1.2.1. Pressurized water reactors

The designers of pressurized water SMRs cumulatively mention the following inherent and passive safety design features as facilitated by smaller reactor capacity and size:

— Integral design of the primary circuit with in-vessel location of steam generators and control rod drives, to eliminate large diameter piping, minimize reactor vessel penetrations, and prevent large-break loss of coolant accidents (LOCA) and reactivity initiated accidents with control rod ejection, as well as to limit the scope of small and medium-break LOCA;

— Compact modular loop-type designs with reduced piping length, an integral reactor cooling system accommodating all main and auxiliary systems within a leaktight pressure boundary, and leak restriction devices, all to prevent LOCA or limit their scope and hazard;

— A primary pressure boundary enclosed in an enveloping shell with low enthalpy slow moving water, intended to prevent LOCA or limit their scope and hazard;

— Increased thermal inertia at a reasonable reactor vessel size, contributing to long response time in transients and accidents;

— Enhanced levels of natural convection, sufficient to passively remove decay heat from a shutdown reactor over an indefinite time;

— In-vessel retention of core melt through, for example, passive external cooling of the reactor pressure vessel;

— Compact design of the primary circuit and the containment, to facilitate protection against missiles and aircraft crash.