Each US nuclear reactor has a planned shutdown every two years or so [1]. It's a crazy couple weeks, particularly since many outside technical crews brought in to work on the shutdown are already scheduled to go on to the next shutdown, and if the dates isn't hit, costs go sky high. Not to mention you've got a billion and half dollars worth of power generation sitting idle while it's down.
Since US plants are old, some of the control systems are purely mechanical. Think PID loops, but built out of spinning metal.
According to the legends circulating around Sunday afternoon barbecues near my local power plant, these require super-human skill to tune. Hordes of fresh engineering graduates wielding Matlab has been broken by the challenge, resulting in dreaded shutdown overruns. Also the difference between a well tuned control system and a oldish one makes a lot of financial difference in the operation of the plant for the next two years. Making a new digital control system would require an unthinkable recertification from the NRC, and is a total no-go.
And so the US nuclear infrastructure relies on a handful of old, retired engineers known as "witchdoctors". They show up for a week or less of a shutdown, tune the spinny bits to perfection, do it on schedule, every time. Vice-presidents tremble at their every whim. The fee for a tuning was rumored to be $250,000, and the witchdoctors then disappears back to one of his three homes in the Caribbean, or to some inaccessible mountain retreat.
(I, of course, cannot verify the truth of any of this. Just things I've heard.)
[1] https://www.roadtechs.com/shutdown/sched/outage-chart.html