If i remember properly buildcraft supports nuclear reactors. Just use compressors with water cells or pumps to get snow and compress it again to get ice and pipe it in to the reactor. Automate it all with pipes and you should be set, if you need external cooling that bad on a reactor you should be able to net enough energy gain to make it worthwhile. But then again I don't play with reactors that much so you will have to figure it out.
Yes, and I've tried it, but it is not very efficient: you need min 20% of the energy you produce with the nuclear reactor to create Ice Blocks, and it's a lot of compressor/redstone engine/tube/cable.
RedPower CASUC is 10 time more efficient (and cheaper)