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20iSHS 2019
Moscow, Russia
PEROVSKITE-LIKE MATRIX FOR IMMOBILIZATION OF HIGH-LEVEL RADIOACTIVE WASTE PRODUCED BY SHS METHOD
A. O. Semenov*", M. S. Kuznetsov", and O. Yu. Dolmatov"
National Research Tomsk Polytechnic University, Tomsk, 634050 Russia
*e-mail: semenov_ao@tpu.ru
DOI: 10.24411/9999-0014A-2019-10151
Active evolution of nuclear programs leads to the accumulation of high-level radioactive waste (HLW) [1]. The necessity of isolation of the most long-lived and biologically hazardous radionuclides from the environment for a long time and decreasing of the storage sites service life lead to the development of innovative technologies and creation of perspective materials for immobilization of waste and subsequent storage during the time for reduction of radioactivity to acceptable level. The growing necessity to isolate the most long lived and biologically hazardous radionuclides coupled with a shorter storage sites service life require new innovative technologies and materials for waste immobilization and further storage within the period of time needed for radioactivity reduction to acceptable levels for radioactive decay. Currently the most frequently used technology for the radioactive waste disposal is radionuclides vitrifying into aluminophosphate or borosilicate glasses [2]. Meanwhile. vitreous matrix materials are not an ideal solution in terms of time and storage conditions. Glass cannot ensure stable and safe HLW storage during several thousand years due to its chemical instability and tendency to spontaneous crystallization. Alternative materials for immobilization are crystalline matrices which are synthetic analogues of naturally occurring geologically stable minerals. Such minerals compounds (perovskite, monazite, zirconolite, pyrochlore, etc.) are able to reliably hold/contain highly active fractions of radioactive waste for a long time [3]. The application of these matrixes is complicated by the lack of industrial manufacturing techniques. One of the alternative methods of matrix production is self-propagating high-temperature synthesis (SHS). The advantages of SHS are the possibility of obtaining materials with desired properties, high purity of the final product, low energy consumption and the ability to control all the stages of the synthesis process [4]. The present paper shows experimental studies of perovskite-like matrix production by SHS-mode. A mixture of industrially manufactured powders of neodymium and aluminum oxides was added to nickel and aluminum powders following the stoichiometric ratio:
Al + Ni + Nd2O3 + Al2O3 = 2NdAlO3 + NiAl
where Nd2O3 was a simulator of the trivalent actinide fractions of radioactive waste because of the ionic radius of Nd is similar to different isotopes of actinides.
Reagents powders were mixed/prepared in an Erweka cubic mixer. The initial batch was pressed using a hydraulic laboratory press PGL-12 into cylindrical samples with a diameter of 25 mm and a height of 12-15 mm, but under/with different pressures: 15, 20, 25, 30, and 40 MPa to obtain samples with different densities. The synthesis was conducted under residual pressure (300 Pa) on an experimental installation for pyrometric studies of the SHS process which included a SHS-reactor.
We prepared a series of laboratory experiments on varying the content of the Nd2O3-AhO3 additive from 10 to 40 wt % in the basic mixture of Ni-Al system and pressing pressure of 10-30 MPa to study the effect of density of the batch components and the dilution ratio on synthesis and phase composition of the immobilization matrix.
XV International Symposium on Self-Propagating High-Temperature Synthesis
During the heating process a burning wave was initiated at the edges of samples and propagated over the entire sample volume if initial samples had a system density of 4.8-5.2 g/cm3 (10-30 MPa) and a dilution ratio no higher than 30 wt % as well as initial temperature of 650-700 K (depending on the preparation conditions of the initial mixture). We observed local combustion areas without further wave generation followed by synthesis attenuation when the batch of Ni-Al was diluted by more than 30 wt % of Nd2O3-AhO3 regardless of the density of the mixture due to excessive dilution as well as energy consumption of the neodymium aluminate synthesis reaction.
Thermomechanical destruction of samples is observed when the density of the system is higher than 5.27 g/cm3 (pressing pressure of 40 MPa) due to the non-stationarity of the combustion wave propagation. It can be explained by excessive density of the initial mixture which leads to a significant growth in the specific energy yield of reactions occurring in a unit volume of the sample.
On the next stage of research, the synthesized samples were subjected to XRD analysis to study the composition. The results of after synthesis phase formation for different percentages of the additive are summarized in Table 1.
Table 1. Results of XRD analysis.
No. P, MPa Additive content Nd2O3+Al2Ü3, x, wt % Phase formation, wt %.
Nd2O3 AI2O3 Ni2Al3 Al3Ni NiAl NdAlO3
1 10 10 1.2 8.3 32.4 21.1 21.2 15.7
2 20 10 2.3 7.2 35.2 16.2 23.4 15.6
3 30 10 2.6 4.5 31.7 1.1 33.5 26.6
4 10 20 8.3 7.4 27.3 16.2 24.3 16.5
5 20 20 2.8 6.6 31.7 15.6 25.4 17.9
6 30 20 4.9 2.7 27.2 - 27.2 37.9
7 10 30 13.8 12.6 19.9 22.2 15.4 16.1
8 20 30 8.1 7.1 26.2 14.3 19.6 24.7
9 30 30 2.8 4.7 26.8 - 24.2 41.5
According to Table 1, all samples with different synthesis conditions contained neodymium aluminate phases NdAlO3 from 15 to 44 wt %. The maximum perovskite content was obtained in the sample with 30 wt % additives and the pressing pressure of 30 MPa which equaled the density of 5.15 g/cm3. Under a pressing pressure of 10-20 MPa, the phases of unreacted reagents in Nd2O3 equaled to 1.2-13.86 wt %; AhO3 6.6-12.6 wt %. The proportion of unreacted products at the same density increases with the growth of the initial mixture dilution by the Nd2O3-AhO3 system. However, if the applied pressing pressure rises, the number of unreacted diluent phases reduces with the perovskite phase formation. Thus, for example, when the additive value is x = 10 wt % and the pressing pressure rises from 10 to 30 MPa, the synthesized neodymium aluminate phase in the sample increases by 9 wt %; at x = 20 wt % and x = 30 wt % by 21% and by 25 wt % respectively. This fact is explained by the combustion growth temperature during the synthesis process. In addition, the change in the degree of dilution also affects the formation of the nickel aluminide NiAl phase: Ni2Al3 and NiAb phase formation grows whereas NiAl phase with dilution decreases.
The content of Ni2Al3 and NiAb phases can lead to an increase in the strength characteristics of the matrix material due to the distortion of their crystal lattices compared to NiAl.
The further stage of the experiments intends to determine the leaching rates of the samples during the simulation of long-term storage in geological formations. The obtained samples were placed in the central channel №2 of the IRT-T nuclear research reactor for irradiation by the fast neutron flux (0 = 1014 n/cm2-s and a set of maximum fluencies (9.2 • 1019 n/cm2). Fast
ISHS 2019 Moscow, Russia
neutrons emit energy during elastic collisions. which cause numerous atomic displacements. Irradiation acts similarly to recoil nuclei and simulates radiation damage/impact from a-particles and behavior of recoil nuclei in crystal structures. This fluence corresponds approximately to 20000 years of storage.
The leachability of the samples was tested at 90°C and is based on the IAEA MCC-1 static test for monolithic samples with a known geometric surface.
Figure 1 represents/shows the rates of samples leaching under P = 30 MPa and with different dilution ratios.
15 ------
0 5 10 15 20 25 30
time,day
— x=10%;-x=20%;— x=30%
Fig. 1. Leaching rate of the simulator after irradiation in the nuclear core in IRT-T reactor.
Values of average leaching rates for all samples are (1.98-2.34)-10-9 g/(cm2day) and do not exceed the requirements of 10-7 g/(cm2day), which is 104 times lower than the known values for borosilicate and phosphate glasses.
Thus, the material can be used as the immobilizing matrix for actinide fraction of radioactive waste.
1. J. Zhang, Nuclear Fuel Reprocessing and Waste Management (Modern Nuclear Energy Analysis Methods Book 2), Singapore: World Scientific Publishing Company, 2018, 308 p.
2. K.D. Kok, Nuclear Engineering Handbook (Mechanical and Aerospace Engineering Series 60), Boca Raton: CRC Press, 2016, 1000 p.
3. R.E. Masterson, Nuclear Engineering Fundamentals: A Practical Perspective, Boca Raton: CRC Press, 2017, 987 p.
4. E.A. Levashov, A.S. Mukasyan, A.S. Rogachev, D.V. Shtansky, Self-propagating high-temperature synthesis of advanced materials and coatings, Int. Mater. Rev., 2017, vol. 62, pp. 203-239.
