Few specifics have been supplied by Matt Groening regarding the Springfield Nuclear Power Plant. The nuclear assembly was cited as a Fissionator 1952 Slow-Fission Reactor, but beyond that, we can only guess. It presents all the appearances of the pressurized water reactor plant with which we are somewhat familiar, as these are the most common types in the US.

He is the safety inspector at the Springfield nuclear power plant.

It's not a company per-se, it's The Springfield Nuclear Power Plant

If you mean the Springfield in the Simpsons, it's Mr. Burn's nuclear power plant. If you mean one of the hundreds of other springfields in the world, you'll have to specify which one.

what is the existing of nuclear power plant?