In December 2009 the winner of a competitive tender by the United Arab Emirates for a large LWR was announced. The proponent of a higher priced, and losing, entry was quoted as blaming the loss on the fact that superior safety is expensive. Au contraire! In a plant properly designed according to the Safety-by-Design approach the increased level of safety is accompanied by a decrease in cost, as elaborated by Carelli.9 In fact, such a simpler plant is safer (there are fewer ‘things’ which can go wrong) and at the same time cheaper (there are fewer ‘things’).

This inverse connection between safety and cost has not been fully exploited by reactor designers, not going much further than the elimination of large LOCAs and associated piping systems. Rather, relying on proven technology, supply chain and infrastructure of the large PWRs has been taken by traditional LWR vendors as the sure path to economic competiveness. This conventional, sure-footed approach completely misses the fact that the iPWR is not just a smaller PWR, but it is a completely different design, with its own individual challenges and rewards. If properly identified, addressed and fully exploited the iPWR does indeed yield both increased safety and decreased cost. The systematic approach adopted in the IRIS design is in principle applicable to all iPWRs in general. It starts by eliminating the following major components and systems present in large LWRs:

• all large piping to/from the reactor vessel;

• steam generator pressure vessel;

• canned motors and seals of primary pumps;

• pressurizer vessel and pressurizer spray system;

• vessel head penetrations due to external control rods drive mechanisms (CRDMs);

• vessel bottom penetrations and seals due to in-core instrumentation;

• all active safety systems;

• high-pressure emergency core cooling system;

Also, the following major components have been reduced:

• shielding;

• number and complexity of passive safety systems;

• number of valves;

• size of containment and nuclear building;

• number of NSSS buildings (from two or more to one);

• number of large forged components (from approximately a dozen to one).

The only added major component/system is the seismic isolators.

In most iPWR designs the control rod drive mechanisms are located above the core, with the steam generators arranged outside in an annular configuration against the vessel. This leaves a coolant downcomer between the core and the vessel which is a quite effective shielding of the vessel wall and provides another iPWR intrinsic cost reduction. In fact, in some designs like IRIS the downcomer is large enough to reduce by several orders of magnitude the neutron fluence to the vessel wall, practically eliminating the vessel embrittlement and allowing increased plant lifetime. Also, the routine personnel exposures, and thus the ALARA costs, are reduced because the radiation level outside the iPWRs vessel is significantly decreased in respect to traditional LWRs.

The economy of scale, where the capital cost per unit power increases as the plant size decreases obviously applies and does favor larger plants. But traditional LWRS and iPWRS are on different, roughly parallel, cost versus power curves, with the iPWR being, even significantly, below because of the simpler and more economic design. In addition, SMRs in general enable economy of multiples versus single monolithic plants by relying on bulk/serial components fabrication (e. g. many small serial steam generators versus a few one-of-a-kind), accelerated learning and multiple units savings.

Modular construction and multiple modules deployment yield shorter construction schedule, module deployment tailored to demand with reduced spin reserve (i. e. reduced requirement for purchase power).

Quantification of the various factors is shown in Table 3.2, which compares the IRIS SMR (335 MWe) as part of a four-unit deployment against a single large PWR of 1340 MWe. The SMR starts with a large (1.7) penalty factor due to the economy of scale, but the other factors unique to SMRs in general (factors 2 through 5) and iPWRs in particular (factor 6) bring the estimated penalty for the iPWR down to 1.05, which is within the uncertainty level.

Two other factors have a significant financial impact favoring the SMRs in general: improved cash flow and reduced capital at risk. Construction of large plants takes a long time, with no income until completion. On the other hand, staggered deployment of the smaller SMRs enables ‘bootstrapping’ with the first unit generating income to support construction of the second unit and so on. As a result, the maximum cash outflow, or capital at risk, is significantly reduced as shown in Figure 3.2, which compares the cash flow of the same four 335 MWe IRIS plants (1340 MWe total) deployed every three years against a single 1340 MWe large PWR. An in-depth discussion will be found in Carelli et al.10 and Boarin and Ricotti11 iPWR economics is the subject of Chapter 10.

Finally, a critical consideration for iPWRs, which has a very significant impact in terms of both economics and public acceptance: a direct consequence of reducing the probability of catastrophic accidents to the order of E-8 is that the plant can

be licensed with a drastically reduced emergency planning zone, in principle to the plant boundary, although in practice to a few kilometers.12