Not sure what you mean by coupon quality but I'll take a shot. There isno specific reuiqrment for quality when qualifying anything for nukes. You do however have to have absolute traceability.
There for ethere is nothing special about performencae quals. Standard ASME IX methods are fine.
However, with nukes the most difficult part of WPS quals is the impact testing. And specifically the comparison of HAZ to unnaffected base metal.
The latest Section II revs have considerably lessened this problem but most nuke work is done with plants licensed to older revs. And lots of luck getitng them to recert the plant.
Modern steel making processes have created steels of extremely high quality and therefore it will be very difficult for your HAZ to equal or exceed the unnaffected BM.
The secret is to actually use lesser quality steel for your qualification. Large grained stuff as opposed to the generally understood high quality fine grained stuff. In other words SA-36 as opposed to SA-516. And stay away from normalized andnormalized and tempered stuff too. Use hot rolled plate for oyur quals.
The grains in the HAZ will refine with proper heat input and will demonstrate greater lateral expansion than the BM.

Junbao Zhang: Try reading the subparagraphs following NB-4366... NB-4367 and NB-4368... you'll probably not need to ask this question... One thing very, very important in all code work, but most especially in nuclear work is to read ALL of the requirements, not just one part.