Probably from the 80s. Paperback in English. The tone of the story was humorous.
I would swear the title was something like NUKEE or N.U.K.I.E. or some variation. but haven't found anything with searches on various websites.
A massive, very advanced, fully automated, Breeder/Reactor Nuclear Power plant is designed and built. (The excess [neutrons?] from the reactor used to generate power, are used to enrich more uranium and/or plutonium so that it can be used as more fuel. (more or less self sustaining)
The plant was supposed to be controlled by 2 A.I. computers. The A.I.s were supposed to be based on 2 famous Nuclear scientists. (pretty sure one was supposed to be Madam Curie. The other maybe Einstein or Rutherford.)
However, hackers got in and substituted two different A.I.s.: a 30s gangster (like Al Capone?) and a notorious bad girl. (Someone like Marie Antoinette, or Mae West)
Previously the plant was only supposed to enrich/make more fuel as needed. Unfortunately, the NEW A.I.s start flirting and running the breeder reactor at full power...
This becomes a problem because they are producing too much power and enriched nuclear fuel. Nobody can shut them down and everyone has to start using more and more power so it does not overload and meltdown
In the end everyone is counting down to when it will overload because they can't use enough power.....
The janitor who has been cleaning the place decides to fix the toilet that has had an "Out of Order" sign since the plant was opened. He fixes it and decides to use it. When he finishes, he flushes it. BUT its water is coming from some critical system (i.e. emergency cooling water supply) When the system detects the lowered pressure on this critical system it initiates a reactor scram and shuts down the plant...