The philosophy of containment for integral reactors is the same as that for loop type designs The containment has the following functions

• Protection from the effects of internal events, especially, retention of radioactivity released from the core

• Protection from external impacts

Single and duel containment designs are possible The low discharge rate from LOCA in integral reactors, results in pressure suppression systems being very attractive The guard vessel concept where the size of the leak tight containment is minimized to a shell which fits closely around the RPV has been developed for integral reactors It provides simplicity of design and has positive benefits in accident management

The guard vessel is usually made of steel but could also be made of pre — strressed concrete However, the emphasis on improved safety characteristics of integral reactors has led to a preference for steel, rather than steel lined concrete as it allows for ultimate heat removal by heat transfer through the steel All containment designs share the general objective of plant size reduction

The guard vessel concept can be applied to integral reactors, but not to loop type reactors since the whole of the primary circuit including CVCS is compact and can be contained in a vessel of reasonable size The advantages are

• Very effective containment of radioactive species both in normal operation and in upset conditions

• Possibility of reduced specification in terms of pressure and volume of the containment

• The additional protection the guard vessel provides, appears in some conditions to allow construction of nuclear plants closer to centers of population

For marine reactors, a water filled containment giving pressure suppression and elimination of the additional weight of a shield has been adopted for some designs.