nuclear scientist
A scientist specializing in nuclear physics or nuclear engineering.
He is a famous nuclear scientist.
原子力科学者 is a broader term that can include nuclear engineers and researchers in nuclear energy, while 核科学者 often emphasizes the scientific study of the atomic nucleus itself.
Compound of 核 (kaku, 'nucleus') + 科学 (kagaku, 'science') + 者 (sha, 'person').