Some major highlights of the passive safety design features in the IRIS design, structured in accordance with the various levels of defence in depth [II-6, II-7], are shown below.

Level 1: Prevention of abnormal operation and failure

The IRIS safety-by-design™ systematic approach is the basis for effective Level 1 prevention of many initiating events; correspondence between design features and initiating events prevented is the following:

(A) Integral design of primary circuit with no large diameter piping:

• Elimination of large break LOCAs;

• Elimination of loss of seal (head, pump) LOCAs;

• Elimination of control rod ejection accidents;

• Elimination of concerns related to high pressure safety injection (HPSI) systems;

(B) Increased natural circulation due to large, tall vessel:

• Reduced severity of loss of flow (LOFA) accidents;

(C) Large thermal inertia due to increased water inventory:

• Prevention of core uncovery in small and medium break LOCAs;

• Reduced requirements for heat removal systems;

• Reduced concerns related to loss of feedwater;

(D) Other specific design solutions:

• Elimination of the possibility of a reactor coolant pump shaft break.

Level 2: Control of abnormal operation and detection of failure

IRIS will use state of the art plant control and protection systems to monitor and control plant operations; it will also incorporate advanced diagnostics/prognostics systems. The contribution of passive systems at this level would be as follows:

• Slower progression of a loss of heat sink accident (LOHS) due to large thermal inertia.

Level 3: Control of accidents within the design basis

Level 3 safety functions are contributed to by the following passive safety features/systems:

(A) Passive emergency heat removal system (EHRS):

• Control of LOHS;

(B) Increased natural circulation due to large, tall vessel:

• Control of loss of flow accidents (LOFA);

(C) Steam generator system designed for full primary pressure:

• Significantly reduced severity and simple mitigation of steam generator tube rupture (SGTR) accident.

Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of consequences of severe accidents

The following passive safety features/systems of IRIS contribute to achieving the objective at this DID

level: [40]

Level 5: Mitigation of radiological consequences of significant release of radioactive materials

Level 5 safety functions are contributed to by the following passive safety features/systems:

(A) Small fuel inventory:

• Reduced radioactivity release;

(B) High design pressure containment plus pressure suppression system plus reduced core power density plus increased thermal inertia:

• Slower progression of accidents and increased retention of fission products;

• Low leakage rate containment;

• Deposition of radionuclides in auxiliary building.