Pressurised water reactors (PWRs, see a brief description in Box 4.1) constitute the majority of nuclear power reactors currently in operation, accounting for 61% of the total reactor fleet in the world [4.3]. PWRs also constitute the majority among the power reactors being currently constructed. In 2010, out of 60 new nuclear power units under construction, 54 were with PWRs [4.4].

Basic characteristics of advanced SMR designs addressed in the present section1 are summarised in Table 4.1. In contrast to the PWR SMR currently available for deployment (and discussed above), some of the advanced SMRs do not always follow the conventional PWR layout. Generally speaking, the PWR designs shown in Table 4.1 could be divided in two major categories: Self-pressurised PWRs with in-vessel steam generators and compact modular PWRs[13] [14].

Westinghouse SMR 800/225 In batches/24 months

• Self-pressurised PWRs with in-vessel steam generators. The self-pressurised PWR with in-vessel steam generators, also known as the integral design PWR, are represented by the CAREM-25 and CAREM-300, SMART, IRIS, IMR[16], mPower, NuScale, and NHR-200 (see example at Figure 4.1). These designs differ from conventional PWRs in that they have no external pressurisers and steam generators, with steam space under the reactor vessel dome acting as a pressuriser and steam generators being located inside the reactor vessel. Some of these designs, namely, the CAREM, the IRIS, the IMR, the mPower, and the NuScale also

use the in-vessel (internal) control rod drives. CAREM-25, IMR, NuScale, and NHR-200 use natural circulation of the primary coolant in normal operation mode and have no main circulation pumps. Other designs use in-vessel canned pumps.

• Compact modular PWRs. The compact modular SMRs, referred to as the “marine derivative” Russian designs in [4.2], appear to be similar to a conventional PWRs. However, the modules hosting the reactor core and internals, the steam generators, the pressuriser, and the coolant pumps are compactly arranged, and linked by short pipes with leak restriction devices. The pipes are mostly connected to the hot branch, and all primary coolant systems are located within the primary pressure boundary, so that the primary coolant system is sometimes referred to as “leak-tight”. The designs belonging to this group are VBER-300, KLT-40S (described in Section 3), and ABV. The ABV holds an intermediate position between the two groups as it has internal steam generators and uses natural convection of the primary coolant but employs an external gas pressuriser.

The general characteristics of advanced PWR SMRs could be summarised as follows:

• All of the PWR-based SMRs in Table 4.1 are land-based, with the exception of the ABV. This reactor was developed as barge-mounted but could also be based on land. The VBER-300 is land-based but could also be configured to operate on a barge.

• The electric output varies between 15 and 350 MWe. The NHR-200 is a dedicated reactor for heat production. The targeted availability factors are typically around 90% or even higher.

• The plant operational lifetime is in line with that of a modern conventional PWR: generally 60 years, with 50 years for the ABV and 40 years for the NHR-200. [17]

• The refuelling intervals are longer, the bum-up levels are higher and the plant lifetime is longer, compared to the currently available SMRs. Some of the advanced PWR SMRs offer greater flexibility in capacity deployment (e. g. multi-module plant configurations).

• Of the designs presented, the ABV is a factory fabricated and fuelled reactor designed for 12 years of continuous operation, and the core of the mPower is refuelled after 4.5-5 years of continuous reactor operation. Other designs rely on partial core refuelling in batches. The refuelling intervals are mostly between two and four years. IRIS is being designed for a 4-year refuelling interval (with an 8-year refuelling interval being considered as an option), while CAREM provides for annual refuelling.

• The SMART, the ABV, and the NHR-200 target distributed deployment, while for all other designs both concentrated and distributed deployment are targeted. Twin-unit option is provided for the IRIS, IMR, and VBER-300. The ABV is a twin-unit barge-mounted reactor. The mPower and the NuScale are being designed for multi-module plants of flexible capacity.

• The primary pressure is set to 15-16 MPa in most cases (as in a conventional large PWR). However it is ~12 MPa for the CAREM, ~13 MPa for the mPower, ~11 MPa for the NuScale, and only 2.5 MPa for the NHR-200.

• The fuel is typically UO2 with less than 5% enrichment in 235U (as in large light water reactors). The exception is the ABV which, similar to the KLT-40S, uses cermet fuel with uranium enriched in 235U to slightly less than 20%.

• The average projected fuel burn-up is between 30 and 70 MWday/kg, but typically around 40 MWday/kg or slightly above.

• Several of the designs offer compact containments with maximum dimensions less than 15-25 m. These are the IRIS, the IMR, the ABV, the NuScale, and the NHR-200. For the ABV, all primary containment dimensions are within 7.5 m.

• The plant surface areas, where indicated, vary and depend on plant configuration[18]. The minimum areas are indicated for the ABV (6 000 m2 on the coast and 10 000 m2 in the bay) and NHR-200 (8 900 m2). In other cases the areas are between ~100 000 and 300 000 m2, with a substantial reduction in the relative size of the area needed for twin or multi-module units.