RU2201629C2 - Method for immobilizing concentrates of transplutonium or transplutonium and rare-earth elements in cermet - Google Patents

Abstract

FIELD: recovery of high-radioactivity liquid wastes. SUBSTANCE: method involves mixing of concentrate of transuranium or transuranium and rare-earth elements with solution of ceramic- forming flong (concentrated orthophosphoric acid) and solution of metal-forming flong (nickel nitrate), calcination of mixed up solutions, and compaction of calcination product by hot pressing in inert environment. Orthophosphoric acid in terms of quinquivalent phosphor oxide is added in equimolar amount relative to transplutonium or transplutonium and rare-earth elements. Concentrate is immobilized in dense nonporous metal cermet characterized in throughput metal (electron) conductivity. EFFECT: facilitated procedure without impairing most important characteristics of cermet. 2 ex, 3 tbl

Description

 The invention relates to the field of processing liquid high-level waste (HLW) generated by hydrometallurgical methods for the regeneration of irradiated nuclear fuel.

Currently, the most rational way of handling liquid HLW is their fractionation, which leads to the production of concentrates of cesium-137, strontium-90, transplutonium and rare earth elements (TPE and REE) or their sum (TPE + REE) [1]. The feasibility of fractionating liquid HLW is due to three main reasons:
1. The possibility of industrial use of individual fractions of HLW (cesium-137 concentrate is the feedstock for obtaining photon radiation sources, and strontium-90 concentrate is a radioactive heat source).

 2. The difference in the requirements for long-term stability of the curing forms of individual fractions of HLW (the most stringent are presented to the localization of TPE due to their high radiotoxicity and long half-lives).

 3. Fractionation can significantly reduce the volume of cured HLW forms that require very reliable and long-term isolation from the biosphere, in particular TPE or (TPE + REE) fractions.

 For the curing of unfractionated liquid HLW, the two-stage processes of their vitrification are most widely used. An alternative to glass for fixing HLW is a ceramic with a higher thermal and thermodynamic stability. When non-fractionated HLW is included in ceramics, preference is given to polycrystalline materials - a set of mutually compatible stable solid solutions and / or individual crystalline phases in which individual elements or groups of elements that are part of liquid HLW are securely fixed [2].

 Obviously, it is more expedient to include a separate fraction of liquid HLW - TPE concentrate or (TPE + REE) in single-phase ceramics with a uniform distribution of elements over its volume, eliminating the possibility of its cracking due to local overheating.

 The most thermally and thermodynamically stable form of curing liquid HLW is cermet, which, unlike glass and ceramics, has high thermal conductivity due to the presence of through (bulk) metal conductivity. A known method of incorporating unfractionated HLW into cermet - cermet [3], which is a composition of polycrystalline ceramics dispersed in a metal matrix. The ceramic component of the cermet is a mixture of crystalline phases: non-hydrogen-reduced metal oxides, as well as aluminosilicates and titanates, for which liquid HLW is mixed with the necessary components. The cermet metal matrix is an alloy based on iron of hydrogen-reduced metals both already contained in liquid HLW and specially added to the latter in order to obtain alloys with high thermal conductivity and corrosion resistance.

The process of obtaining cermet includes the following main stages:
- mixing liquid HLW with the necessary additives;
- calcination of the mixture in the urea melt at a temperature of 800 ^o C;
- the formation of a metal matrix by reduction at a temperature of 800 ^o With hydrogen corresponding metal oxides;
- compaction of the cermet by hot pressing or "cold" pressing followed by sintering in a non-oxidizing (reducing or inert) atmosphere.

 The disadvantages of cermet are the polyphase of its ceramic component, the multi-stage nature and complexity of the hardware design of the process of its preparation.

 Closest to the claimed is a method of curing a TPE or (TPE + REE) concentrate in cermets with through metal (electronic) conductivity: TPO or (TPE + REE) monoaluminates with a perovskite-type structure dispersed in a metal nickel matrix [4].

The method involves the following main stages:
- mixing of two solutions: TPE concentrate or (TPE + REE) and aluminum nitrate in an equimolar ratio in terms of metal oxides with at least an equimolar amount of nickel nitrate solution in terms of metallic nickel relative to the sum of TPE or (TPE + REE) oxides and alumina, i.e. in molar ratios
(1-x) [0.5M ₂ O ₃ • 0.5Al ₂ O ₃ ] • xNi,
where M ₂ O ₃ - oxides of TPE or (TPE + REE),
x ≥ 0.5;
- calcination of a mixed solution in an air atmosphere;
- compaction of the obtained calcine by hot pressing (GP) in an inert atmosphere.

The disadvantages of the prototype are the high temperature of calcination of the mixed solution, due to the relatively high decomposition temperatures of TPE nitrates (400 ^o C) and especially REE nitrates (800 ^o C and above) and the synthesis temperatures of TPO monoaluminates or (TPE + REE) (800 ^o C) with the structure perovskite type, as well as their relatively low sintering ability [5].

 The objective of the present invention is the curing of the concentrate TPE or (TPE + REE) in having a dense non-porous metal-ceramic having through metal (electronic) conductivity and simplifying the process of its production without compromising its most important characteristics.

To solve the problem, instead of aluminum nitrate (see prototype), it is proposed to use concentrated orthophosphoric acid Н ₃ РО ₄ in an equimolar amount in terms of phosphorus pentoxide Р ₂ O ₅ with respect to TPE or (TPE + REE) oxides, as a ceramic-forming material. .e. M ₂ About ₃ : P ₂ O ₅ = 1, and the process of obtaining cermet to carry out, as described in the prototype.

When concentrated orthophosphoric acid is added to the TPE or (TPE + REE) concentrate, a stable colloidal solution of their orthophosphates in nitric acid is formed:
M (NO ₃ ) ₃ + H ₃ PO ₄ -> MPO ₄ + 3HNO ₃ .

Calcination of a mixture of a colloidal solution of TPE orthophosphates or (TPE + REE) and a true solution of nickel nitrate is accompanied by complete dehydration of the former at a temperature of 300 ^o C through the stage of formation of crystalline hydrates:
MPO ₄ • nH ₂ O (t) -> MPO ₄ + nН ₂ O (n = 0.5-3)
and thermal decomposition of the second at a temperature of 300-350 ^o With to Nickel oxide:
Ni (NO ₃ ) ₂ (t) -> NiO + 2NO ₂ + 0.5O ₂ ,
and the GP process of the obtained calcinate is the thermal dissociation of nickel oxide into metallic nickel and oxygen:
NiO (t) -> Ni + 0.5O ₂ .

 The final product of compaction of calcine by the GP method in an inert atmosphere is cermet: synthetic monazite (TPE orthophosphates or (TPE + REE) with a monoclinic structure) dispersed in a nickel metal matrix.

A prerequisite for solving this problem is compliance with the stoichiometry of the components of the ceramic component of cermet, i.e. the molar ratio of M ₂ O ₃ : P ₂ O ₅ = 1, otherwise
when M ₂ O ₃ : P ₂ O ₅ <1, the amount of the metal component of cermet decreases due to the interaction of an excess of phosphoric acid with nickel nitrate with the formation of nickel phosphates;
at M ₂ O ₃ : P ₂ O ₅ > 1, the chemical stability of cermets decreases, since excess (free) TPE or (TPE + REE) oxides not bound to orthophosphates, as is known [5], are relatively easily hydrolyzed, and then dissolve, especially under hydrothermal conditions.

The advantages of the method compared with the prototype are the ability to cure the TPE concentrate or (TPE + REE) in a dense non-porous cermet having through metal (electronic) conductivity and without impairing its most important characteristics, simplifying its production process by reducing the calcination temperature from 800 to 300-350 ^o FROM.

Example 1
The method was tested in laboratory conditions on an acidic (1 M HNO ₃ ) solution of europium nitrate (50 g / l in terms of europium oxide), which is an electronic analogue of americium, the main component of TPE concentrate.

Concentrated orthophosphoric acid (600 g / l in terms of phosphorus pentoxide) was added to a solution of europium nitrate to a molar ratio of Eu ₂ O ₃ : P ₂ O ₅ = 1, and then a solution of nickel nitrate (200 g / l in terms of metallic nickel) to a molar ratio of EuPO ₄ : Ni = 1.

The mixture of solutions was calcined in air at a temperature of 300 ^o C for 0.5 h, and calcinate, the concentration of the residual nitrate ion in which was less than 0.1 wt.%, Was compacted by GP in an argon atmosphere in the following mode: temperature 1200 ^o C, pressure 29.4 MPa (300 kg / cm ² ), holding time 1 h.

The obtained hot-pressed cermets had the following characteristics:
phase composition: europium phosphate EuPO ₄ with a monoclinic structure and metallic nickel;
through metal (electronic) conductivity;
open porosity - 0;
the rate of leaching of europium into distilled water at a temperature of 90 ^o C ~ 10 ^-8 g / cm ² • day;
capacity for europium oxide 57.6 wt.%;
Nickel capacity (nickel consumption) 19.2 wt.%.

Example 2
The method was tested in laboratory conditions on an acidic (1 M HNO ₃ ) model concentrate (TPE + REE), the chemical composition of which (in terms of metal oxides) is given in table. 1, but in which the oxides of americium and curium were replaced by equimolar amounts of oxides of their electronic analogues - europium and gadolinium, respectively.

Concentrated orthophosphoric acid was added to the model concentrate (TPE + REE) to a molar ratio of M ₂ O ₃ : P ₂ O ₅ = 1, and then a solution of nickel nitrate to a molar ratio of MPO ₄ : Ni = 1.

 The mixture of solutions was calcined and calcined was compacted as described in Example 1. The concentration of residual nitrate ion in the calcine was less than 0.1 wt.%.

The obtained hot-pressed cermets had the following characteristics:
phase composition: REE MPO ₄ orthophosphates with a monoclinic structure and metallic nickel;
through metal (electronic) conductivity;
open porosity - 0;
the rate of REE leaching into distilled water at a temperature of 90 ^o C ~ 10 ^-8 g / cm ² • day;
capacity for REE oxides 55.8 wt.%;
Nickel capacity (nickel consumption) 20.0 wt.%.

The effect of the amount of nickel in cermets on the nature of its conductivity for two systems:
1. (1-x) EuPO ₄ • xNi or (1-x) 0,5Eu ₂ O ₃ • (1-x) 0,5P ₂ O ₅ • xNi;
2. (1-x) MPO ₄ • xNi or (1-х) 0,5М ₂ О ₃ • (1-х) 0,5Р ₂ O ₅ • xNi, where М - REE, and х - molar fraction of nickel in cermets are given in table. 2 and 3, respectively.

From the table. 2 and 3 it is seen that the formation of cermets with through metal (electronic) conductivity occurs at a molar ratio of EuPO ₄ (MPO ₄ ): Ni ≤ 1.

The obtained samples of hot-pressed cermets had the following characteristics:
qualitatively identical phase composition: europium orthophosphate or REE orthophosphate with a monoclinic structure;
through metal (electronic) conductivity at a molar ratio of EuPO ₄ (MPO ₄ ): Ni ≤ 1;
open porosity - 0;
the leaching rate of europium or REE in distilled water at a temperature of 90 ^o With 10 ^-9 -10 ^-8 g / cm ² • day;
with a molar ratio of EuPO ₄ (MPO ₄ ): Ni ≤ 1, the maximum capacity for europium oxide or REE oxides is 57.6 or 55.8 wt.%, respectively, and the minimum capacity for nickel (nickel consumption) is 19.2 or 20.0 wt. % respectively.

As can be seen from the examples, the proposed method in comparison with the prototype allows you to cure the concentrate TPE or (TPE + REE) in a dense non-porous cermet and simplify the process of obtaining it by lowering the temperature of calcination from 800 to 300-350 ^o With no degradation of the most important characteristics of the final form of curing , in particular, through metal (electronic) conductivity, chemical resistance, high capacity for TPE oxides or oxides (TPE + REE), which can reach 67.7 or 56.0 wt.%, respectively, and low nickel capacity (consumption nickel) 14.6 or 19.9 wt.% (minimum), respectively.

Sources of information
1. Galkin B.Ya., Yessimantovski VM, Lazarev LN, Lyubtsev RI et al. Extraction of Cesium and Strontium, Rare Earth Elements, and TRU from Liquid Volative Waste by Means of an Extractant, Based on a Dicarboxylate of Cobalt. Int. Conf. on Chem. Extraction "ISEC-88", Moscow, 1988.

 2. Harker A.B. Tailored ceramics, in Radioactive Waste Forms for the Future / edited by Lutze W. and Ewing R.C. (North-Holland, Amsterdam), 1988, p. 335-392.

 3. Aaron W.S., Quinby T.C., Kobisk E.H. Cermet High-Level Waste Forms. Report ORNL / TM-6404 (Oak Ridge National Laboratory, Oak Ridge, TN), 1978.

 4. RF patent 2138866, prior. 04/09/98, publ. 09/27/99, bull. 27.

 5. Compounds of rare earth elements. Systems with oxides of I-III groups / Arsenyev P. A, Kovba L.M., Bagdasarov Kh.S. et al. - M .: Nauka, 1983, 278 p. (Chemistry of rare elements).

Claims (1)

 A method of curing a concentrate of transplutonium or transplutonium and rare earth elements in cermets with through metal conductivity, comprising mixing a concentrate of transplutonium or transplutonium and rare earth elements with a solution of a ceramic-forming matrix material and a solution of a metal-forming matrix material - nickel nitrate, calcining the calcine with a calcine mixture and calcining the mixture characterized in that as a ceramic-forming m -symmetric material using concentrated phosphoric acid in an equimolar amount, based on the phosphorus pentoxide with respect to oxides or transplutonium transplutonium and rare earth elements.

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