Atucha II is a heavy water cooled and moderated nuclear reactor utilizing natural uranium as fuel. Vertical coolant channels encompass each fuel assembly, separating moderator from coolant. A moderator tank is also present around the core. A reactor pressure vessel keeps nearly the same pressure for moderator and coolant. The heavy water accomplishes the moderator function if the temperature is lower than the coolant temperature. The thermal power generated in the moderation process preheats the feed-water (light water flow). A system encompassing the complexity of CANDU and PWR was built, i.e. Atucha II. An unprecedented and unrepeatable endeavor took place, as described in the book. This confirmed, independently of the original designers, the validity of the design choices and the safety margins.