The integral reactor coolant system (Integral RCS) is characterized by:

• Entire RCS located in a single pressure vessel;

• No additional pressure vessels, connecting loop piping, or supports.

The integral reactor vessel includes:

• Axial flow, fully immersed coolant pumps with high temperature bearings and high temperature sealed rotor and stator windings;

• Helical-coil, once through steam generators (SGs);

• Internal control rod drive mechanisms (I-CRDMs) designed for in-vessel environment;

• Pressurizer and related heaters.

IRIS three tier safety concept

The overall approach to safety in the IRIS is represented by the following three tier approach:

(1) The first tier is safety-by-design™ [II-3], which aims at eliminating by design the possibility for an accident to occur, rather than dealing with its consequences. By eliminating some accidents, the corresponding safety systems (passive or active) become unnecessary as well;

(2) The second tier is provided by simplified passive safety systems, which protect against the accident possibilities still remaining and mitigate their consequences;

(3) The third tier is provided by active systems which are not required to perform safety functions (i. e., are not safety grade) and are not considered in deterministic safety analyses, but do contribute to reducing core damage frequency (CDF).

First Tier

The first tier is embodied in the IRIS ‘safety-by-design’™ philosophy [II-3]. Nuclear power plants consider a range of hypothetical accident scenarios. The IRIS ‘safety-by-design’™ philosophy is a systematic approach that aims — by design — to eliminate altogether the possibility for an accident to occur, i. e., to eliminate accident initiators, rather than having to design and implement systems to deal with the consequences of an accident. It should be noted that integral configuration is inherently more amenable to this approach than a loop type configuration, thus enabling safety improvements not possible in a loop reactor. To consider only the most obvious example, loss of coolant accidents caused by a large break of external primary piping (large break loss of coolant accidents — large break LOCAs) are eliminated by design since no large external piping exists in IRIS. Additionally, in cases where it is not possible or practical to completely eliminate potential initiators of an accident, safety-by-design™ aims at reducing the severity of the accident’s consequences and the probability of its occurrence. As a result of this systematic approach, the eight Class IV design basis events [II-3] (potentially leading to the most severe accidents) that are usually considered in light water reactors (LWRs), are reduced to only one in the IRIS, with the remaining seven either completely eliminated by design, or their consequences (as well as probability) reduced to a degree that they are no longer considered Class IV events [II-1, II-2].

Second Tier

The second tier consists of passive safety systems needed to cope with remaining potential accidents. Because of safety-by-design™, they are fewer and simpler than in typical passive loop type LWRs [II-1]. Notably, the elimination of the possibility for some accidents to occur enables simplifications of the IRIS design and passive safety systems, resulting simultaneously in enhanced safety, reliability, and economics. In other words, increased safety and improved economics support each other in the IRIS design.

Third Tier

The third tier has been addressed within the probabilistic risk assessment/probabilistic safety assessment (PRA/PSA) framework. In fact, PRA was initiated early in the IRIS design, and was used iteratively to guide and improve the design safety and reliability (thus adding ‘reliability by design’). The PRA has suggested modifications to reactor system designs, resulting in reduction of the predicted core damage frequency (CDF). After these modifications, the preliminary PRA level 1 analysis [II-4] estimated the CDF due to internal events (including anticipated transients without scram, ATWS) to be about 2 x 10—8, more than one order of magnitude lower than in typical advanced LWRs [II-1]. A subsequent evaluation [II-5] of the large early release frequency (LERF) also produced a very low value, of the order of 6 x 10—10, which is more than one order of magnitude lower than in typical advanced loop LWRs [II-1], and several orders of magnitude lower than in present LWRs.