The word "反应堆" is formed by combining the characters "反应", meaning "reaction", and "堆", meaning "pile". "反" contributes the sense of "counter" or "opposite", while "应" contributes "response", together denoting a reactive process. "堆" adds the concept of a physical accumulation or structure, resulting in a term that specifically refers to a device where nuclear reactions are sustained, such as a reactor core.