SMR: What are mini nuclear reactors? | The scientists respond | Science | The USA Print

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In English they are called Small Modular Reactors (SMRs) or Small Nuclear Reactors. Although the most used term is SMR because, in addition to being small, they are conceived as modular. What defines the difference between what we call small, medium or large is the power they generate. Below 300 megawatt equivalents (MWe), which is about a third of the capacity of the normal reactors they use today, is considered SMR. This is the definition of International Atomic Energy Agency (IAEA).

All SMRs out there so far are prototypes in various stages of development, but none are in use yet. The countries that are developing them are the United States, which is betting heavily on them, China, Russia, Canada, South Korea and the United Kingdom. These small nuclear reactors have been used before, although not as a source of electricity. For example, they are the ones used in nuclear submarines.

A conventional nuclear power plant like the ones in operation today requires a huge financial investment, according to the Spanish Nuclear Society, between 4,000 and 5,000 million euros. And they also have other added problems such as safety and waste management. SMRs are proposed as a cheaper and easier to manage alternative. The designs include a first module with a power of less than 300 MWe, to which additional modules with similar characteristics can eventually be added.

One of the first differences between SMRs and current nuclear power plants has to do with their construction. A nuclear power plant cannot be built anywhere, as it requires suitable geological characteristics or, in the case of those that use water as a coolant (boiling or pressurized water reactors), a nearby water source such as a lake, river or the sea, an SMR, being smaller, will not require as much cooling water. For its construction it is necessary to move an entire industry to the place chosen for the location, with the high production costs that this entails. Small reactors can be built in factories that can be located anywhere. Once built, it will be possible to move them to the place where the mini nuclear power plant will be located. This will allow them to be installed, for example, in isolated or distant places where it is not profitable to install a conventional nuclear power plant, or where it is difficult to get the general electrical network to reach them. Its use is also considered in desalination plants, heating systems, or in industrial applications.

The main difference between the small reactors and the nuclear reactors that are now in operation has to do with their operation. A 700 or 1000 MW reactor is complicated to manage, both from the point of view of safety and from the point of view of radioactive waste.

From the environmental point of view, it will be necessary to see how the prototypes respond, although they are expected to be safer for several reasons. The first is that their small size allows a better choice of the place where they are installed from the point of view of geological stability. Also from the point of view of a more operational safety, since for these smaller reactors passive safety systems can be designed that cannot be used in a conventional nuclear power plant. Since these passive systems depend on physical phenomena, which do not require human action, it will be easier to “turn off” the system in an emergency, through systems such as natural circulation, convection, gravity or self-pressurization. Additionally, many designs include a steel container that will encase the entire reactor and act as an additional engineering barrier.

According to the information that the developers of these prototypes are making public, waste management will be easier for the simple reason that it will be produced in less quantity, although it is clear that in the case of having three modular reactors (900 MWe), its waste would be equivalent to that of a conventional plant. On the other hand, these new designs have the advantage over conventional ones that the recharging time is less frequent, so that it can be done every three to seven years, instead of the current one or two years. Some SMRs are designed to last up to 30 years without recharging. This will simplify the operation of the plants and the management of waste.

In general, the 700 MWe reactors can roughly adjust production according to the electricity demand at that time. Proponents of SMRs estimate that they will be able to adjust their production in an even more flexible way than current nuclear power plants, coordinating it with that of renewable energies, for example wind or solar, which are variable energy sources that depend on the weather and the time. of the day Its defenders argue that, when combined with renewable sources, they would increase the efficiency of these in a hybrid energy system.

Different types of modular reactors are being considered. There are prototypes of light water, which use pressurized water as a coolant. There are also other possible coolants such as some gases or molten metals (fast neutron reactors), as well as molten salts. Another aspect that is of interest in SMRs is the use of thorium as an alternative fuel, especially as a result of the prototypes that some countries are already developing. This is a second major line of action within nuclear reactors for peaceful uses, where alternative fuels to uranium are sought, which is currently used in nuclear reactors and whose presence in the earth’s crust is limited.

Maria Villa Alfageme She researches the use of radioactive tracers in the environment and is a Full Professor in the Department of Applied Physics II at the University of Seville.

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