How do small modular nuclear reactors work, and how safe are they if S’pore decides to use them?

Small modular reactors (SMRs) are compact nuclear power plants with a capacity of up to 300 megawatts, roughly one-third the size of traditional nuclear reactors. They generate electricity through nuclear fission, a process that splits heavy atoms like uranium, releasing an immense amount of energy compared to fossil fuels.

Unlike conventional reactors, SMRs contain less nuclear material, which could result in a smaller release of radioactive materials in case of emergencies. This characteristic may allow SMRs to be placed closer to industrial areas or densely populated regions, given that their emergency planning zones could be smaller, even confined to the plant's boundaries.

One of the key safety features of SMRs is their passive safety systems, which rely on natural phenomena like gravity to function, reducing the need for human intervention. For instance, some designs use natural circulation to remove heat from the reactor core, preventing accidents and minimizing potential damage.

The majority of SMRs generate heat by splitting uranium atoms in the reactor core, converting this heat into steam that drives turbines to produce electricity. This method is similar to that used in most conventional nuclear power plants, making it a mature and well-understood technology.

SMRs generate heat using water-cooled systems, which can store and absorb heat during emergencies without requiring active cooling or operator input. They also employ the same type of nuclear fuel as conventional power plants, which has a long history of safe operation and a well-understood safety record.

One example of SMRs in action is the KLT-40S reactors powering Russia's world's first floating nuclear power plant since 2020. These reactors use inert helium gas to extract heat from nuclear fuel, which can then be used for electricity generation or heating industries like steel production and clean hydrogen generation. Some models allow for refuelling without shutting down the reactor.

Another example is China's HTR-PM, which entered commercial operation in 2023. This reactor uses graphite reflectors to regulate heat transfer, preventing excessively high fuel temperatures.

Molten salt reactors (MSRs) are another type of SMR that employs liquid salt to transfer heat. These reactors operate at low or atmospheric pressures, eliminating the risk of high-pressure ruptures. Molten salt fuels naturally expand as they heat up, reducing fuel density and slowing the reaction, which passively shuts down the core if temperatures rise beyond design limits.

Similarly, lead-cooled fast neutron reactors use liquid metals like sodium or lead to carry heat away from the reactor core. These reactors operate at low pressures, eliminating the risk of high-pressure ruptures, and can produce less long-term nuclear waste. The BREST-OD-300 is a lead-cooled fast neutron reactor currently under construction.