Uranium metal heated to 250 to 300 °C reacts with hydrogen to form Uranium hydride 2U + 3H2 → 2UH3. The high temperature moderator material Uranium hydride poses a significant enhancement in small reactor design by thermalizing (slowing down) neutrons and decreasing the required fuel mass for a system. The intimate contact between the uranium and the hydrogen of the fuel results in a self-moderated reactor fuel. The reactor must be designed through control of the hydrogen-to-uranium ratio, to be slightly under-moderated. Hydrogen works well as a neutron moderator because its mass is almost identical to that of a neutron. This means that one collision will significantly reduce the speed of the neutron because of the laws of conservation of energy and momentum. A principal difference between oxide and hydride fuels is the high thermal conductivity of the latter. This feature greatly decreases the temperature drop over the fuel during operation, thereby reducing the release of fission gases to the fraction due only to recoil.

1. The development of neutron-based hydrogen imaging and crystallographic characterization that allows us to understand fundamental diffraction behaviors and to observe changes in hydrogen distribution as a function of temperature and

2. Subsequent neutron multiplication (reactivity) effects of changes in hydrogen distribution using measurement-based cross sections in a sample micro reactor design. The main conclusions from the work are that: (1) hydrogen does not redistribute significantly below temperatures of 800°C in Uranium hydride and (2) hydrogen redistribution affects the reactivity slightly but not significantly.

The behavior of hydrogen in the fuel is the source of phenomena during operation that are absent in oxide fuels. Because of the large heat of transport (thermal diffusivity) of H in ZrHx, redistribution of hydrogen in the temperature gradient in the fuel pellet changes the initial H/Zr ratio of 1.6 to ∼1.45 at the center and ∼1.70 at the periphery. Because the density of the hydride decreases with increasing H/Zr ratio, the result of H redistribution is to subject the interior of the pellet to a tensile stress while the outside of the pellet is placed in compression.