2.11.3.1 Introduction

Zirconium hydride is used as a material for neutron reflectors in fast reactors. The evaluation of the ther­mal conductivity, elastic modulus, and other basic properties of zirconium hydride is extremely impor­tant for assessing the safety and cost-effectiveness of nuclear reactors. Metal hydrides, ofwhich zirconium hydride is a typical example, are also very interesting because they exhibit unique properties and shed light
on some fundamental aspects of physics. As part of work on metals such as zirconium, Yanamana et al. have successfully created crack-free, bulk-scale metal hydrides, and systematically investigated their funda­mental properties — particularly at high tempera­tures. Here, we present an outline of the results on the fundamental properties of zirconium hydride. Figure 6 shows the zirconium-hydrogen binary phase diagram.19

2.11.3.2 Production of Zirconium Hydride20

We used polycrystalline (grain size: 20-50 pm) ingots of high-purity zirconium as the starting material for producing hydrides. The main impurities present in the zirconium were O (0.25 wt%), H (0.0006 wt%), N (0.0024 wt%), C (0.003 wt%), Fe (0.006 wt%), and Cr (0.008 wt%). The hydride was generated with high-purity hydrogen gas (7 N) at a prescribed pres­sure, using an advanced ultra-high vacuum Sieverts instrument. Details of the instrument configuration are given in Figure 7.

The procedure for synthesizing hydrides varies according to the type of metal. This is due to the phase transition, from metal to hydride that is accom­panied by a massive increase in volume due to hydro­genation, and to differences in the strength of the hydride. Figure 8 shows the external appearance of zirconium hydride substances produced by the author’s group.

Weight percent hydrogen 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Figure 6 Binary phase diagram of the zirconium-hydrogen system. 5 and e represent the face-centered cubic 5-phase hydride and the face-centered tetragonal e-phase hydride, respectively. Adapted from Zuzek, E.; Abriata, J. P.; San-Martin, A.; Manchester, F. D. Bull. AlloyPhase Diagrams 1990, 11 (4), 385-395.

Low temperature incubator (inner temperature: 298 K)

Turbo-molecular pump

Oil rotary vacuum pump

Ionization vacuum gauge

Liquid nitrogen trap

Compressed hydrogen gas cylinder

Reactor for high temperature (quartz glass)

Electric resistance furnace (<1273 K)

Figure 7 Schematic diagram of advanced Sieverts instrument.

2.11.3.3 Lattice Parameter20

Zirconium hydride or deuteride described here was all fcc_C1 (8) ZrH2-x or ZrD2-x single-phase crys­tals with a fluorite structure. The lattice parameters at ambient temperature of zirconium hydride or deu­teride are plotted in Figure 9, as a function of hydro­gen content (CH). The lattice parameter of zirconium hydride increases slightly with increasing hydrogen content, according to the following formula:

a(nm) = 0.4706 + 4.382 x 10-3 x CH(H/Zr).

Figure 8 Bulk-scale zirconium hydride.

2.11.3.4 Elastic Modulus and Hardness20,21

Figure 10 illustrates the hydrogen content depen­dence of the elastic modulus of zirconium hydride or deuteride, determined using an ultrasonic pulse echo method. The elastic modulus of zirconium hydride is higher than that of the pure metal, and decreases slightly with increasing hydrogen content. The hydrogen content dependence of the elastic modulus of zirconium hydride is expressed by the following equations (E: Young’s modulus, G: Shear modulus, and B: Bulk modulus):

E(GPa) = 187.7 — 33.28 x Ch(H/Zr)

G(GPa) = 73.59 — 14.19 x CH(H/Zr)

B(GPa) = 130.0 — 2.329 x CH(H/Zr)

Figure 11 illustrates the hydrogen content depen­dence of the Vickers hardness of zirconium hydride and deuteride. The graph clearly shows that the Vickers hardness of the hydride is higher than that of pure zirconium, and that it decreases slightly with increasing hydrogen content. Generalizing these results, we can conclude that increasing the hydrogen content has the effect of making zirconium hydride and deuteride plastically ‘softer.’ The relationship between the hardness and hydrogen content depen­dence for zirconium hydride is expressed by the following formula:

0.479

0.478

ф

ф

E

ra

ra

CL

0.477 —

Figure 9 Hydrogen content dependence of the lattice parameter of zirconium hydride and deuteride.

Figure 11 Hydrogen content dependence of the Vickers hardness of zirconium hydride and deuteride.

Hy(GPa) = 7.190 — 2.773 x CH(H/Zr)