1.1. BACKGROUND

1.1.1. Rationale and developments in Member States

According to classifications adopted by the IAEA, small reactors are reactors with an equivalent electric output of less than 300 MW; medium sized reactors are reactors with an equivalent electric power of between 300 and 700 MW [1].

Small and medium sized reactors (SMRs) are not intended to benefit from economics of scale. In most cases, deployment potential of SMRs is supported by their ability to fill niches in which they address markets or market situations different from those of currently operated large-capacity nuclear power plants, e. g., situations demanding better distributed electrical supplies or a better match between capacity increments and investment capability or demand growth, or more flexible siting and greater product variety [2, 3].

It is important to note that the term small or medium sized reactor does not necessarily mean small or medium sized nuclear power plant. Like any nuclear power plants, those with SMRs can be built many at a site, or as twin units. In addition to this, innovative SMR concepts provide for power plant configurations with two, four, or more reactor modules. Units or modules can be added incrementally over time, reaping the benefits of experience, timing, and construction schedules (see Fig. 1), and creating an attractive investment profile with minimum capital at risk.

Sometimes it is perceived that SMRs are meant to address users in countries which currently either do not have a nuclear infrastructure, or which have it on a small scale, and which are contemplating either introduction or significant expansion of nuclear power for the first time. However, this is not the case — most innovative SMR designs are intended to fulfil a broad variety of applications in developed and developing countries alike, irregardless of whether they have already embarked on a nuclear power programme or are only planning to do so [1-3].

Finally, it should be emphasized that SMRs are not the only prospective nuclear option; it must be recognized that a diverse portfolio of reactors of different capacities and applications are required if nuclear power is to make a meaningful contribution to global sustainable development. The anticipated role of SMRs

within the global nuclear energy system could be to increase the availability of clean energy in usable form in all regions of the world, to broaden access to clean, affordable and diverse energy products and, in this way, to contribute to the eradication of poverty and support of a peaceful and stable world.

In 2008, more than 45 innovative[1] SMR concepts and designs were developed within national or international research and development (R&D) programmes involving Argentina, Brazil, China, Croatia, France, India, Indonesia, Italy, Japan, the Republic of Korea, Lithuania, Morocco, the Russian Federation, South Africa, Turkey, the USA, and Vietnam [2, 3].

Innovative SMRs are being developed for all principal reactor lines and some non-conventional combinations thereof. The target dates of readiness for deployment range from 2010 to 2030.

Strong reliance on inherent and passive safety design features has become a trademark of many advanced reactor designs, including several evolutionary designs [4] and nearly all innovative SMR designs [2, 3]. Reactors with smaller unit output require adequate defence in depth to benefit from more units being clustered on a site or to allow more proximity to the user, specifically when non-electrical energy products are targeted and the user is a process heat application facility such as a chemical plant.

This report is intended to present state of the art design approaches with the aim to achieve defence in depth in SMRs. Preparation of this report is supported by IAEA General Conference resolution GC(51)/14/B2(k) of September 2007.