Walnut hulls or shells will decompose slowly in a compost pile because they are so woody in nature.  All parts of black walnut contain the allelopathic chemical, juglone, which is toxic to some plants.

Walnut leaves can be composted because the juglone toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. Mulch or woodchips from black walnut are not recommended for plants sensitive to juglone without composting those woodchips first.  The same holds true for shells and hulls. However, composting the material for a minimum of six months allows the chemical to break down to a safe level even for plants sensitive to juglone, like tomatoes.  

If you have an active and productive compost pile that has a good balance of air, water, and bacteria, the juglone that causes issues for tomatoes and other plants will be readily broken down and the resulting compost will be safe to use on any plants.  Especially if that material is composted for 6 or more months.  

If you have concerns or feeling a little timid about using it, use the compost sparingly or in only one area at first.  If you are noticing juglone toxicity symptoms on your tomatoes growing with that compost, save that pile of compost for another use in the landscape near plants that are more tolerant of juglone.  If you don't see any issues after testing it out, then its good to use!  

The degree of symptoms varies with species but for those that are sensitive, they often yellow, grow poorly or look stunted, and may eventually die. Plants that are not as sensitive may yellow slightly or grow more slowly, but are rarely killed.

The short answer is juglone readily breaks down when composted and black walnut plant material composted for 6 or more months should be safe to use around any plant. 

Happy gardening!

--Aaron