You would be better protecting it with a harder varnish than polyurethane varnish.
Poly is tough (floors) not hard. The shellac is a great moisture barrier and I would suggest garnet shellac on walnut.
Note if you use a premixed "orange shellac" like Zinsser you will be apply a shellac that has not been de-waxed and poly will not stick; the other varnishes will.
I would suggest a dewaxed shellac like Zinsser seal coat or a garnet mixed fresh from flakes; then a phenolic resin varnish like Waterlox Original in either satin or gloss.
Note: Gloss will highlight every flaw; satin will hide many flaws.
Scott
Finishing is an 'Art & a Science'. Actually, it is a process. You must understand the properties and tendencies of the finish you are using. You must know the proper steps and techniques, then you must execute them properly.