As nuclear power generation has become established since the 1950s, the size of reactor units has grown from 60 MWe to more than 1600 MWe, with corresponding economies of scale in operation. At the same time there have been many hundreds of smaller reactors built both for naval use (up to 190 MW thermal) and as neutron sources, yielding enormous expertise in the engineering of small units. The International Atomic Energy Agency (IAEA) defines 'small' as under 300 MWe, but in general today 500 MWe might be considered an upper limit to 'small'.
Today, due partly to the high capital cost of large power reactors generating electricity via the steam cycle and partly to the need to service small electricity grids under about 4 GWe,a there is a move to develop smaller units. These may be built independently or as modules in a larger complex, with capacity added incrementally as required (see section below on Modular construction using small reactor units). Economies of scale are provided by the numbers produced. There are also moves to develop small units for remote sites.
Generally, modern small reactors for power generation are expected to have greater simplicity of design, economy of mass production, and reduced siting costs. Many are also designed for a high level of passive or inherent safety in the event of malfunctionb.
The most advanced modular project is in China, where Chinergy is preparing to build the 210 MWe HTR-PM, which consists of twin 250 MWt reactors. In South Africa, Pebble Bed Modular Reactor (Pty) Limited and Eskom have been developing the pebble bed modular reactor (PBMR) of 200 MWt (80 MWe). A US group led by General Atomics is developing another design – the gas turbine modular helium reactor (GT-MHR) – with 600 MWt (285 MWe) modules driving a gas turbine directly, using helium as a coolant and operating at very high temperatures. All three are high-temperature gas-cooled reactors (HTRs) which build on the experience of several innovative reactors in the 1960s and 1970s.
Another significant line of development is in very small fast reactors of under 50 MWe. Some are conceived for areas away from transmission grids and with small loads; others are designed to operate in clusters in competition with large units.
Already operating in a remote corner of Siberia are four small units at the Bilibino co-generation plant. These four 62 MWt (thermal) units are an unusual graphite-moderated boiling water design with water/steam channels through the moderator. They produce steam for district heating and 11 MWe (net) electricity each. They have performed well since 1976, much more cheaply than fossil fuel alternatives in the Arctic region.
Also in the small reactor category are the Indian 220 MWe pressurised heavy water reactors (PHWRs) based on Canadian technology, and the Chinese 300 MWe PWR such as built at Qinshan Phase I and in Pakistan. These designs are not detailed in this paper simply because they are well-established. Furthermore, India is now focusing on 450 MWe and 700 MWe versions of its PHWR, and China on PWRs of 1000 MWe and higher.