China
Africa
Explore chapters and articles related to this topic
Capture Pumps
Published in Igor Bello, Vacuum and Ultravacuum, 2017
Zirconium is a chemically very active NEG. Zirconium is a grayish-white ductile metal with a melting point of 2158 K. To a large extent, zirconium is used in nuclear plants to moderate nuclear reactions, where a diffusion process of deuterium takes place. The introduction of deuterium impurities into zirconium makes it brittle. At high deuterium concentration or generally hydrogen concentration, zirconium may lose mechanical integrity, start to crack, and chip off. Hydrogen chemisorption occurs with the formation of zirconium hydride. Whenever gas impurities are chemisorbed, zirconium turns from a ductile to a brittle material. Zirconium sorbs hydrogen at 673 K and desorbs it at 1153 K. The chemically active gases such as nitrogen, oxygen, carbon monoxide, and carbon dioxide are, however, sorbed at 1673 K. Therefore, zirconium should be used in two independent strip loads that are heated to two different temperatures to chemisorb both hydrogen/water vapor and other reactive gases.
Structural Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
Turning the focus to zirconium, it will be seen from Figure 15 that zirconium is typical among the exothermic occluders of hydrogen. Unalloyed zirconium and zirconium-rich alloys take up to 450 ppm hydrogen in solid solution at temperatures of 500°C; the solubility decreases rapidly with decreasing temperature, becoming 65 ppm hydrogen at 300°C and perhaps 0.05 ppm hydrogen at 20°C. In the fabricated condition, zirconium alloy structural components in the nuclear reactor core contain about 10 ppm hydrogen. During reactor service, additional hydrogen from the corrosion process can diffuse into the tubes, and when present in quantities above the solubility limit, it precipitates as a brittle, solid zirconium hydride. The hydride forms either as single platelets or as elongated groups of platelets. These hydride platelets will have an embrittling effect. The severity of hydride embrittlement depends not only on the volume fraction of hydride present, but also on its distribution and morphology, particularly the orientation of platelets with respect to any residual or applied stress. Oriented hydrides in zircaloy fuel cladding tubes may have a deleterious effect on the in-reactor service of the tube. They may be performance limiting. When the hydride platelets are parallel to the radial direction of the tube, a drastic decrease of ductility may occur. As a result, studies of hydride orientation are of major importance.
Feasibility of Using Poisons to Suppress the Positive Temperature Reactivity Coefficient in Hydride-Moderated Reactors
Published in Nuclear Science and Engineering, 2023
Vedant K. Mehta, Zachary A. Miller, Dasari V. Rao
Among the various metal hydrides, zirconium hydride (ZrHx) and yttrium hydride (YHx) are of particular interest due to their excellent volumetric hydrogen retention capacities at and above core operating temperatures (). The ZrHx moderator is superior to the YHx moderator due to its higher hydrogen density per unit volume and lower zirconium neutron absorption cross section, thus requiring less moderator mass. However, at higher temperatures (1000 K), YHx has lower partial pressure than the ZrHx system, making it more feasible for higher-temperature designs. Therefore, many self-regulating reactor and microreactor designs will aim to utilize either of these two hydrides for neutron moderation.1 Utilization of metal hydrides results in three major engineering challenges: (1) metal hydrides are prone to hydrogen diffusion due to the inherent temperature gradient generated in the core3; (2) similar to diffusion, hydrogen can migrate outside the active core regions, potentially reducing the discharge time of the core4; and (3) due to neutron upscattering in the epi-thermal energy region, reactors can also be exposed to positive temperature reactivity coefficients (TRCs) from metal hydrides.