Zombie / Orphan open files

Hi

This is a quick general NFS server question.

Does the NFSv4x  specification require or recommend that:   the NFS server, after some reasonable time, 
should / must close orphan / zombie open files ?

On several NAS platforms I have seen large numbers of orphan / zombie open files "pile up" 
as a result of Kerberos credential expiration.

Does the Red Hat NFS server "deal with" orphan / zombie open files ?

Thanks

Andy Romero
Fermilab