Hello , i don't know the specifics about walnuts but why did you wash them ?
If they have been down a long time then they may not keep anyway , but any that you know are down within a day or so , just collect , brush off any soil/muck and allow any dampness to dry off and store in a dry , cool airey room/shed . Like spuds or carrots etc , wash them and they don't store as well .