Nuclear reactors are often regulated by water to control excess heat. However, micro-reactors are cooled using Helium gas. This has the advantage of eliminating the possibility of phase changes like water turning into gas which generates pressure. It is also more efficient for generating electricity since water does not need to be moved around the reactor.
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Smaller self-contained nuclear reactors that can be manufactured and shipped. Multiple micro-reactors can be combined for higher energy needs. It is believed to be safer because it can be manufactured and shipped (along with self-regulating Tri-structural isotropic particle fuel (TRISO) fuel) rather than custom built on-site as nuclear reactors are today. They use helium gas to cool the reactor which is also safer in case of excess heat and more efficient for generating electricity.