Atkins has bought US firm Nuclear Safety Associates for up to $27M (£16M).
The UK-headquartered consultancy paid an initial $15M for the engineering and technical services firm, with up to $12M to follow dependent on performance.
NSA has 130 staff, and reported revenue of $25M for the year to 31 December 2013.
Atkins chief executive officer Uwe Krueger said: “This acquisition is another step towards our strategy to be one of the world’s leading energy consultancies.
“NSA will enhance our presence in North America, the world’s largest nuclear market, and its safety and security skills will strengthen our international nuclear offering.”