Good info from HydroDave if hired as an STE, but it sounds like your job offer is to be an engineer in the fluids and mechanical code (C2320 at the public yards)....
You'll have to complete a basic system familiarization, and then you'll be assigned a particular mechanical or fluid system to become the expert on. If there are problems with that system (a leaky valve, improper system response, etc), you'll be answering the mail that documents the condition to perform the corrective actions IAW all applicable tech manual and reactor plant manual requirements as well as radiological controls manual, environmental regulations, etc. As you progress in seniority, you'll take on more systems. You might also be on call to support testing and evolutions, or assigned to shift during them.
Likely hiring on at the GS-12/13 level and progressing towards GS-15 based on length of service. Upper management positions would be 10-15 years out dependent upon attrition and your acumen for the business.