核心问题
He held a demanding and highly paid position with a company that built nuclear reactors. 我查了许多英汉词典，均未查到上句中的 with 的用法和词义。
词典解答
上句中的 with 是一种较新的用法, 表示“在……单位(工作)”, “是……的成员”或“参加……”。例句见 with 。
此种用法的 with 具有与 in, at 或 to 基本相同的含义:
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Kate is a conscientious teacher at the Normal Training College. (Cronin) 凯特是师范学院一位有责任心的教师。
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He took a post as surgeon to the New Century Oil Company. (Cronin) 他在新世纪石油公司找到了一个外科医生的职务。