Small-sized nuclear power plants - What’s behind SMR reactor concepts?

27.09.2019

When it comes to the further development of reactor concepts, the abbreviation SMR comes up again and again. This article explains the meaning of the term, gives an overview of some of the best-known concepts, and presents the work of GRS on this topic.

Definitions of SMR

There are two common definitions of SMR among experts. The International Atomic Energy Agency (IAEA), for example, groups small and medium-sized reactors together under the term Small and Medium Sized Reactors. Reactors with a capacity of up to 300 megawatts are referred to as small reactors and medium-sized reactors are those having an electrical capacity of between 300 and 700 megawatts.

The second definition was originally coined in North America. Here, SMR stands for Small Modular Reactor. The underlying definition - compared to the IAEA’s definition - takes the modular character of the plants into account. Concepts of this type provide the essential components of a primary circuit all in one single module. In addition to transportability, this is based on the idea of manufacturing everything together in a single factory and minimising work on the construction site. Individual modules with a low output can then be combined to form a larger power plant.

SMR concepts so far

Most SMR concepts so far can be summarised under the definition of the IAEA. These are usually modified light water reactors. They are mainly used to produce electricity in confined spaces, for example on icebreakers, in submarines or on aircraft carriers. Another example of a light water-cooled SMR is the floating Akademik Lomonossow nuclear power plant. The Russian energy group Rosenergoatom plans to use it to supply the remote town of Pewek in Siberia as well as offshore gas and oil drilling platforms with electricity and heat from the end of 2019.

Novel SMR concepts

However, the SMR label also includes reactor concepts of which, in addition to electricity production and the modular character of the components, other aspects are of interest to the developers. Due to their high core exit temperatures, some of the concepts are intended not only for power generation but also for providing process steam or process heat for industrial applications. The use of transmutation to reduce radioactive waste, the breeding of fuel or the burning of weapons-grade fissile material are also discussed as possible further applications of certain SMR concepts.

In addition, manufacturers are also hoping for economic advantages when using factory-manufactured SMR modules, citing lower investment costs, shorter production times, and the possibility of serial production of individual modules. The construction, operation and dismantling of such plants are also estimated to be less labour-intensive than is the case with conventional nuclear power plants.

SMR concepts whose functionality is not based on the principle of a light water reactor can be subdivided according to the coolant used:

Heavy-water-moderated and -cooled SMR concepts
So-called heavy water contains the hydrogen isotope deuterium. Deuterium absorbs fewer neutrons than light hydrogen and is therefore a more efficient moderator. Instead of enriched uranium, natural uranium could therefore also be used as fuel in heavy-water-moderated SMRs.

Gas-cooled SMR concepts
Gas-cooled SMR concepts rely on the use of gases such as helium or carbon dioxide as coolants. Compared to other SMR types, they reach much higher coolant temperatures (up to 1,000 °C) and could therefore be used to generate process heat in the chemical or petrochemical industry. Low-temperature processes, e.g. for district heating, could be used downstream with this concept.

Liquid-metal-cooled SMR concepts
Lead, lead-bismuth and sodium are to be used as coolants in SMR concepts with liquid metal cooling. The metals are characterised by a high boiling temperature and a high heat capacity. Uranium is to be used as fuel in combination with plutonium or other transuranic elements. In order to prevent contaminated primary coolant from reacting with the water-steam cycle, these concepts usually provide for an intermediate cycle. Core outlet temperatures are expected to be around 750 °C. Steam and heat could then be extracted within a temperature range from 500 °C to 700 °C for further purposes.

Molten-salt-cooled SMR concepts
Finally, other SMR concepts envisage molten salts both as coolants and as fuel carriers. It is assumed that the melts tested to date will be stable up to temperatures of 1,400 °C. Due to the heat transport properties of the molten salts, it should be possible to build reactors with significantly smaller dimensions for the same power in comparison to gas-cooled reactors. The high operating temperatures should allow high efficiencies and heat extraction for industrial high-temperature processes.  

Novel SMRs and safety

Developers see safety advantages that new SMR concepts have over large nuclear power plants. Among other things, their passive safety functions, which in some cases are also used in conventional nuclear power plants, are cited as decisive. These systems do not require electrical energy for their activation and operation, but are driven by gravity, for example. In certain SMRs, passive safety systems are intended to allow automatic shutdown without the need for an external power supply or human intervention. Cooling should also be possible passively by gravity, convection and evaporation and thus without the use of electrically driven pumps to circulate the coolant. The latter, however, would require a certain design height.  

By using alternative coolants and passive safety systems, some SMR concepts can exclude certain accident scenarios that have to be considered in conventional nuclear power plants. In addition, the damage potential of an assumed major accident compared to large nuclear power plants is reduced to the extent that SMRs are meant to be loaded with significantly lower amounts of nuclear fuel – hence the amount of radioactive material that could be released into the environment in case of an accident would be correspondingly lower. However, these potential advantages are offset by new safety-related challenges, depending on the concept. For example, the concept of a sodium-cooled SMR must ensure that the metallic sodium will not come into contact with oxygen because it ignites easily. The highly corrosive effect of molten salts also places special demands on the properties of the materials to be used, for example, in the construction of coolant lines. Finally, innovative concepts also have to take into account the very little operating experience compared to classic light water reactors. Many of the safety improvements developed for light water reactors are down to the evaluation of events. Naturally, corresponding empirical values are not available for a number of the new SMR concepts.

Work of GRS relating to SMRs

As part of its research and expert activities, GRS has been looking at the safety of SMR concepts at the conceptual level. In a study funded by the Federal Ministry of Economics and Technology, for example, the research needs for the further development of existing simulation codes used for the assessment of the safety of nuclear power plants were examined.

For various reasons, an assessment of the safety of SMRs is only possible to a limited extent. It is quite possible to make statements as to whether a certain SMR concept is plausible in its safety-related design and complies with recognised principles. However, no reliable statements can be made on the basis of concepts alone as to whether a plant constructed on the basis of the concept would be capable of being licenced in terms of the current state of the art in science and technology or in accordance with the applicable rules and regulations. Such assessments require a great deal of information that cannot be obtained from a concept alone or which is not yet available. This primarily includes the concrete technical implementation of the concept, for example the exact technical characterisation of the safety-relevant components and the materials used. However, the safety of a nuclear installation also depends, for example, on the characteristics of the specific site, e.g. with regard to seismic activity or possible flooding and the like.

Find out more

GRS report on studies into SMR concepts (German language)
IAEA publication „Deployment Indicators for Small Modular Reactors“
IAEA publication "Status of Small and Medium Sized Reactor designs"
atw (Vol. 64, 2019): "SMRs - Overview on international Developments and Safety Features"