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Russian Metallurgy (Metally)

, Volume 2018, Issue 8, pp 716–721 | Cite as

Thermodynamic Analysis of the Oxidation of Radioactive Graphite in the Na2CO3–K2CO3–Sb2O3 Melt in an Argon Atmosphere

  • N. M. BarbinEmail author
  • I. A. Sidash
  • D. I. Terent’ev
  • S. G. Alekseev
Article
  • 6 Downloads

Abstract—Graphite is used as a neutron moderator and reflector. Moreover, graphite can be used as sealants and bearings in reactors. The graphite mass in a reactor is 1–2 ths t. A large amount of radioactive graphite wastes forms when graphite reactors are taken out of service. The existing methods of processing radioactive graphite are based on its isolation from the environment. These methods cannot substantially decrease the volume of radioactive graphite wastes. As a result, the processing of irradiated reactor graphite by oxidation in molten salts can be considered as an alternative reclamation method, which can decrease the volume of radioactive graphite wastes. The oxidation of radioactive graphite in the Na2CO3–K2CO3–Sb2O3 melt in an argon atmosphere is thermodynamically simulated using the TERRA software package. The data obtained are used to analyze the distribution of elements over condensed and gas phases. Heating of the system to 1073 K is found to cause the evaporation of the condensed compounds of antimony and cesium. Upon heating to 1273 K, the condensed compounds of potassium, sodium, and chlorine evaporate. Heating to 1373 K leads to the evaporation of the condensed compounds of nickel. Heating to 1673 K brings about the evaporation of the condensed compounds of uranium, calcium, and strontium. Heating to 1773 K causes the evaporation of the condensed compounds of plutonium, beryllium, americium, and europium. At temperatures above 1773 K, only a vapor–gas phase exists in the system.

Keywords:

thermodynamic simulation radioactive graphite radionuclides heating oxidation melts carbonates 

INTRODUCTION

Graphite is used as a neutron moderator and reflector. Moreover, graphite can be used as sealants and bearings in reactors. The graphite mass in a reactor is 1–2 ths t.

After operation in the nuclear industry, graphite is contaminated with radionuclides; as a result of the loss of sealing in fuel elements, the graphite packing can be contaminated with nuclear fuel and the products of its fission [1].

A large amount of radioactive graphite wastes forms when graphite reactors are taken out of service. The existing methods of processing radioactive graphite are based on its isolation from the environment. These methods cannot substantially decrease the volume of radioactive graphite wastes. As a result, the processing of irradiated reactor graphite by oxidation in molten salts can be considered as an alternative reclamation method, which can decrease the volume of radioactive graphite wastes.

Data on the behavior of the radionuclides that are present in irradiated reactor graphite during oxidation in molten salts using antimony oxide are very scarce.

EXPERIMENTAL

We used the TERRA software package, which is intended to calculate the phase compositions and the thermodynamic and transport properties of arbitrary systems [2, 3], to perform a thermodynamic analysis. This software package is successfully used in physics and chemistry [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

Table 1 gives the initial system composition used for a thermodynamic simulation, which was carried out at an initial pressure of 1 atm, an initial temperature of 273 K, and a final temperature of 3273 K. The temperature step was 100 K.

Table 1.  

Initial system composition

Phase

Phase composition

Content, wt %

Gas

Ar

89.29

Condensed

Sb2O3

2.68

Salt

K2CO3

2.68

 

Na2CO3

2.68

Radioactive graphite

C

2.68

U

2.14 × 10–4

Cl

3.5 × 10–5

Ca

5 × 10–6

Ni

1.52 × 10–7

Cs

6.25 × 10–8

Pu

1.34 × 10–6

Be

2.23 × 10–7

Am

1.61 × 10–7

Sr

1.79 × 10–8

Eu

1.96 × 10–7

RESULTS AND DISCUSSION

Figures 1–7 show the element balances in the radioactive graphite–Na2CO3–K2CO3–Sb2O3 system in an argon atmosphere.

Fig. 1.

Distributions of (a) carbon and (b) antimony over phases upon heating.

Figure 1a shows the carbon distribution over equilibrium phases. In the temperature range 273–473 K, the condensed C content decreases to ~78% because of an increase in the gas CO2 content to ~5%, and condensed Na2CO3 (~10%) and K2CO3 (~7%) are also present. An increase in the temperature to 1273 K causes a decrease in the condensed C (~40%), Na2CO3, and K2CO3 contents because of an increase in the gas CO content to ~60%. In the range 1273–3273 K, the contents of gaseous CO and condensed C do not change.

The antimony distribution over equilibrium phases is shown in Fig. 1b. In the range 273–773 K, the condensed Sb2O3 oxide content decreases because of an increase in the metallic antimony content to ~100%. As the temperature increases to 1073 K, the contents of the Sb4 and Sb2 vapors increase to ~95 and ~5% because of a decrease in the condensed Sb2O3 content. As the temperature increases to 1573 K, the contents of the Sb2 and Sb vapors increase to ~82 and ~6%, respectively, because of a decrease in the contents of the Sb4 and Sb3 vapors to ~5 and ~7%, respectively. A further increase in the temperature to 3273 K results in an increase in the antimony vapor content to ~100% because of a decrease in the Sb2 vapor content.

Figure 2a shows the potassium distribution over equilibrium phases. In the temperature range 273–973 K, the entire potassium is in the form of condensed K2CO3 (~100%). As the temperature increases to 1273 K, the K vapor content increases to ~100% due to a decrease in the condensed K2CO3 content. An increase in the temperature to 3273 K leads to an increase in the ion K+ content to ~5% because of a decrease in the K vapor to ~95%.

Fig. 2.

Distributions of (a) potassium and (b) sodium over phases upon heating.

Figure 2b shows the sodium distribution over equilibrium phases. In the temperature range 273–973 K, almost the entire sodium is in the form of condensed Na2CO3 (~100%). As the temperature increases to 1273 K, the Na vapor content increases to ~100% because of a decrease in the condensed Na2CO3 content. In the range 1273–3273 K, sodium is in the form of Na vapor (~100%).

Figure 3a shows the uranium distribution over equilibrium phases. In the range 273–1173 K, uranium is in the form of condensed UO2 oxide (~100%). An increase in the temperature to 1673 K leads to an increase in the \({\text{UO}}_{3}^{ - }\) ion content (~100%) because of a decrease in the condensed UO2 content. An increase in the temperature to 2273 K results in an icosahedron in the condensed U2C3, UO, and U contents to ~61, ~23, and ~4%, respectively, due to a decrease in the \({\text{UO}}_{3}^{ - }\) ion content to ~12%. An increase in the temperature to 2473 K leads to an increase in the condensed UO and U contents to ~58 and ~27%, respectively, due to a decrease in the ion \({\text{UO}}_{3}^{ - }\) content and the condensed U2C3 content (~15%). As a result of an increase in the temperature to 3273 K, the condensed U content increases to ~97% because of a decrease in the condensed UO content to ~3%.

Fig. 3.

Distributions of (a) uranium and (b) chlorine over phases upon heating.

Figure 3b shows the chlorine distribution over equilibrium phases. In the range 273–1073 K, the condensed NaCl and the KCl vapor contents increase to ~17 and ~4% because of a decrease in the condensed KCl content to ~79%. An increase in the temperature to 1273 K results in a decrease in the condensed KCl and NaCl contents because of an increase in the KCl and NaCl vapor contents to ~81 and ~19%, respectively. An increase in the temperature to 2773 K leads to a decrease in the KCl vapor content to ~64% due to an increase in the NaCl and Cl vapor contents to ~31 and ~5%, respectively. As a result of an increase in the temperature to 3273 K, NaCl and KCl vapor contents decrease to ~21 and ~37% because of an increase in the Cl vapor content to ~37% and the Cl ion content to ~5%.

Figure 4a shows the calcium distribution over equilibrium phases. In the range 273–673 K, almost the entire calcium is in the form of condensed CaCO3. An increase in the temperature to 973 K leads to an increase in the condensed CaO content to ~100% because of a decrease in the condensed CaCO3 content. When the temperature increases to 1673 K, the Ca vapor content increases to ~100% due to a decrease in the condensed CaO content. A further increase in the temperature to 3273 K does not lead to changes: calcium is present in the form of Ca vapor.

Fig. 4.

Distributions of (a) calcium and (b) plutonium over phases upon heating.

Figure 4b shows the plutonium distribution over equilibrium phases. In the temperature range 273–1273 K, almost the entire plutonium is in the form of condensed PuO2 (~100%). As a result of an increase in the temperature to 1673 K, the PuO vapor content increases to ~20% and the condensed Pu2O3 content increases to ~65% because of a decrease in the condensed PuO2 content to ~15%. An increase in the temperature to 1773 K leads to an increase in the PuO and Pu vapor contents to ~96 and ~4%, respectively, due to a decrease in the condensed PuO2 and Pu2O3 contents. As a result of a further increase in the temperature to 3273 K, the Pu vapor content increases to ~100% because of a decrease in the PuO vapor content.

Figure 5a shows the beryllium distribution over equilibrium phases. In the temperature range 373–1473 K, almost the entire beryllium is in the form of condensed BeO (~99%). An increase in the temperature to 2573 K leads to an increase in the BeO and Be vapor contents to ~87 and ~11% due to a decrease in the Be3O3 and Be2O2 vapor contents to ~1 and ~1%, respectively. As a result of a further increase in the temperature to 3273 K, the Be vapor content decreases to ~8% and the BeC2 vapor content increases to ~92%.

Fig. 5.

Distributions of (a) beryllium and (b) nickel over phases upon heating.

Figure 5b shows the nickel distribution over equilibrium phases. An increase in the temperature to 873 K leads to an increase in the metallic Ni content to ~100% due to a decrease in the condensed NiO content. As a result of an increase in the temperature to 773 K, the condensed NiO content decreases to ~1% and the metallic Ni content increases to ~99%. In the range 773–973 K, nickel is in the form of metallic Ni (~100%). When the temperature increases to 1373 K, the condensed Ni content decreases because of an increase in the Ni vapor content to ~100%. A further increase in the temperature to 3273 K does not cause changes: nickel is present as Ni vapor (~100%).

Figure 6a shows the cesium distribution over equilibrium phases. In the temperature range 273–873 K, the condensed Cs2CO3 and Cs vapor contents increase to ~50 and ~3%, respectively, due to a decrease in the condensed CsCl content to ~47%. An increase in the temperature to 1073 K leads to a decrease in the condensed Cs2CO3 and CsCl contents and to an increase in the Cs vapor content to ~100%. Upon a further increase in the temperature to 3273 K, the Cs vapor content decreases to ~80% and the Cs+ ion content increases to ~20%.

Fig. 6.

Distributions of (a) cesium and (b) americium over phases upon heating.

Figure 6b shows the americium distribution over equilibrium phases. In the range 273–1073 K, almost the entire americium is in the form of condensed AmO2. An increase in the temperature to 1573 K leads to an increase in the condensed Am2O3 content to ~100% because of a decrease in the condensed AmO2 content. Upon a further increase in the temperature to 1773 K, the condensed Am content increases to ~100% because of a decrease in the condensed Am2O3 content. In the range 1773–3273 K, the entire americium is in the form of Am vapor.

Figure 7a shows the europium distribution over equilibrium phases. In the range 273–473 K, almost the entire europium is in the form of condensed Eu2O3. An increase in the temperature to 1073 K leads to an increase in the condensed EuO content to ~100% because of a decrease in the condensed Eu2O3 content. When the temperature increases to 1673 K, the Eu vapor content increases to ~100% due to a decrease in the condensed Eu2O3 and EuO contents. A further increase in the temperature to 3273 K does not cause changes: europium is in the form of Eu vapor (~100%).

Fig. 7.

Distributions of (a) europium and (b) strontium over phases upon heating.

Figure 7b shows the strontium distribution over equilibrium phases. In the range 273–773 K, almost the entire strontium is in the form of condensed SrCO3. An increase in the temperature to 1273 K leads to an increase in the condensed SrO content to ~100% due to a decrease in the condensed SrCO3 content. As the temperature increases to 1573 K, the condensed SrO content decreases and the Sr vapor content increases to ~100%. A further increase in the temperature to 3273 K does not cause changes: strontium is in the form of Sr vapor (~100%).

CONCLUSIONS

We analyzed the element distribution in the radioactive graphite–Na2CO3–K2CO3–Sb2O3 system in an argon atmosphere and found that the carbon content decreases upon heating of the system. Heating to 1073 K leads to the evaporation of the condensed compounds of antimony and cesium. Upon heating to 1273 K, the condensed compounds of potassium, sodium, and chlorine evaporate. Heating to 1373 K leads to the evaporation of the condensed compounds of nickel. Heating to 1673 K brings about the evaporation of the condensed compounds of uranium, calcium, and strontium. Heating to 1773 K causes the evaporation of the condensed compounds of plutonium, beryllium, americium, and europium. At temperatures above 1773 K, only a vapor–gas phase exists in the system.

Notes

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Copyright information

© Pleiades Publishing, Ltd. 2018

Authors and Affiliations

  • N. M. Barbin
    • 1
    • 2
    • 3
    Email author
  • I. A. Sidash
    • 2
  • D. I. Terent’ev
    • 2
  • S. G. Alekseev
    • 2
    • 4
  1. 1.Ural State Agrarian UniversityYekaterinburgRussia
  2. 2.Ural Institute of GPS MChSYekaterinburgRussia
  3. 3.Ural Federal UniversityYekaterinburgRussia
  4. 4.Scientific–Engineering Center Reliability and Resource of Large Systems and Machines, Ural Branch, Russian Academy of SciencesYekaterinburgRussia

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