Fine-grained SrWO 4 and NaNd(WO 4 ) 2 ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.

— Na x Sr 1 – 2 x Nd x MoO 4 ( x = 0–0.5, Δ x = 0.1) solid solutions with the scheelite structure have been synthesized for the first time and their crystallographic parameters have been determined as functions of composition and temperature. Their thermal expansion coefficients have been determined in the temperature range 25–1000°C: α a = (12.9–14.9) × 10 –6 °C –1 and α c = (24.9–25.9) × 10 –6 °C –1 . Using spark plasma sintering at t max ≈ 872–985°C and τ ≈ 650–750 s, we have prepared ceramics with relative densities in the range 97–99%.

Spark Plasma Sintering has been used to obtain first ever fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure and high density (95.8–99%) that are promising as materials for nuclear waste immobilization. It has been demonstrated that the densification intensity in tungstates at the initial sintering stage is determined by the agglomeration degree of input powders. The activation energy of fine-grained ceramics with the scheelite structure has been established to correlate at high temperatures with the activation energy of grain-boundary diffusion. Rapid sintering of fine-grained NaNd(WO4)2 ceramic results in internal tensile stress fields that lead to lowered hardness and fracture toughness of the ceramic.

The Spark Plasma Sintering method was used to produce high-density ceramics from tungstates SrWO 4 and NaNd(WO 4 ) 2 with scheelite structure. These compounds are proposed as possible matrices for the consolidation of radwaste components. Powder samples were obtained by coprecipitation method and studied by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). After sintering, the samples retained their phase identity (scheelite structure). The total duration of sintering was ~ 13-15 min, the relative density was reached ~ 92, 99%.

Mineral-like matrix Nax(Ca/Sr)1-2xNdxWO4 (x = 0, 0.1 and 0.2) for radwaste is proposed. Powder samples of complex tungstates have been synthesized by coprecipitation from aqueous solutions of Na, Ca, Sr, Nd salts. Crystallographic characteristics of compounds have been calculated and a monotonous change observed in them along with some x value changes. Monophasic ceramics with high relative density (97–100%) from these powders have been obtained through Spark Plasma Sintering (SPS). SPS of tungstates has been shown to occur in multiple stages, with the sintering kinetics at the final stage of sealing controlled by diffusion both in the crystal lattice and along grain boundaries. Activation energies have been determined for sintering of tungstates at different x values. The reduced activation energy of sintering at the third heating stage (at high temperatures) could be ascribed to the grain growth.