Hard types to crack from banks to nuclear power plants (4)

I believe the answer is:

nuts

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'plants' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nuts that I've seen before include "balls" , "Barmy (colloq)" , "Mad (slang)" , "Bolt securers" , "Crazy (sl.)" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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