chemical endurance of polymeric compounds containing radioactively contaminated spent vacuum oil

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Page 1: Chemical endurance of polymeric compounds containing radioactively contaminated spent vacuum oil

ISSN 1066-3622, Radiochemistry, 2013, Vol. 55, No. 4, pp. 450–453. © Pleiades Publishing, Inc., 2013. Original Russian Text © T.S. Volkova, I.G. Tananaev, V.S. Volkov, O.M. Slyunchev, 2013, published in Radiokhimiya, 2013, Vol. 55, No. 4, pp. 374–377.

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Chemical Endurance of Polymeric Compounds Containing Radioactively Contaminated Spent Vacuum Oil

T. S. Volkova*, I. G. Tananaev, V. S. Volkov, and O. M. Slyunchev

Mayak Production Association, pr. Lenina 31, Ozersk, Chelyabinsk oblast, 456780 Russia; * e-mail: [email protected]

Received December 5, 2012

Abstract—Oil–absorbent suspension was incorporated in a matrix material based on epoxy–4,4'-isoprop-ylidenediphenol resin under laboratory conditions. The following powder materials were used as absorbents: magnesium and calcium oxides, Taunit, Aerosil, high-reactivity carbon mixture, and N910 synthetic polymer. Experiments were performed with radioactively contaminated spent VM-4 vacuum oil from chemical-metallurgical production. The weight fraction of oil in the compounds was varied from 19 to 39%. The chemi-cal endurance of the compound samples under the conditions of prolonged keeping in aqueous medium was examined. The results obtained show that all the polymeric samples ensure sufficiently strong fixation of oil and α-emitting nuclides, preventing their release into the leaching medium. The degree of leaching of oil and α-emitting nuclides from all the examined compounds in 90 days did not exceed 0.23 and 0.10%, respectively.

Keywords: immobilization, spent vacuum oil, absorbent, oil–absorbent suspension, matrix material

Management of spent technical oils (STOs) is an urgent problem at the Mayak Production Association. Immobilization of the accumulated wastes in matrices of various types using the sorption method is consid-ered as one of the most promising solutions of this problem. The suggested approach involves solidifica-tion of oil in two steps: preparation of a mixture of an absorbent with the sorbed oil and subsequent mixing with a host material to obtain a chemically durable compound.

Previously [1] we demonstrated the efficiency of the sorption method for STO solidification. For exam-ple, without an absorbent up to 16% oil can be incor-porated into a polymeric matrix, with acceptable char-acteristics of the compounds obtained. At the oil weight fraction in the composite exceeding 16%, the system undergoes practically full segregation, with the formation of a separate oil phase.

Diverse powder materials can be used as oil absor-bents. In our previous studies [1–5], we tested for this purpose magnesium and calcium oxides, Aerosil, Taunit, high-reactivity carbon mixture (HRCM), and N910 syn-thetic polymer [6]. The compounds obtained met the requirements of GOST (State Standard) R 51 883–2002 for compression strength (no less than 50 kg cm–2).

In this study we tested the chemical endurance of polymeric compounds containing radioactively con-taminated oil under the conditions of prolonged keep-ing in water.

DOI: 10.1134/S1066362213040188

EXPERIMENTAL

In the first step of our study, we measured the ra-dioactive contamination of STO by the procedure in-volving incineration to obtain ash residue.

As experimental object we used spent VM-4 vac-uum oil contaminated with Pu and Am radionuclides. The specific activity of α-emitters in the oil was 1.5 × 106, and that of β-emitters, 3.5 × 105 Bq kg–1.

Then, STO was solidified in two steps. First, based on the previously determined [3–5] absorpion ability (the oil weight absorbed by unit weight of a material before the onset of formation of separate oil phase), we prepared an oil–absorbent mixture with a definite weight ratio of the components. It should be noted that the oil in this mixture was fully sorbed by the absor-bent, i.e., there was no individual oil phase.

The following powder materials were used as oil absorbents (the amount of VM-4 vacuum oil that can be absorbed by the material, g g–1, is given in parenthe-

Page 2: Chemical endurance of polymeric compounds containing radioactively contaminated spent vacuum oil

CHEMICAL ENDURANCE OF POLYMERIC COMPOUNDS 451

RADIOCHEMISTRY Vol. 55 No. 4 2013

RESULTS AND DISCUSSION

ses): HRCM (20.0), MgO (4.0), Taunit (3.0), CaO (1.5), Aerosil (12.5), N910 synthetic polymer (6.0).

The second step of the process consisted in solidifi-cation of the absorbent with the sorbed oil. As a host material we used a mixture of ED-20 epoxy–4,4'-iso-propylidenediphenol resin and an amine curing agent, polyethylenepolyamine, in 10 : 1 weight ratio. The waste incorporation into the polymeric matrix was per-formed by a sequential procedure involving stepwise mixing of the absorbent containing the sorbed oil with the resin, followed by mixing with the curing agent. The resulting compound was thoroughly mixed and allowed to harden in a mold in air for 14 days.

The final step of the study was testing of the poly-meric samples obtained for chemical endurance in ac-cordance with GOST (State Standard) 52126–2003 [7]. Samples of size 2 × 2 × 2 cm (open geometric sur-face area approximately 24 cm2) were placed in cruci-bles, and 100 cm3 of distilled water was added. Tests were performed at 25°С. The contact solution (leaching medium) was replaced from time to time with fresh water. The leaching medium after the re-placement was acidified with 1 cm3 of concentrated HNO3 to exclude the radionuclide sorption on the flask walls. The total test time was 90 days.

In the leaching medium, we monitored the oil con-tent and the total activity of α- and β-emitting nuclides. The oil concentration was determined by IR spec-trometry. The specific activity of the sum of α- and β-emitting nuclides was determined by direct measure-ment of the emitted flux, taking into account the weight of the substance in the source. Measurements were performed with a Tennelec XLB automatic radi-ometer (Canberra). From the specific activities of the leaching media, we calculated the rate and degree of component leaching.

The leaching rate R, g cm–2 day–1, was calculated by the formula

R = mΔt/(m0,ΔtSsΔt), (1)

where mΔt is the activity of the component washed out from the compound in time interval Δt, g (Bq); m0,Δt, initial activity of the component in the compound for the given time interval Δt, g g–1 (Bq g–1); Ss, geometric surface area of the sample, cm2; and Δt, time interval between replacements of the leaching medium, days.

The degree of leaching of the components from the sample S, %, was calculated by the formula

S = (m/m0) × 100, (2)

where m is the total amount of a component washed out from the sample by nth day, g (Bq); m0, initial amount of the component in the compound, g (Bq).

Following the above-described procedure, we in-corporated the radioactively contaminated spent vac-uum oil into the polymeric host material. The composi-tions of the compounds obtained are given in the table. The weight fraction of oil in the samples was varied from 19 to 39%, which is considerably higher than in STO solidification without absorbent (16%). The abso-lute value of the initial activity of the polymeric sam-ples (without taking into account their weight) varied from 2600 to 5000 Bq for α-emitters and from 600 to 1200 Bq for β-emitters.

It should be noted that the samples after 90-day leaching had no visible signs of failure (dissolution), cracks, or fractures.

The data on the rate and degree of leaching of oil and of α- and β-emitting nuclides from the compounds in the course of the standard 90-day test are shown in Figs. 1–6, and the mean leaching rates of the compo-nents are given in the table.

As can be seen, all the polymeric samples ensure

Compositions and characteristics of polymeric compounds incorporating STO Compound

no. Absorbent Oil : absorbent, g g–1

Oil weight fraction, %

Mean leaching rate in 90 days, g cm–2 day–1 Density, g cm–3 oil α-emitters β-emitters

1 HRCM 20.0 39.0 7.08 × 10–7 4.39 × 10–7 4.06 × 10–5 0.99 2 N910 5.0 35.0 4.17 × 10–7 3.26 × 10–7 1.28 × 10–5 0.97 3 MgO 3.0 34.0 8.18 × 10–7 1.03 × 10–7 4.79 × 10–5 0.97 4 Taunit 3.0 24.0 8.97 × 10–7 1.03 × 10–7 5.26 × 10–5 1.03 5 Aerosil 9.0 24.0 4.17 × 10–7 1.48 × 10–7 1.35 × 10–5 1.03 6 CaO 1.5 19.0 1.05 × 10–6 1.09 × 10–7 6.65 × 10–5 1.05

Page 3: Chemical endurance of polymeric compounds containing radioactively contaminated spent vacuum oil

VOLKOVA et al. 452

RADIOCHEMISTRY Vol. 55 No. 4 2013

Fig. 1. Degree of oil leaching S from polymeric compounds as a function of time. (1) HRCM, (2) MgO, (3) Taunit, (4) CaO, (5) Aerosil, and (6) N910.

sufficiently strong fixation of oil, preventing its release into the leaching medium. The degree of leaching in 90 days from all the compounds tested did not exceed 0.23%, and the leaching rate was in the range from 4.8 × 10–7 to 2.2 × 10–4 g cm–2 day–1.

Calcium oxide exhibits the weahest fixation ability;

Fig. 3. Degree of leaching of α-emitting nuclides S from polymeric compounds as a function of time. (1) HRCM, (2) MgO, (3) Taunit, (4) CaO, (5) N910, and (6) Aerosil.

Fig. 2. Rate of oil leaching R from polymeric compounds as a function of time. (1) HRCM, (2) Taunit, (3) CaO, (4) MgO, (5) Aerosil, and (6) N910.

Fig. 4. Rate of leaching of α-emitting nuclides R from polymeric compounds as a function of time. (1) Taunit, (2) CaO, (3) N910, (4) HRCM, (5) MgO, and (6) Aerosil.

Fig. 5. Degree of leaching of β-emitting nuclides S from polymeric compounds as a function of time. (1) HRCM, (2) MgO, (3) Taunit, (4) CaO, (5) N910, and (6) Aerosil.

Fig. 6. Rate of leaching of β-emitting nuclides R from polymeric compounds as a function of time. (1) MgO, (2) CaO, (3) Aerosil, (4) HRCM, (5) Taunit, and (6) N910.

the degree of oil leaching from samples of compound 6 was 0.23 %. Taunit is inferior in retention properties to magnesium oxide, Aerosil, HRCM, and N910.

The strongest fixation of oil was attained for com-pound 1 with HRCM as oil absorbent.

α-Emitting nuclides are also retained by the absorb-

Page 4: Chemical endurance of polymeric compounds containing radioactively contaminated spent vacuum oil

ACKNOWLEDGMENTS

REFERENCES

CHEMICAL ENDURANCE OF POLYMERIC COMPOUNDS 453

RADIOCHEMISTRY Vol. 55 No. 4 2013

ing materials sufficiently strongly. The highest degree of leaching of α-emitters was observed with compound 1 containing the largest amount of oil (39.0 wt %).

The oil content of compounds 4 and 5 is equal, but the degree of leaching of α-emitters from samples of compound 4 was appoximately 1.5 times lower than from those of compound 5. Thus, Taunit used as absor-bent exhibits better retaining power than Aerosil.

Taunit has also certain advantage over CaO but is inferior in sorption properties to MgO (degree of leaching of α-emitters from samples of compounds 3, 4, and 6 was 0.022%).

Comparison of the retaining power of MgO and N910 polymer shows that MgO is preferable (the de-gree of leaching from samples of compounds 2 and 3 was 0.073 and 0.022%, respectively).

The highest degree of leaching of β-emitting nu-clides (14.5%) is observed from samples of compound 6 with CaO used as absorbent. This compound is also characterized by the lowest weight fraction of oil, compared to the other compounds studied.

In the ability to retain β-emitting nuclides, Taunit is inferior to Aerosil, magnesium oxide, HRCM, and N910.

The leaching rate of β-emitters from the samples studied varies from 9.9 × 10–3 to 3.5 × 10–5 g cm–2 day–1. The level prescribed by regulations is <10–3 g cm–2 day–1 for 137Cs and 90Sr.

Thus, MgO, HRCM, and N910 exhibit the highest ability to retain radionuclides. The indubitable advan-tage of MgO is its ready availability and low cost.

1. Volkova, T.S., Tananaev, I.G., and Slyunchev, O.M., Vopr. Radiats. Bezopasn., 2012, no. 4, pp. 3–9.

2. Volkova, T.S. and Slyunchev, O.M., in VI otraslevaya nauchno-prakticheskaya konferentsiya molodykh spetsia-listov i aspirantov “Molodezh’ YaTTs: nauka, proizvod-stvo, ekologicheskaya bezopasnost’”: Sbornik dokladov (VI Branch Scientific and Practical Conf. of Young Spe-cialists and Postgraduate Students “Youth for Nuclear Fuel Cycle: Science, Production, Environmental Safety”: Coll. of Papers), Zheleznogorsk, Krasnoyarsk krai: Gorno-Khi-micheskii Kombinat, November 8–11, 2011, pp. 59–61.

3. Volkova, T.S., Slyunchev, O.M., and Kozlov, P.V., Khim. Tekhnol., 2012, vol. 13, no. 7, pp. 441–447.

4. Volkova, T.S., Tananaev, I.G., Slyunchev, O.M., and Koz-lov, P.V., Khim. Tekhnol., 2012, vol. 13, no. 8, pp. 507–511.

5. Volkova, T.S., Slyunchev, O.M., and Volkov, V.S., Abstracts of Papers, Chetvertaya konferentsiya molo-dykh uchenykh i spetsialistov s elementami nauchnoi shkoly “Raduga-2011: Obrashchenie s radioaktivnymi otkhodami. Problemy i resheniya” (Fourth Conf. of Young Scientists and Specialists with Elements of Sci-entific School “Rainbow-2011: Radioactive Waste Management. Problems and Solutions”), Sergiev Posad, October 5–7, 2011, pp. 28–29.

6. Nochar Petrobond Absorbent Polymer Tritiated Oil So-lidification. Deactivation and Decommissioning Focus Area, DOE/EM-0598, US Department of Energy, Office of Environmental Management, 2001.

7. GOST (State Standard) R 52 126–2003: Radioactive Wastes. Determination of the Chemical Endurance of Solidified High-Level Wastes by Prolonged Leaching.

The study was supported by the Russian Founda-tion for Basic Research (project no. 12-03-31664).