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Hydrolytic and thermal stability of magnesium potassium phosphate compound for immobilization of high level waste

  • Sergey E. Vinokurov
  • Svetlana A. Kulikova
  • Boris F. Myasoedov
Article
  • 44 Downloads

Abstract

The samples of the magnesium potassium phosphate (MPP) compound have been synthesized during solidification of high level waste (HLW) surrogate. The compound consists of a main phosphate phases Mg1.1Na0.35K0.45PO4 × (4–6)H2O and MgCs0.5Na0.2K0.3PO4 × (5–6)H2O. Differential leaching rates of 239Pu, 152Eu and 90Sr from the MPP compound after heat treatment (450 °C) are 7.8 × 10−9; 1.7 × 10−7 and 8.8 × 10−6 g cm−2 day−1, respectively. The coefficient of thermal expansion of MPP compound—(11.6 ± 0.3) × 10−6 °C−1; coefficient of thermal conductivity averages 0.5 W m−1 K−1. The properties of MPP compound meet the regulatory requirements for solidified HLW.

Keywords

Magnesium potassium phosphate compound High level waste Immobilization Thermal stability Leaching rate Leaching mechanism 

Introduction

HLW generated from the reprocessing of spent nuclear fuel (SNF) from reactor plants must be transferred to a stable solidified form suitable for long-term and environmentally safe disposal. The MPP compound is a promising material for solidification of liquid radioactive waste (LRW) [1, 2, 3, 4, 5]. Earlier in Ref. [1] we showed the efficiency of solidification of high salt liquid intermediate level waste (ILW). At the same time, higher requirements for hydrolytic and thermal stability for matrices for HLW immobilization are demanded for the reason of possible significant heating (up to 300–400 °C) of the compound due to heat release of radionuclides of HLW. The present article is concerned with the study of the possibility of solidification of HLW in the MPP compound.

Experimental

Chemicals and procedures

All experiments were performed in a glove box at ambient atmospheric conditions. The chemicals used in the experiments were of no less than chemically pure grade. Samples of MPP compound were obtained after solidification of the surrogate of industrial HLW, obtained after the reprocessing of SNF of the water–water energetic reactor (WWER-1000). Preliminary preparation of the HLW surrogate and binding components (MgO, KH2PO4) was previously reported in Ref. [1]. The chemical and radionuclide composition of the prepared HLW surrogate is presented in the Table 1. The density of HLW surrogate is 1280 g l−1, pH 7.0 ± 0.1, salt content—about 484 g l−1.
Table 1

Characteristics of the HLW surrogate (uncertainties for data—3%)

Specific activity of nuclides*, Bq l−1

Metal content, g l−1

239Pu—2.6 × 108

Na—83.9; Sr—3.0; Zr—5.6; Mo—0.8;

152Eu—1.7 × 108

Pd—4.1; Cs—7.4; Ba—1.2; Nd—28.2;

90Sr—5.6 × 107

Fe—0.8; Cr—2.3; Ni—0.4; U—2.1

*Nuclides were added to the HLW surrogate individually

Samples of the MPP compound were obtained at room temperature in accordance with the following ratio of compound components, wt%: MgO: KH2PO4: H3BO3: HLW surrogate = 11.0: 32.9: 1.2: 31.9. Earlier in Ref. [2], we found that the wollastonite (CaSiO3) adding into MPP compound leads to an increase in mechanical and thermal stability. Therefore, wollastonite (FW-200, Nordkalk) with a particle size of 0.07–0.16 mm was used as the mineral filler of the MPP compound. The filling of the obtained samples by the salts of HLW surrogate was 10.0 wt%, and by wollastonite—23 wt%.

As a result, cubic samples of MPP compound with dimensions of 2 cm × 2 cm × 2 cm were prepared after solidification of the HLW surrogate. The samples were kept for 15 days to attain compressive strength at ambient atmospheric conditions.

Methods

The density of MPP compounds was determined by measuring and weighing the samples. The structure of the obtained samples of MPP compound was studied by scanning electron microscopy (SEM) (LEOSupra 50 VP, Carl Zeiss) with X-ray spectral microanalysis (X-MAX 80, Oxford Inst.).

The hydrolytic stability of the compound samples was determined according to the semi-dynamic test GOST R 52126-2003 [6]. Test conditions: 23 ± 2 °C, monolithic sample, bidistilled water (pH 6.2 ± 0.1), periodic replacement of the leaching agent. The radionuclide content in solutions after leaching was determined by radiometric methods: 239Pu—alpha-spectrometry (Alpha Analyst, Canberra); 152Eu—gamma-spectrometry (multi-channel gamma spectrometer Canberra, high-purity germanium detector); 90Sr—liquid-scintillation spectrometry (SKS-07P-B11(10), GreenStar, scintillator Optiphase Hi Safe III). The calculation of differential (LRdif) leaching rate of MPP compound components was given in Ref. [1]. The leaching mechanism of the compound components from the samples was evaluated according to the model [7], described by the linear relationship of log (Bi) from log (t), where Bi—the total yield of the element from the compound during contact with water, mg m−2; t—the contact time, days. The calculate procedure of Bi is given in [1, 8]. The following mechanisms of element leaching from the compound correspond to various values of the slope in this equation: > 0.65—surface dissolution; 0.35–0.65—diffusion transport; < 0.35—surface wash off (or a depletion if it is found in the middle or at the end of the test) [8].

Thermal stability of the obtained compound was studied after heat treatment at 450 °C according to NP-019-15 [9]. Heat treatment of the samples was carried out in the muffle furnace SNOL 30/1300 (UMEGA, Lithuania) during 4 h, heating rate 7 °C min−1, cooling was a natural.

The mechanical strength of the compounds was determined using a universal test machine AG–X Plus (Shimadzu, Japan). The thermophysical characteristics of the compound samples were determined. The coefficient of thermal expansion (CTE) of the samples was determined by dilatometry using a horizontal dilatometer (DIL 402 C, Netzsch, Germany) in the polythermal conditions up to 800 °C (heating rate—5 °C min−1). The coefficient of thermal conductivity (CTC) was determined using of the laser flash unit (LFA 457/2/G MicroFlash, Netzsch, Germany) in the temperature range from 20 up to 500 °C.

Results and discussion

The density of the obtained MPP compound samples was 2.00 ± 0.05 g cm−3.

According to the SEM data, the MPP compound samples obtained after the solidification of the HLW surrogate consist of a basic phosphate phase of the average composition Mg1.1Na0.35K0.45PO4 × (4–6)H2O (Phase #1 in Fig. 1a), which is an analog of the natural mineral of the K-struvite [10]. At the same time, it was shown that separate inclusions of the compound are enriched in cesium and have an average composition of MgCs0.5Na0.2K0.3PO4 × (5–6)H2O (Phase #2 in Fig. 1a). In addition, the following phases are uniquely identified: wollastonite CaSiO3, which was added as a mineral filler (Phase #3 in Fig. 1a); KNO3 (Phase #4 in Fig. 1a), formed during the replacement of potassium with alkali metals of the HLW surrogate and previously identified at the immobilization of high salt LRW [1].
Fig. 1

SEM micrograph of the MPP compound samples before (a) and after heat treatment at 450 °C (b)

The structure of the MPP compounds changes after heat treatment at 450 °C (Fig. 1b). The normalized oxygen content in the investigated sections of the samples decreases, which indicates the partial removal of the crystallization water. Thus, the composition of the main phase of the heat treated compounds (Phase #5 in Fig. 1b) corresponds to Mg1.1Na0.35K0.45PO4 × (0.2–0.5)H2O. The same effect was observed for particles enriched in cesium: the obtained samples have the composition MgCs0.5Na0.2K0.3PO4 × (1.5–2.0)H2O (Phase #6 in Fig. 1b). It is noted that wollastonite (Phase #3 in Fig. 1b) remains unchanged, while KNO3 phase is not detected, which probably decomposed at 450 °C (KNO3 decomposition temperature > 400 °C [11]).

The hydrolytic stability to radionuclides leaching from the MPP compound samples was studied, including after heat treatment (at 450 °C). Kinetic curves of the dependence of the differential leaching rate of radionuclides from the MPP compound samples are shown in Fig. 2, and the mechanism of their leaching are present in Fig. 3 and in Table 2.
Fig. 2

Kinetic curves of the dependence of the differential leaching rate of radionuclide from the MPP compound samples obtained (a) and after their heat treatment at 450 °C (b)

Fig. 3

Logarithmic dependence of the yield of immobilized radionuclide from the MPP compound samples obtained (a) and after their heat treatment at 450 °C (b)

Table 2

The leaching mechanism of the radionuclides from the MPP compound with immobilized HLW surrogate

Components of the MPP compound

Contact time of the samples with water, days

Lines slope (Fig. 3)

Leaching mechanism

MPP compound

239Pu

0–15

0.28

Wash off

15–90

−3.07

Depletion

152Eu

0–15

0.61

Diffusion

15–90

−1.27

Depletion

90Sr

0–90

0.59

Diffusion

MPP compound after heat treatment at 450 °C

239Pu

0–90

−0.61

Wash off and depletion

152Eu

0–90

0.31

Wash off and depletion

90Sr

0–90

0.60

Diffusion

The values of LRdif of radionuclides from the samples on the 90th day of contact with water (Fig. 2): for 239Pu, 152Eu and 90Sr—1.0 × 10−9;1.0 × 10−8 and 9.6 × 10−7 g cm−2 day−1, respectively. The reported values of LRdif correspond to the requirement for glass-like compound for HLW immobilization [9] (leaching rate of 239Pu and 90Sr—≤ 1.0 × 10−7 and 1.0 × 10−6 g cm−2 day−1, respectively). It has been established that the radionuclides leaching rate increases by almost an order of magnitude after heat treatment of compounds at 450 °C, at the same time, it below than the regulatory requirements: LRdif of 239Pu, 152Eu and 90Sr—7.8 × 10−9; 1.7 × 10−7 and 8.8 × 10−6 g cm−2 day−1, respectively.

The leaching mechanism of 239Pu and 152Eu from the MPP compound samples in the first 15 days is due to surface wash off (slope is 0.28) and diffusion (0.61), respectively, and in the next 75 dayssurface depletion (− 3.07 and − 1.27, respectively). The heat treatment of the compound samples does not substantially affect the mechanism of leaching of radionuclides from the samples: the leaching of 239Pu and 152Eu occurs due to surface wash off (− 0.61 and 0.31, respectively). Since europium is an actinides (III) analogue, we could assume that a slow-soluble mixed orthophosphate (Eu, Am)PO4, which is an analog of natural minerals (monazite, phabdophane [2]) is formed. The behavior of 90Sr at the leaching of compound samples both before and after heat treatment is due to diffusion from the inner layers of the compounds (0.59 and 0.60, respectively).

The compressive strength of the compound samples was 33.0 ± 3.0 MPa, after heat treatment of the samples at 450 °C, it decreases to 10.5 ± 0.5 MPa, while satisfying the requirements for vitrified HLW (9–13 MPa) [9].

Thermophysical properties (CTE, CTC) of materials for immobilization of HLW should facilitate the heat removal during storage of solidified waste to avoid excessive heating of the compound. CTE of samples at heating in the range (250–550) °C is 11.3 × 10−6 °C−1, and at cooling in the range (550–250) °C is 11.9 × 10−6 °C−1 (Fig. 4), which corresponds to the regulatory requirements for a glass-like compound (8–15) × 10−6 °C−1 [9].
Fig. 4

Thermal expansion of the MPP compound samples with immobilized HLW surrogate

The CTC of MPP compound in the interval (20–500) °C averaged 0.5 W m−1 K−1, which is lower than the required normalized values for a glass-like compound (0.7–1.6) × 10−6 W m−1·K−1. However, CTC can be increased by introducing substances with a high thermal conductivity, for example graphite (CTC > 200 W m−1 K−1).

Conclusions

As a result of the performed studies, the high hydrolytic and thermal stability of the MPP compound is shown, which allows it to be considered as a promising material for HLW solidification. At the same time, this technology will be energy-saving, not requiring expensive high-temperature electric furnaces, the liquidation of which at the end of their service life is a major radioecological problem that has not yet been resolved. We will test the MPP compound for the immobilization of real HLW to assess the prospects for its industrial use.

Notes

Acknowledgements

The study was carried out through the Russian Science Foundation Grant (Project No 16-13-10539).

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2018

Authors and Affiliations

  • Sergey E. Vinokurov
    • 1
  • Svetlana A. Kulikova
    • 1
  • Boris F. Myasoedov
    • 1
  1. 1.Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of SciencesMoscowRussia

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