I am a radioprotection safety expert at CERN and would like to start a blog on radioprotection safety issues. Is this permitted?

This may be problematic for several reasons: a statement in a blog is deemed to be public and, in view of your role at CERN, could be misconstrued as official communication of CERN or could reflect on CERN should your personal opinion deviate from official policy. As a general rule, you need prior permission before making a public statement concerning CERN or its activities, unless it is an area in respect of which you have clearly allocated responsibility and you have informed your hierarchy.

You should first talk to your hierarchy, describe the issues you intend to post on your blog and discuss whether and to what extent you will need prior clearance. 

Regardless of the specific content, your general obligations of tact and discretion as a CERN contributor will continue to apply. Finally, running the blog should not interfere with your work hours.

(External communication)