Current large PWRs utilize two to four separate large steam generators; one in each coolant loop. These steam generators are either U-tube or once-through type heat exchangers. In either case, the higher pressure primary water flows inside the steam generator tubes and the lower pressure secondary fluid is outside the tubes. In the U-tube steam generator design, dry slightly saturated steam is delivered to the turbine generator. In the once-through steam generator design, super-saturated steam is delivered to the turbine generator (NRC, 2006).

Current steam generators range up to 70 feet (21 m) tall and contain 3000 to 16 000 tubes welded to a tube sheet. The steam generator tube sheet and tubes are part of the RCS boundary. A steam generator tube rupture provides a short circuit path for primary coolant to escape containment with the secondary fluid (NRC, 2009). Steam generator tube issues include tube denting, wastage, thinning, corrosion, flow — induced vibrations, cracking and deformation of U-tube bend or of support plates, tube leakage and fractures (Bonavigo and De Salve, 2011).

The active iPWR designs that are the most developed are planning one of two principal steam generator heat exchanger designs. The least traditional iPWR heat exchanger design is a once-through helical coil steam generator. The second iPWR heat exchanger design is a once-through straight-tube steam generator. No iPWR vendors have publically indicated an intention to use a vertical U-tube type steam generator, which will eliminate the most significant current steam generator tube concerns regarding cracking and deformation of the U-tube bend. Helical coil steam generators provide additional heat transfer surface in a limited amount of space and the helical fluid flow generates less flow-induced vibration. Helical coil steam generators also reduce thermal stress on the feedwater and steam headers generated by thermal expansion of the tubes. Likewise, there is little flow-induced tube vibration in once-through straight-tube steam generators. Therefore, fewer tube supports would be required, which limits low fluid flow areas and the associated corrosion concern.

However, once-through straight-tube designs do generate larger thermal stress on the feedwater and steam headers due to thermal expansion of the tubes. Helical coil steam generators will likely be more costly to produce than once-through straight-tube steam generators because of the increased complexity of the design, but the cost and complexity may be worth the reduction in stress on the steam generator headers and the increase in heat transfer area compared to the once-through straight-tube steam generator design.

Variations on implementation of these designs exist. For example, the NuScale iPWR design plans to utilize two separate but intertwined helical coil steam generators with the high pressure primary fluid outside the steam generator tubes and the lower pressure secondary fluid inside the steam generator tubes (NuScale, 2012). The SMART iPWR is designed for eight separate mini helical coil steam generators in the downcomer space around the reactor pressure vessel riser section (Lee, 2010). Likewise, the CAREM iPWR is designed for 12 separate mini helical coil steam generators in the downcomer space (Mazzi, 2011). The SMART iPWR and CAREM iPWR utilize a more traditional design approach with the primary fluid inside the steam generator tubes. All three of these designs will produce superheated steam.

The Generation mPowerTM is currently designed for a single once-through straight — tube steam generator that completely surrounds the central riser section. The primary fluid will flow inside the tubes and the secondary fluid will flow in the shell. The secondary fluids will enter and exit low in the steam generator shell; superheated steam will be produced (Kim, 2010). The Westinghouse iPWR design will also use a single once-through straight-tube steam generator that completely surrounds the central riser section. However, in this application a separate steam drum, outside the reactor pressure vessel, will be used to remove entrapped moisture in the steam. Dry steam with minimal moisture content will be delivered to the turbine generator instead of superheated steam employed in other iPWR designs (Memmott et al, 2012).

The Holtec SMR-160 uses a steam generator and superheater that are directly flanged to the top section of the reactor pressure vessel. This has the advantage of providing more direct access to the reactor fuel during the refueling process. The Holtec design plans to exchange reactor fuel in a single cartridge replacement (Oneid, 2012).