Early in 2011, there were about two dozen SMR design development projects ongoing worldwide. About twelve advanced SMRs currently being developed have reached advanced design stages and could in principle be implemented as FOAK or prototype plants before 2020. In some cases, pre-licensing negotiations or a formal licensing process have been initiated.

The majority of these near-term advanced SMRs are of PWR type, but there is one indirect cycle high temperature gas cooled reactor (using superheated steam in the power circuit), one advanced heavy water reactor (AHWR, being developed in India), two lead-bismuth cooled fast reactors, and one sodium cooled fast reactor.

PWRs constitute the majority of advanced SMR designs currently developed in the world. All of them could be divided in two design families:

• self-pressurised PWRs with in-vessel steam generators;

• compact modular PWRs (which are all Russian designs, sometimes referred to as “marine derivative” designs).

The gross electric output varies between 15 and 350 MW. The near-term advanced SMR projects fitting into the first group are CAREM-25 (Argentina), SMART (Republic of Korea), IRIS[78] (United States), Westinghouse SMR (United States), mPower (United States), and NuScale (United States). The Russian marine-derivative designs are KLT-40S, ABV, and VBER-300.

The self-pressurised PWR with in-vessel steam generators, also known as the integral design PWR, 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 also use the in-vessel (internal) control rod drives.

The compact modular SMR appears to be similar to 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 (nozzles) 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”.

Barge-mounted advanced SMRs are all Russian designs. The KLT-40S and the ABV would be implemented first as barge-mounted twin-unit plants. The ABV is being considered for a land-based plant. The VBER-300 is land-based but could be configured to operate on a barge. All non-Russian SMRs are land-based plants.

There is only one near-term advanced SMR in the advanced heavy water reactor category. This is the Indian AHWR which is being design to operate on uranium-thorium or plutonium-thorium fuel. The AHWR is a pressure tube vertical type direct cycle plant with natural circulation of the coolant in all circuits and all operation modes. The primary coolant is boiling light water.

Among the near-term non water cooled advanced SMRs, the most advanced is the Chinese high — temperature gas cooled reactor HTR-PM which is an indirect cycle reactor employing the steam generators and a Rankine cycle with reheating for power conversion. The indirect cycle efficiency of the HTR-PM is remarkably high, 42%, due to steam reheating. The HTR-PM is intended to produce only electricity.

In addition to this, there are three non water cooled fast reactors in the advanced SMR category which target deployment in the near term. These designs include the sodium-cooled 4S (Japan) and the lead-bismuth cooled SVBR-100 (the Russian federation) and New Hyperion Power Module (United States). All of these designs operate at a nearly atmospheric primary pressure and employ in­vessel steam generators or primary heat exchangers. The 4S has an intermediate heat transport system. Regarding advanced SMRs — fast reactors it is noted that all of them incorporate a high degree of innovation related to long refuelling interval and, therefore, only the prototype plants could be expected by 2020.