodes would do it. Using more isotopes than external cooling can compensate for would allow the reactor to be gaining heat even while its off, so if you stuff it full of isotopes that offer no benefit and pour lava on the reactor to negate cooling, that would do it too. So really, unless you blow it up on purpose, this chicken kill switch should prevent ALL meltdowns during normal operation.
Have fun with it!