What is a bone density scan test, and does it require radiation? is it harmful?

Yes and no: Yes it requires radiation. The bone mineral density test (which is not a "bone scan" which is something entirely different) requires x-radiation. But no, it is not harmful in and of itself. The radiation dose is around 1/10 the dose from a chest radiograph. That said, radiation doses are cumulative, so each DXA adds a little to your total dose accumulation and risk.

Safe: Bone density scans usually use radiation to measure the density of bone. The dose of radiation is much less than that from a chest x-ray. It is not considered harmful.