While reviewing the search engine log for this blog I found lots of search regarding Nullable Primary Key. It is not possible. This post is especially to clear the Not Nullable Primary Key Property. The Allow Nulls property can’t be set on a column that is part of the primary key. All columns that are part of a table’s a primary key must contain aggregate unique values other than NULL.
Reference : Pinal Dave (http://blog.SQLAuthority.com) , BOL