The objective of this report is to analyze the basic engineering problems involved in the design of a pressurized demonatration reactor to be moderated and cooled with heavy water. Two fundamental concepts of this reactor, here referred to as the HWER, are as follows: (1) a tank-type reactor, in which the moderator pressure is maintained at or near atmospheric conditions, and (2) a pressurized vessel-type reactor in which the moderator pressure is maintained at 1000 to 1300 psi. The fuel and primary coolant channels in either case will be designed for an internal pressure pressure of 1000 to 13000 psi.