Hydrogen-moderated self-regulating nuclear power module

The hydrogen-moderated self-regulating nuclear power module (HPM), also referred to as the compact self-regulating transportable reactor (ComStar), is a type of nuclear power reactor using hydride as a neutron moderator. The design is inherently safe, as the fuel and the neutron moderator is uranium hydride UH3, which is reduced at high temperatures (500–800 °C) to uranium and hydrogen.

Source: Wikipedia — Hydrogen-moderated self-regulating nuclear power module (CC BY-SA 4.0)