No it isn't. The purpose of a nuclear reactor is to take highly radioactive material found in nature, cook off a bunch of its radioactive energy (producing power in the process), and create less-radioactive material as a by-product. The waste that comes out at the end is less radioactive than the material we started with, albeit much more concentrated. But that makes it easier to handle and store, compared to the CO2 and other greenhouse gases that are spewed out all over the place.

If we dumped all that radioactive waste into the ocean (BAD IDEA), mother nature would dilute it for us and spread it out until it disappears into the natural background radiation. The drawback is that it'll be dangerous to everything that encounters it before it's diluted, so we can't do that.