FRiend, you are suffering from a “failure of imagination”.
Just ‘cuz you can’t imagine how it could be done doesn’t mean it can’t be done. Never underestimate your enemy.
Anyone who witnessed the NYC Twin Towers majesty before 9/11 could not believe those towers could possibly collapse, but they did. For some, the fall was beyond comprehension.
US gov’t “in charge” of air attack safety favors terrorists piloting aircraft into either spent fuel pools or reactor buildings.
Or what about a Level 4 biological or poisoning attack in the control room, where all the operators were killed and rescue for the plant came too late?
What about loss of temperature control via a hack attack on SCADA systems, a common tactic now? Or failure of backup power during a widespread power outage?
What about... things we haven’t imagined could happen?
That it could be successfully be accomplished is laughable because the physics of the problem make any attempt a guaranteed to be fatal to the thief in a matter of a few hours at most which is far less time than would be necessary to accomplish the task.
Yes insider sabotage of power reactors is a possibility that the industry is well aware of and counter measures are in place and are continually upgraded.