No. It's not. At all. It's two completely different processes.
A nuclear power plant uses fission to heat water. The unstable nuclear elements break apart (the decay) which releases energy. That energy heats the water. The hot water is pumped to a heat exchanger where it drives a steam engine. Yes, nuclear power plants are very fancy steam engines.
The sun is a giant ball of gas that due to gravity is collapsing in on itself, shoving hydrogen atoms together with such force that they combine and turn into helium. The byproduct of this shoving together is light and heat. There's no water or steam involved in this process.
Other than producing heat, there's nothing similar to the processes of a nuclear reactor and the sun.
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