The IRIS design significantly increases defence in depth by adding as the first layer of safety an inherent elimination of as many accidents as practical through the safety-by-design™ philosophy [II-3], as previously described. The postulated accident scenarios eliminated include: [39]

The postulated accidents whose severity or consequences are reduced include:

• Small/medium break LOCAs;

• Steam generator tube rupture;

• Steam line break;

• Feed line break;

• Reactor coolant pump seizure.

Passive safety systems

The passive safety systems in IRIS are fewer and simpler than in typical passive LWRs [II-1]. Their function is to protect against remaining possible accidents and mitigate their consequences.

When compared with typical passive LWRs, the IRIS’s safety systems are not novel. Most of them are similar to those in the AP600/AP1000 but simplified and fewer in number, while the pressure suppression system is similar to that of a BWR [II-1].

Active systems

In IRIS, no active safety grade systems are required. However, active non-safety-grade systems, while not assumed available in deterministic safety analysis, may be used (if available) to help mitigate accidents, and thus enhance defence in depth (DID) and contribute to reducing the probability of core damage in the PRA analysis. The active, non-safety related features include:

(1) Standby diesel generators which provide power to DID systems in the event that normal plant alternate current (AC) power supplies are not available1;

(2) A startup feedwater system that can provide feedwater to the steam generators in order to remove core decay heat, in the event that the normal feedwater system is unavailable;

(3) Functioning of the normal plant cooling water systems (service water and component cooling water) can provide support for other DID components as well as remove core decay heat;

(4) The chemical and volume control system normal make-up pumps with their boric acid tank as suction source can provide high pressure make-up water to the RCS in the event of a small loss of coolant accident;

(5) The normal residual heat removal pumps with their in-containment water source can provide low pressure make-up water to the RCS and heat removal capability when RCS pressure is reduced;

(6) Diverse means of containment cooling are provided to significantly reduce the chance of containment failure.