This is about nuclear reactor and not nuclear warhead. Nuclear reactors are used to produce electricity and/or heat and to breed radioactive elements.
Either powered by actual expensive artificial isotopes with enough activity to power an engine, OR they actually broke the ban and had it running on pu239 (which would also make its fuel reserve a nice yield booster)
https://en.wikipedia.org/wiki/Nuclear_reactor
"A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction."
Nuclear engine is a device used to initate and control a self-sustained nuclear chain reaction.
This is in contrast to nuclear bomb which initiates but does not control the chain reaction. Once the reaction is started nothing else can be done with it (ie. there is no control involved other than setting up initial parameters).