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Problems with both. Lots of problems. First off, over cooling can cause problems of its own. You change the nuclear reaction with the temperature. Water aims to bring it down to a certain temperature, not freeze the nuclear reaction, because that could cause untold issues. Second, transporting both of these is expensive and extremely difficult. We're talking about using very large amounts of these, and both liquid nitrogen and dry ice don't like to be in those states. They're both gasses have been forced into a different state by both high pressure and extremely low temperatures. Transporting small amounts of the stuff is highly costly. Transporting enough to cool an entire nuclear reactor is unheard of. Third, when you use these two, they create new chemical reactions. We know that H20 has no tremendous reaction with a nuclear reaction beyond cooling it down. It's a lot less certain what condensed CO2 and Nitrogen gas could do. The main problem with both is that they can be highly flammable, and I don't see that being a benefit to anyone.

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