RU2243609C2 - Method for ceramic solidification of concentrates of transplutonium and rare-earth elements - Google Patents

Abstract

FIELD: recovery of liquid radioactive wastes.

SUBSTANCE: proposed method includes calcination of concentrate of transuranium and rare-earth elements in environment of ceramic-forming flong. Used as ceramic-forming flong is concentrated solution of chromium nitrate or concentrated solution of chromium nitrate doped with concentrated solution of copper nitrate in proportions required to form solid solutions of M _{1 - x} Cu _x CrO ₃ , where M are transuranium and rare-earth elements; x = 0 0.20. Calcinate is compacted in the open using hot isostatic pressing method.

EFFECT: enhanced reliability of ceramic immobilization of radioactive wastes.

1 cl, 2 tbl, 2 ex

Description

The present invention relates to the field of the processing of 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 ceramic, which has a higher thermal and thermodynamic stability. When non-fractional 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, 3].

It is obvious that it is more expedient to include a separate fraction of liquid HLW - a concentrate (TPE + REE) in a single-phase ceramic with a uniform distribution of elements over its volume, eliminating the possibility of its cracking due to local overheating, and with a high oxide capacity (TPE + REE), eliminating the need for their separation and curing them in a small volume of ceramics.

Closest to the claimed is a method of curing a concentrate (TPE + REE) in a single-phase ceramic - zirconia stabilized with oxides (TPE + REE) in the form of a cubic solid solution with a fluorite type structure, уМ ₂ О ₃ · (1-y) ZrО ₂ , where M = (TPE + REE), and y = (0.25 0.35) [4].

The method involves the following main stages

- mixing two solutions: a concentrate (TPE + REE) and a ceramic-forming matrix material (KM) - zirconyl nitrate in a molar ratio (0.20-0.35) :( 0.80-0.65) in terms of metal oxides;

- calcination of a mixed solution in an air atmosphere;

- compaction of calcine by hot pressing to obtain single-phase ceramics with the following characteristics:

- phase composition: zirconia stabilized by oxides (TPE + REE) in the form of a cubic solid solution with a fluorite type structure;

- thermal conductivity at a temperature of 20 ° C - λ ²⁰ ≤ 3.4 W / (m · K);

- the rate of REE leaching into distilled water at a temperature of 90 ° С - r ⁹⁰ ~ 10 ⁸ g / (cm ² · day);

- mass capacity for oxides (TPE + REE) - E _m = 0.41-0.59 kg / kg of ceramics;

- volumetric capacity for oxides (TPE + REE) - Еν = 2.54-3.71 kg / dm ^{3 of} ceramics;

- KM consumption in terms of metal (zirconium) - G _Zr = 1.07-0.50 kg / kg of oxides (TPE + REE).

The disadvantage of the prototype is that it proposed for the curing of the concentrate (TPE + REE) single-phase ceramic is a typical dielectric (insulator) having low conductivity (thermal and electrical conductivity), which can lead to cracking of the ceramic due to the occurring between its center and the wall of the temperature gradient due to heat during radioactive decay (TPE + REE).

In addition, the single-phase ceramic proposed in the prototype has a relatively low capacity (especially bulk) for oxides (TPE + REE) and a relatively high KM consumption per metal (zirconium).

The objective of the present invention is the curing of the concentrate (TPE + REE) without separation into a dense non-porous stable single-phase ceramic with high thermal conductivity, high mass and volumetric capacity for oxides (TPE + REE) and low consumption of CM and dopant.

To solve this problem, instead of zirconyl nitrate (see prototype), it is proposed to use chromium nitrate or it with an alloying additive - copper nitrate in the proportions necessary for the formation of solid solutions M _1-x Cu _x CrO ₃ , where x = (0 -0.20), with a perovskite-type structure, calcination is carried out as described in the prototype, and calcification is compacted in an air atmosphere by hot isostatic pressing (HIP).

Calcination of a mixture of solutions: concentrate (TPE + REE), chromium nitrate and copper nitrate is accompanied by their thermal decomposition to metal oxides:

and the HIP process of calcined in air in the atmosphere — by the interaction between them with the formation of single-phase ceramics - solid solutions of mixed chromates (III) (TPE + REE) and copper (II) with high thermal conductivity:

из-за природной дефектности. Легирование M^IIICrO₃ оксидом двухвалентного металла M^IIO способствует дальнейшему окислению

из-за возникающего дефицита положительных зарядов и приводит к образованию смешанных хроматов (III) РЗЭ и M(II): M $\genfrac{}{}{0ex}{}{\text{III}}{\text{1-x}}$ М $\genfrac{}{}{0ex}{}{\text{II}}{\text{x}}$ СrО₃ с более высокой полупроводниковой проводимостью р-типаIt is known [5] that chromates (III) REE M ^III CrO ₃ , especially LaCrO ₃ , used as starting material for the manufacture of high-temperature electrically conductive and electrode elements, have high p-type semiconductor conductivity, which is due to partial oxidation

due to natural defectiveness. Doping of M ^III CrO ₃ with bivalent metal oxide M ^II O promotes further oxidation

due to the resulting deficit of positive charges and leads to the formation of mixed chromates (III) REE and M (II): M $\genfrac{}{}{0ex}{}{\text{III}}{\text{1-x}}$ M $\genfrac{}{}{0ex}{}{\text{II}}{\text{x}}$ CrO ₃ with higher p-type semiconductor conductivity

From a number of alloying metal (II) oxides (ЩЗМ, Ni, Сu, Zn, Cd, Pb) the most effective, increasing the thermal conductivity of single-phase ceramics $\genfrac{}{}{0ex}{}{\text{III}}{\text{1-x}}$ M $\genfrac{}{}{0ex}{}{\text{II}}{\text{x}}$ CrO ₃ and its sintering by the HIP method - copper (II) oxide.

The rationale for the choice of the claimed molar ratio of components in single-phase ceramics M _{1 x} Cu _x CrO ₃ (x = 0-0.20) is shown in the examples and table. 2.

The advantage of the method compared to the prototype is the ability to cure the concentrate (TPE + REE) without separation into a dense non-porous stable single-phase ceramic - solid solutions M _1-x Cu _x CrO ₃ (x = 0-0.20) with a perovskite-type structure - s high thermal conductivity, high mass and especially volumetric capacity for oxides (TPE + REE), as well as low consumption of matrix metals.

EXAMPLE 1

The method was tested in laboratory conditions on an acidic (1M 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.

To the model concentrate (TPE + REE), a concentrated solution of chromium nitrate (200 g / l in terms of chromium oxide) was added as KM to a molar ratio of M ₂ O ₃ : Cr ₂ O ₃ = 1.

The mixture of solutions was calcined at a temperature of 700 ° C for 0.5 hours in an air atmosphere.

Calcine in a sealed stainless steel container was compacted using the ISU method in the following mode: temperature 1200 ° С, pressure 300 MPa, holding time 1 h.

The obtained undoped HIP ceramics had the following characteristics:

- phase composition: solid solution of chromates (III) REE - MCrO ₃ with a perovskite type structure;

- apparent density ρ _each = 7.06 kg / dm ³ ;

- open porosity e _o = 0.96%,

- thermal conductivity at a temperature of 20 ° C - λ ²⁰ = 11.6 W / (m · K);

- the rate of REE leaching into distilled water at a temperature of 90 ° С - r ⁹⁰ ~ 10 ^-8 g / (cm ² · day);

- mass capacity of REE oxides - E _m = 0.70 kg / kg of ceramic;

- Volumetric capacity for REE oxides - Eν = 4.94 kg / dm ^{3 of} ceramics;

- KM consumption in terms of metal (chromium) G _Cr = 0.31 kg / kg of REE oxides,

EXAMPLE 2

To the model concentrate (TPE + REE) (see Table 1), a concentrated solution of chromium nitrate (200 g / l in terms of chromium oxide) and as a dopant were added a concentrated solution of copper nitrate (200 g / l in terms of on copper oxide (II)) in the ratio necessary for the formation of a solid solution M _0.80 Cu _0.20 CrO ₃ .

The mixture of solutions was calcined, calcine was compacted as described in example 1.

The obtained copper (II) oxide doped HIP ceramics had the following characteristics:

- phase composition: solid solution of mixed chromates (III) REE and Cu (II) - M _0.80 Cu _0.20 CrO ₃ with a perovskite type structure;

- apparent density ρ _kage. = 6.93 kg / dm ³ ;

- open porosity e _o = 0.28%;

- thermal conductivity at a temperature of 20 ° C - λ ²⁰ = 28.7 W / (m · K);

- the rate of REE leaching into distilled water at a temperature of 90 ° C - r ⁹⁰ ~ 10 ^-8 g / (cm ² · day);

- mass capacity for REE oxides - E _m = 0.63 kg / kg of ceramic;

- volumetric capacity for REE oxides - Eν = 4.37 kg / dm ^{3 of} ceramics;

- KM consumption in terms of metal (chromium) G _Cr = 0.38 kg / kg of REE oxides;

- consumption of dopant in terms of metal (copper) G _Cu = 0.09 kg / kg of REE oxides.

The influence of the degree of doping of ceramics M _1-x Cu _x CrO ₃ obtained in the HIP mode specified in Example 1 on its most important characteristics is given in Table 2.

From table 2 it is seen that in the HIP ceramics M _1-x Cu _x CrO ₃ in the range x = 0-0.20:

- thermal conductivity at a temperature of 20 ° C monotonically increases 2.5 times, and at x> 0.20 decreases;

- apparent density decreases by 1.8%, and open porosity by 3.4 times;

- mass and volumetric capacities are reduced by 10 and 12%, respectively, while remaining very high;

- the consumption of matrix metals (chromium + copper) increases by 1.5 times, while remaining quite low.

The phase compositions of the ceramic samples M _1-x Cu _x CrO ₃ (x = 0-0.02) are solid solutions with a perovskite-type structure, and the rates of REE leaching from these samples into distilled water at a temperature of 90 ° С - r ⁹⁰ ~ 10 ^{- 8} g / (cm ² · day).

As can be seen from the examples and table 2, the proposed method allows to cure the concentrate (TPE + REE) without separation into a dense non-porous stable single-phase ceramic M _1-x Cu _x CrO ₃ (x = 0-0,02) with high thermal conductivity λ ²⁰ = 11.6-28.7 W / (m · K), comparable thermal conductivity of metals such as scandium - 15.5; yttrium - 15.8; REE - 10.0-20.9; titanium - 21.9; zirconium - 29.5 W / (m · K).

The proposed single-phase ceramic M _1-x Cu _x CrO ₃ (x = 0-0,02) is superior to single-phase ceramic in the prototype yM ₂ O ₃ · (lY) ZrO ₂ , where y = (0,20-0,35):

- thermal conductivity λ ²⁰ , W / (m · K), not less than 3.4 (x = 0) - 8.4 (x = 0.20) times;

- by mass capacity E _m , where kg M ₂ About ₃ / kg of ceramic:

- minimum (y = 0.35) by 7 (x = 0.20) - 19 (x = 0)%;

- maximum (y = 0.20) by 54 (x = 0.20) - 71 (x = 0)%;

- by volumetric capacity Eν kg M ₂ O ₃ / dm ³ ceramics:

- minimum (y = 0.35) by 12 (x = 0.20) - 33 (x = 0)%;

- maximum (y = 0.20) by 63 (x = 0.20) - 94 (x = 0)%;

and allows to reduce the consumption of matrix metals G _(Cr-Cu) , kg / kg M ₂ About ₃ (chromium and copper compared with zirconium):

- minimum (y = 0.35) by 6 (x = 0.20) - 61 (x = 0)%;

- maximum (y = 0.20) 2.3 (x = 0.20) - 3.5 (x = 0) times.

Sources of information

1. Galkin B.Ya., Yessimantovski V.M., Lazarev L.N., Lyubtsev R.I. 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. Ringwood A.E., Kesson S.E., Reeeve K. D. et. al. SYNROC, ibid, p. 233-334.

4. RF patent No. 20343456, 08/26/92.

5. Compounds of rare earth elements. Komisarova L.N., Pushkina G.Ya., Shatsky V.M. et al. - Moscow: Nauka, 1986, 336 p. (Chemistry of rare elements).

Claims (1)

A method of curing a concentrate of transplutonium and rare-earth elements into ceramics, including its calcination in the presence of a ceramic-forming matrix material and subsequent compaction of calcine by hot pressing, characterized in that a concentrated chromium nitrate solution or a concentrated chromium nitrate solution with a dopant - a concentrated solution are used as a ceramic-forming matrix material copper nitrate in the ratios required for the formation of solid solutions M _1-x Cu _x CrO ₃ , where M is the transplutonium and rare earth elements, x = 0-0.20, and hot pressing of the calcine is carried out by the isostatic method.