While modularization deals with a design and fabrication methodology, design factor is related to the specific and peculiar features and enhancements of a given design concept, in order to meet operating requirements with optimized safety, simplicity and economics. Large plants have been optimized for their particular power output. In designing a plant with smaller output, it does not necessarily make sense to just scale down a large system. Usually SMRs are not a mere re-sizing of larger units; they do not represent a way back, but, on the contrary, a further progress in the technology evolution path.

At a smaller size, different design concepts might be possible, which could lead to a more significant capital cost reduction than simple application of the scaling laws from large design would predict (Hayns and Shepherd, 1991). SMR economic rationale also lies on the enhanced passive safety features and design simplifications, often enabled by a small plant scale. The 300-400 MWe safe integral reactor (SIR) in the 1990s and the international reactor innovative and secure (IRIS) in early 2000s paved the way to the understanding of an innovative technological and economic paradigm.

Most Gen III+ reactor designs include some features that may be regarded as passive (i. e. relying on physical laws and not on human intervention for the activation), but small-scale plants can take maximum advantage of such features, due to their physically smaller size or lower power densities, and consequential lower power output. As a result, the elimination of some engineered safety systems might be possible and/or the safety downgrading of some other components. Revised, simplified and more cost-effective plant layout becomes possible, with favourable impact on costs (Carelli et al., 2008a, 2008b).

Along with such design-related cost benefits, the SMR exploit the economics of small ‘mass production’. SMRs are conceived to take the maximum advantage from standardization and economy of replication (Kutznetsov and Lokhov, 2011), also referred to as the ‘economy of multiples’ paradigm. Moreover, SMRs may encompass a broad range of reactor unit sizes. In principle, the lower the size, the higher the loss of economy of scale to be compensated, and the loss of cost effectiveness in terms of generation cost (Figure 10.13).

SMRs rely on the ‘economy of multiples’ but also on the ‘economy of small’ in the sense that design-related cost savings are necessary to recover economic

competitiveness. The smaller the reactor unit size, the higher must be the design cost savings in order to have the same generation costs as LRs. Some general considerations on cost reduction by design may be drawn from several innovative SMR features, such as the integration of primary loop into the reactor vessel, with the elimination of large loss-of-coolant accident (LOCA), the wide use of passive safety systems with natural circulation of coolant in case of accident, and the elimination of some active components and safety systems. Nevertheless, the design-saving factor that is expected to decrease construction costs of SMRs is strictly dependent on the specific reactor concept. A more reliable estimate could come from a bottom-up cost analysis, referred to the specific plant layout and technical features. In the absence of this information, the economic analysis may consider the design-saving factor as a ‘target’ value to be achieved in order to equalize the SMR and LR projects’ profitability. Thus the economic analysis might offer the manufacturer a sort of indication on a technical and economical goal for the SMR design (Boarin and Ricotti, 2011b). As a consequence, ‘very small’ reactors (VSR) must come up with additional saving factors (Figure 10.14). Rather, VSRs do not really compete in the same SMR playground since they have other unique requirements, e. g. emphasis on total capital cost, rather than on cost per KW installed, and may have unique applications, such as very small or scattered user areas.