Boiling water reactors (BWRs) are second to PWRs in global deployment, accounting for nearly 21% of all currently operated reactors. However, in 2010, out of 60 new nuclear power units under construction, only 2 were BWRs [4.4].

BWRs are single circuit, direct cycle plants. The coolant is boiling light water. Saturated steam condensation cycle (Rankine cycle) is used for energy conversion.

A conventional state-of-the-art BWR (e. g. the ABWR [4.5]) is self-pressurised and includes the reactor pressure vessel hosting the reactor core and the steam separators and dryers, the bottom

mounted external control rod drives, and the bottom mounted external canned recirculation pumps. There are no BWRs in the small and medium-size range currently available for deployment.

The two advanced BWR SMR designs presented in this report[19] are different from ABWRs in that they use top-mounted external control rod drives (such as in PWRs) and rely on natural circulation of the coolant in all operating modes (i. e., they have no recirculation pumps), see Table 4.2. Proposals to use natural circulation of the coolant are not unique to small or medium-sized BWRs. For example, no recirculation pumps are used in the design of the ESBWR of 1 550-1 600 MWe [4.5].

The designs discussed here are quite different from conventional BWRs[20], and have the following

features:

• The CCR of 400 MWe uses compact high pressure containment with its maximum dimension (height) limited by 24 meters, and with the reactor building structures providing the secondary containment.

By using compact high pressure containment, the CCR aims to reduce the volume and mass of the reactor building and nuclear island components proportionally to the power reduction from a conventional large sized ABWR, an approach to overcome the disadvantage of the economy of scale [4.1].

• The VK-300 of 250 MWe is placed within a conventional large PWR type containment (about 45×60 m) within which a primary protective hull (the primary containment) and a gravity driven water pool are located.

• Both designs are land-based reactors; however, location of the VK-300 on a barge is not excluded.

• The projected plant lifetime is 60 years and the targeted availability factors are above 90% for both designs.

• For the VK-300 the construction duration is five years, while for the CCR it is claimed to be only two years, a minimum among all advanced SMR designs addressed in this study. It is expected that such a short construction period is based on the experience of building the

ABWR[21] and taking benefit of the design compactness to maximise factory fabrication of large reactor modules [4.1].

• Both designs use low enrichment UO2 fuel with partial core refuelling in batches. Twin-unit and multi-module plant options are being considered for the CCR.

• For both designs the main specifications are similar to those of the state-of-the-art BWRs. Notably, a very small plant surface area of 5 000 m2 is indicated for a single module CCR.