IAEA (1997a) defines advanced nuclear plant designs as those designs of current interest for which improvements over their predecessors and/or existing designs are expected. Depending on the amount of modifications implemented, advanced reactor designs can be categorized evolutionary or innovative. An evolutionary design is an advanced design that achieves improvements over existing designs through small to moderate modifica­tions, with a strong emphasis on maintaining design proveness to minimize technological risks. The development of an evolutionary design requires at most engineering and confirmatory testing. An innovative design is an advanced design which incorporates radical conceptual changes in design approaches or system configuration in comparison with existing practice. Substantial research and development efforts, feasibility tests, and a proto­type or demonstration plant are required prior to the commercial deploy­ment of this type of design.

An alternative classification was coined by the Generation IV International Forum GIF (2002), which divided nuclear reactor designs in four genera­tions. The first generation consisted of the early prototype reactors of the 1950s and 1960s. The second generation is largely made up by the commer­cial power plants built since the 1970s and that are still operating today. The

Generation III reactors have been developed in the 1990s and include a number of evolutionary designs that offer improved performance, safety and economics. After the increased interest in nuclear power seen in the first decade of the twenty-first century, additional improvements are being incor­porated into Generation III designs, resulting in several concepts that are actively under development and seriously considered for near-term deploy­ment in various countries. The Generation III designs loosely correspond to what in the ARIS system are called evolutionary designs (IAEA, 2010) and it is expected that they will constitute the bulk of the new nuclear plants built between now and 2030. Beyond 2030, it is anticipated that new reactor designs will address key issues such as the closure of the fuel cycle or pro­liferation concerns while possibly ensuring competitive economics, safety and performance. This generation of designs, the Generation IV, consists of innovative concepts in which substantial development is still needed.

Traditionally, nuclear reactors have been classified depending on the energy of the neutron spectrum they use to produce the fission in the fuel, or depending of the coolant they use to extract the fission energy from the core. With regard to the first criterion, nuclear reactors can be thermal when using low energy neutrons and fast when using much higher energy neu­trons that are not slowed down by a moderator. With regard to the coolant, nuclear reactors can be classified as water-cooled reactors (WCR), gas — cooled reactors (GCR), liquid metal-cooled reactors (LMR) and molten salt-cooled reactors (MSR). Water-cooled reactors at the same time can be classified as boiling water reactors (BWR), in which the core is at relatively low pressure and the coolant is allowed to boil; and pressurized water reac­tors (PWR), in which the core is kept at high pressure and the coolant remains in a liquid state. Water-cooled reactors can also be divided into light water reactors (LWR) and heavy water reactors (HWR) that use deu­terium water. While most HWRs belong to the pressurized water reactor type, and are also termed pressurized heavy water reactors (PHWR), some advanced designs use the boiling water reactor concept. As will be seen in upcoming sections, several advanced designs are what is called (IAEA, 1997a) an integral design, which refers to a reactor design in which the whole reactor primary circuit, including, for instance, pressurizer, coolant pumps, and steam generators/heat exchangers, as applicable, is enclosed in the reactor vessel. Finally, depending on the size of the plant, nuclear designs can be classified (IAEA, 1999a) as small (less than 300 MWe), medium (between 300 and 700 MWe) and large (more than 700 MWe). Although innovative reactor designs do not always fit the following norm, in general it can be said that most water-cooled reactors and gas-cooled reactors are thermal reactors, while most fast reactors are cooled by liquid metals or molten salts.

264 Infrastructure and methodologies for justification of NPPs