International Team Led by Westinghouse,
United States of America

II — 1. DESCRIPTION OF THE IRIS DESIGN

The International Reactor Innovative and Secure (IRIS) is an advanced, integral, light water cooled reactor of medium generating capacity (335 MW(e)), that features an integral reactor vessel containing all the reactor primary system components, including steam generators, coolant pumps, pressurizer and heaters, and control rod drive mechanisms; in addition to the typical core, internals, control rods and neutron reflector [II-1,

I — 2]. This integral configuration allows for the use of a small, high design pressure, spherical steel containment which results in a significant reduction in the size of the nuclear island. Other IRIS innovations include a simplified passive safety system concept and equipment features that derive from the ‘safety-by-design’™ philosophy [II-3]. This design approach allows for elimination of certain accident initiators at the design stage, or when outright elimination is not possible, decreases accident consequences and/or their probability of occurrence. Major design characteristics of the IRIS are given in Table II-1. As part of the IRIS pre-application licensing review by the U. S. Nuclear Regulatory Commission (NRC), the IRIS design team has developed a test plan that will provide the necessary data for safety analysis computer model verification, as well as for verifying the manufacturing feasibility, operability, and durability of new component designs.

IRIS is innovative in design — employing an integrated primary system that incorporates all the main primary circuit components within a single vessel, i. e., the core with control rods and their drive mechanisms, eight helical coil steam generators with eight associated fully immersed axial flow pumps, and a pressurizer, see Fig. II-1.

The integral configuration offers intrinsic design improvements as briefly discussed below: [38]

• Large downcomer: The 1.7 m wide downcomer reduces the fast neutron flux on the reactor vessel by 5 orders of magnitude. This leads to a ‘cold’ (i. e., not activated) vessel with almost no outside dose, no vessel embrittlement, and no need for surveillance. The vessel is essentially ‘eternal’, and decommissioning is simplified;

• Fuel assembly: The same assembly as in standard Westinghouse PWRs is used, but it can provide an extended cycle up to 48 months;

• Maintenance: Intervals between maintenance outages can also be extended up to 48 months, thus enabling uninterrupted operation for up to 4 years.

While leading to a larger reactor vessel, the integral layout results in a smaller containment (as illustrated in Fig. II-2) and overall a more compact site, with a positive impact on safety and economics.