A review entitled "Advancein and Prospect of Moderator Materials(Hydride) for Space Nuclear Reactors" was published in the "International Journal of Energy Research," the top journal in the field of nuclear Science and Technology. This article systematically introduces the research status, main problems, and applications of moderator materials used in space nuclear reactors.
Moderator, also known as neutron moderator. In order for nuclear fission reactions to proceed efficiently, a substance is added to the reactor to slow down the movement of neutrons. Common moderators include light water, heavy water, graphite, metal hydrides, etc. For space nuclear power, the operating temperature is high and the moderator volume is required to be as small as possible. Therefore, moderators such as water and graphite are no longer suitable. Metal hydrides are often used as moderator materials in space nuclear reactors. Currently, zirconium hydride and yttrium hydride have become ideal moderator materials in the power sources of space nuclear reactors that have been built and are being developed. In this review, the current research status of the selection, existing problems, main properties of zirconium hydride and yttrium hydride, as well as their domestic and foreign applications are introduced in detail.
Material selection
In this article, the main performance indicators of several moderator materials, including water, graphite, and metal hydride, are first compared. Based on the principles of having the hydrogen concentration as high as possible, the neutron absorption cross-section as small as possible, the moderating ability as strong as possible, and the volume as small as possible, the two most suitable materials - zirconium hydride and yttrium hydride - were selected.
Table 1 Properties of moderator materials
Summary of main issues
In the application and preparation processes of these two metal hydrides, the main problems are hydrogen loss and hydrogen cracking. The process of hydrogen loss is shown in Figure 1. According to the process principle of hydrogen loss, two main methods to prevent hydrogen loss are summarized - improving the thermal stability of hydrides and preparing hydrogen-resistant coatings; compared with hydrogen loss, The principle of hydrogen cracking is more complex, but according to domestic and foreign research reports, the main means of inhibiting hydrogen cracking are relatively unified. They are all based on slowing down the hydrogen absorption rate.
Figure 1 Schematic diagram of the dehydrogenation process of metal hydrides
Performance study summary
The application of materials is inseparable from the study of basic properties. Any application of materials without basic performance research is a rogue behavior. In the third chapter of this review, a detailed summary of the data on the basic properties of zirconium hydride and yttrium hydride at home and abroad is given, including a summary of the research status of binary phase diagrams, lattice structure and crystal structure of zirconium hydride and yttrium hydride. Summary of changes in grid parameters, relationship curves between thermal diffusivity and hydrogen content, and relationship curves between thermal conductivity and hydrogen content. I won't go into details about the specific data here. Interested friends can check it out in the original article. But at the end of this chapter, the review author made an interesting discovery: As can be seen from Figure 2, the thermal conductivity and thermal diffusivity of yttrium hydride show a downward trend as the temperature increases and are both higher than those of pure metals. This is opposite to the changing rules of metal hydrides such as zirconium hydride, titanium hydride and hafnium hydride. There is no specific explanation for the reason for this phenomenon, but this does not affect the application of zirconium hydride and yttrium hydride.
Figure 2 Thermal conductivity and thermal diffusivity of zirconium hydride and yttrium hydride change with temperature
Application
Chapter 4 of this review summarizes the application and design of zirconium hydride and yttrium hydride as moderator materials for space nuclear reactors. Since the 1960s, the former Soviet Union has conducted research on the power supply of the TOPAZ series of space nuclear reactors. TOPAZ-I and TOPAZ-II both use honeycomb-shaped massive zirconium hydride as the moderator material. The schematic diagram of the core structure of TOPAZ-II is as follows. As shown in Figure 3, there are 37 holes in the round cake-shaped ZrH1.85 for inserting fuel elements (TFE), and the 12 outer cylinders are control drums and safety drums. Among the six ground tests of TOPAZ-II, the longest duration was 14,000h (583 days). Three of the tests were suspended due to fuel element problems, two experiments were suspended due to NaK coolant leakage, and one was suspended due to the moderator ZrH1.85. Aborted due to loss of hydrogen. In addition to the former Soviet Union, France's ERATO plan also uses the design of a zirconium hydride (ZrH1.7) moderator.
Table 2 TOPAZ-II series test conditions
Compared with zirconium hydride, the main advantage of yttrium hydride is that it has a lower high-temperature decomposition pressure and is suitable for high-power and long-life reactor types. However, the use of yttrium hydride as a moderator in high-temperature reactors is still in the technical demonstration stage and there still needs to be more practical experience. In 1991, the United States adopted the design of a yttrium hydride (YH1.8) moderator in the SPACE-R space nuclear reactor, with a design life of 10 years, which is much higher than that of a zirconium hydride moderator. It is worth noting that in the 2020 research report of the Oak Ridge Laboratory (ORNL) in the United States, it was found that ORNL has completed the preparation of various shapes of yttrium hydride, and its application target is directly aimed at space nuclear reactors!
Figure 3 Schematic diagram of the core structure of TOPAZ-II
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