Transition metal hydrides may be used as neutron moderators for small-scale highly stressed nuclear power installations (NPI) at higher temperatures [7, 35]. Zirconium hydride moderator blocks of NRE should provide structural integrity during thrust regime and remain chemically inert under bimodal power regime at power fluxes of 1-1.5kW/m3 at average temperatures of 570K and temperature gradients of ~110K across the block.

Zirconium hydride parts are typically processed by reach-through hydrogen impregnation of the heated metal preforms, or by molding metal hydride pow­ders with subsequent pressure treatment [36]. The terms “reach-through impreg­nation” and “reach-through hydrogenation” mean diffusion-controlled impregnation of metal preforms up to the certain hydrogen volume fraction. Important advantage of a compacted preform hydrogenation method is the possibility of processing pore-free

material, and relative straightforwardness of the process. Among the disadvantages of this method are prolonged cycle time required for preform hydrogen impregnation (up to several weeks for large-sized performs), and the necessity to use autoclave equipment.

Forming the parts from hydride powders offers higher efficiency production of hydride parts and the capacity for composite production. The main disadvantage of the powder-based materials is typically imperfect grain boundaries that lead to poorer corrosion resistance in gaseous media. Reaction of Zr metal with hydrogen proceeds spontaneously and is accompanied by heat generation. Such spontaneous nature of reaction before hydride formation typically leads to cracking of the impregnated metal, caused by stress generation and simultaneous embrittlement. Cracking could be prevented through controlling the impregnation speed that leads to more balanced processes of stress generation and relaxation. The qualitative process and phase formation kinetics description could be obtained from the equilibrium phase diagram of the metal-hydrogen system. e. g. ZrH diagram (see Fig.4.30).

The process may be approximately divided into three stages. First stage involves formation of a в solid solution of hydrogen in zirconium (above the temperature of polymorphic transformation). The second stage involves formation of a hydride phase (at first on a surface. then in the bulk). This stage brings bulk volume changes in the materials that generate stresses possibly exceeding the ultimate strength of the material. The third stage involves compositional homogenization of the hydride phase, accompanied by the redistribution of the stresses (both in the magnitude and in the sign). Reach-through Zr impregnation by H is accompanied by volume change. For hydrides with composition close to MeH2 (e. g. for Zr hydride) this volume change may reach up to 20 %. The magnitude of the volume change influences the stress state level in the material being hydrogenated.

Based upon the data accumulated during research into hydrogen diffusion and creep [37, 38] Research Institute “Luch” has offered an optimized hydrogenation technology.

As the zirconium hydrides have insufficient strength and fracture toughness [36], there is a risk of their brittle fracture under thermal stresses generated under oper­ating conditions. This was the driving force for research into the basic mechanical parameters of zirconium hydride. The thermal stress resistance of zirconium hydride samples processed by reach-through impregnation is characterized by highly specific temperature dependence (see Table4.7).

Thermal stress resistance (TSR) of the samples is practically stable within tem­perature range of -190 to 900 °C despite the twofold strength disparity. The stability of experimentally measured TSR values is confirmed by the calculated R criterion, as the strength increase and E-modulus decrease temperature rise are compensated by significant growth of a linear expansion coefficient. The bulk structure of a sample and surface quality of hydrides after various machining treatments, influence both strength and TSR [36]. Samples after surface polishing demonstrate the largest TSR values.

The formation of hydride composites at introduction of a metal phase to 20- 40vol% reduces martensitic grain nature (Fig.4.31) and increases thermal stress resistance by two to three times, due to a local stresses relaxation, increase of a fracture toughness and the relationship a/E (Table 4.8).