Nuclear Reactor Buildings, MAPP I and II, where India achieved a quantum leap in safety by pioneering the concept of a Double Containment. Here the containment of 1220 mm was split in to two 610 mm thick containments. The inner 610 mm prestressed concrete containment covered with 5 coats of PVC paint on the Inside provides a leak tight environment which can withstand internal pressures and temperatures during a Most Credible Accident. ‘Break out‘ panel was left as incase of RAPP but it was prestressed by ‘Flat Jacks‘ which needs surveillance throughout its life. ‘Break out‘ panel is meant for taking out major equipments if emergency demands. The outer Reinforced Masonry Wall provides a second barrier for additional radiation protection. The annular space between the containments is constantly maintained at below atmospheric pressure (-0.2 Atmospheres) to aspirate any leakage occurring through the inner containment during an MCA. The outer wall of reinforced masonry also protects the inner containment from rain, wind and sun, reduces the risk of corrosion and provides a barrier for external objects. The dousing tank was eliminated and suppression pool was provided. However a vent shaft would operate when there was an increase of pressure in the upper volume and conduct hot air and steam to bubble through a suppression pool at the bottom of the Reactor Building.