The economic characteristics of large water power reactors are known from years of construction and operating experience. The cost of sodium-cooled reactors based on deployments of demonstration units in the late 1900s has led to capital cost estimates of 110-125% that of water-cooled reactors (Waltar et al., 2012). Experience with gas-cooled, and certainly lead/lead-bismuth-cooled, reactors has not been sufficient to allow a comparable projection of overnight capital costs compared to water-cooled reactor experience. Hence, while it is accepted that the capital cost of individual SMR units will be far lower than that of the large-rated reactors employing the same coolant, the capital cost per KWe for SMRs compared to large-rated reactors, although likely larger, is as yet not established. We can only project comparative costs of SMRs employing the various coolants on the basis of the above-noted large-rated reactor experience.

Other potential measures of comparative economic characteristics of variously cooled SMRs are the fundamental parameters of core power density and specific power. The power density, kW/liter, reflects the core volume and hence is often a measure of the vessel containment and plant size necessary for a given power rating. Exceptions do exist if the reactor vessel or containment size is dictated by considerations other than core power density. For example, the SPRISM sodium — cooled fast reactor vessel is sized to accommodate decay heat removal through an air-cooled chimney outside the guard vessel: BWR containments by virtue of their use of in-containment coolant pools for pressure suppression are much smaller than those of PWRs, which control pressure by large air-filled containment volume. The power density is thus a relative indication of capital cost, albeit for plants using comparable design strategies and principal materials. The specific power, kW/kgIHM, reflects the mass of initial heavy metal (IHM) or fuel needed for a given power rating. The specific power is thus a relative indication of fuel cycle cost, but for plants using comparable fuels.

However, it is clear that not all SMRs employing the various coolants of interest use comparable materials or fuels. Hence the relative values of power density and specific power presented in Table 1.5 for various coolants do not necessarily forecast the comparative economic character of reactors employing various coolants. Nevertheless, these parameters provide an insight regarding the significant benefit to sodium-cooled reactors from their high relative parametric values, a benefit which

likely keeps their costs close to water-cooled designs even though they use an exotic liquid metal coolant requiring considerable costly instrumentation and purification systems and their enrichment is much higher than that of water-cooled designs. Furthermore, the low parametric values of the helium-cooled reactor indicate the inherent economic disadvantage of large reactor volume which this reactor coolant type faces. However, unless one considers all aspects of the design, surrogate parameters of cost can be misleading. What is needed is an integrated cost analysis to include the design of the reactor system, all needed safety systems, the power conversion system considering thermal efficiency, operating staff size, maintenance cost, and fuel costs to evaluate the economic competitiveness of any design as measured in cents/kW h of power produced.