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Small Modular Reactors (SMRs) are getting a lot of attention in the nuclear industry because they offer great potential for lower initial capital investment, scalability, and they come in sizes more appropriate for locations unable to accommodate larger 1000+ MW units.  However, there are some big potential advantages that have not been widely discussed that could make SMRs a game-changer.  These advantages are the potential for enhanced safety and security.

Let me explain.

The goal of nuclear plant emergency planning is to protect people from exposure to radiation they might receive during a reactor accident. That radiation exposure would come (mostly) from radioactive gas released into the air from a damaged nuclear plant. There are three physical barriers in all modern nuclear plants that keep radioactive gas inside the reactor: the metal cladding that encases the ceramic uranium fuel pellets, the thick steel reactor vessel and piping and that contains the reactor and coolant, and the concrete and steel containment building than encloses the reactor. For people to be in danger from a reactor accident first the fuel must overheat to create the radioactive gas. Then all three barriers (clad, system piping, and containment building) must be breached to provide a pathway for the radioactive gas to reach the atmosphere. Finally, there has to be a pressure difference to push the gas out of the plant and into the atmosphere. In water cooled reactors like most in use today, the hot water turns to steam and steam pressure builds up inside the containment.  If the containment is breached this pressure pushes the radioactive gas through the hole to the air outside.

With this in mind, small modular reactors offer several big advantages that make them safer:

They are smaller, so the amount of radioactivity contained in each reactor is less. So much less in fact, that even if the worse case reactor accident occurs, the amount of radioactive material released would not pose a risk to the public. In nuclear lingo we say SMRs have a smaller “source term.”  This source term is so small we can design the plant and emergency systems to virtually eliminate the need for emergency actions beyond the physical site boundaries.  Then, by controlling access to the site boundary, we can eliminate the need for off-site protective actions (like sheltering or evacuations).

These smaller reactors contain less nuclear fuel.  This smaller amount of fuel (with passive cooling I’ll mention in a minute) slows down the progression of reactor accidents.  This slower progression gives operators more time to take action to keep the reactor cool.  Where operators in large reactors have minutes or hours to react to events, operators of SMRs may have hours or even days. This means the chance of a reactor damaging accident is very, very remote.

Even better, most SMRs are small enough that they cannot over heat and melt down. They get all the cooling they need from air circulating around the reactor. This is a big deal because if SMRs can’t melt down, then they can’t release radioactive gas that would pose a risk to the public.  Again, this means the need for external emergency actions is virtually eliminated.

Also, some SMRs are not water cooled; they use gas, liquid salt, or liquid metal coolants that operate at low pressures.  This lower operating pressure means that if radioactive gases build up inside the containment building there is less pressure to push the gas out and into the air.  If there is no pressure to push radioactive gas into the environment and all of it stays inside the plant, then it poses no risk to the public.

SMRs are small enough to be built underground. This means they will have a smaller physical footprint that will be easier to defend against physical attacks.  This provides additional benefits of lower construction costs because earth, concrete and steel are less costly than elaborate security systems in use today, and lower operating costs (a smaller footprint means a smaller security force).

In summary, small modular nuclear reactors offer potential safety and security advantages over larger commercial reactors because they can be designed (1) to have smaller source terms, (2) to have accident scenarios that progress more slowly, (3) to be meltdown proof, (4) to operate at lower pressures, and (5) to have smaller security footprints.

These safety and security advantages can result in considerable cost advantages.  A large percentage of a nuclear plant’s operating expenses go into emergency planning and security.  It is possible that four or five SMRs packaged together to provide the equivalent of a large nuclear unit could operate with a smaller staff size and lower costs.  However, because existing rules were written for larger reactors, some changes to NRC regulations will be required for SMRs to take full advantage of their inherent safety and security features.  There are groups already working on these changes.

These safety and security advantages offered by SMRs, when combined with lower initial capital costs, shorter construction times, and scalability, may tip the scales in favor of a new generation of small, factory built modular reactors.

John Wheeler

This Week in Nuclear