Arwel Jones from the National Library of Wales told ITV News that he is hoping that the damage will be limited - "the most valuable stuff is housed in purpose-built cells at the very bottom of the library, and this fire is at the very top."
He admitted to concern that the damage from water pumped onto the building by firefighters would be as bad as the fire damage.
More top news
A North Wales family is raising awareness of chronic tinnitus, after their father took his own life. He suffered for six months.
The report reveals those who regularly consume large amounts of alcohol are more likely to engage in moderate or even problematic gambling.
Powys has been named as being one of the top 20 places to enjoy a long, healthy and safe retirement.