Atomic device runs defectively at core (7)

I believe the answer is:

reactor

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'atomic device' is the definition.
'reactor' can be an answer for 'device' (reactor is a kind of device). I am unsure of the 'atomic' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for reactor that I've seen before include "Apparatus for production of nuclear energy" , "Heart of a nuclear power station" , "device for generating energy" , "Nuclear power plant" , "Nuclear apparatus for producing energy" .)

I've seen this clue in The Sun.

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