The behaviour of Eu, Pu, Am radionuclide at burning radioactive graphite in an oxygen atmosphere. Computer experiments

Be means of the method of computer thermodynamic simulation we studied the behaviour of the europium, plutonium and americium from the combustion of radioactive graphite in oxygen. Europe is in the form of condensed EuOCl, Eu2O3 and vapour EuO. Pluto is in the form of condensed vapour PuO2 and PuO2. Americium is a condensed AmO2, Am2O3 and vapour Am. The basic reactions occurring compounds with europium, plutonium and americium. Equilibrium constants of the reactions have been determined. Introduction In connection with the completion of the operation and as a result of accidents and man-made emergencies, there are a large amount of radioactive graphite waste. Experts estimate that the total number of irradiated graphite in the world is about 250 thousand tonnes. Exactly reactor graphite is the most of the accumulated solid waste [1, 2]. In the Russian Federation there are about seventeen units. The weight of the graphite stack one reactor, depending on its type, is 1.5–2.5 thousand tonnes. The total mass of irradiated graphite in Russia is approximately equal to 60 thousand tonnes [3]. Technologies for waste disposal of nuclear energy, in the majority, based on the principle of isolation of radioactive graphite from the environment. Considered the most appropriate construction of underground storage in solid granite or basalt rocks. Such a method, by far, the safest, cheapest, and is used by many countries in the fifties of the twentieth century. However, this method also has a number of disadvantages, the main of which is the fact that the area for disposal not unlimited, and this disposal method does not require a reduction of waste [4]. Become a promising high-temperature treatment of radioactive waste. Today, there are several ways to burn radioactive graphite, oxidation air, oxygen, oxidation in molten alkali chlorides, carbonates [2, 5, 6]. The criterion for the successful development of the combustion process is acceptable emissions of radioactive elements to preserve the environment [7]. a Corresponding author: NMBarbin@mail.ru This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20158201013 EPJ Web of Conferences Figure 1. The distribution of Eu in the incineration of radioactive graphite. Radioactive elements cannot be incinerated. Either they are part of a non-combustible waste, or evaporate, depending on their volatility. The gaseous components condense on the larger particles in the flue gases are removed scrub system [7]. Objective The main objective of this study is the behaviour of some radioactive elements in combustion of the reactor graphite under oxygen. Computer experiment The behaviour of radioactive elements is not different from that of their non-radioactive isotopes. Studies conducted by thermodynamic modelling. Thermodynamic modelling is a thermodynamic analysis of the equilibrium state of the system as a whole (complete thermodynamic analysis) [8, 9]. Calculation methods are developed based on variation principles of thermodynamics. One of the most advanced and efficient programs that implement such thermodynamic calculations is a software package TERRA, which is a step further development of the software package ASTRA [10, 11]. Calculations of the equilibrium phases and characteristics are carried out using the reference database on the properties of individual substances [12, 13]. Results and discussion Fused EuOCl is predominant compound in the temperature range from 375 to 800 K, then the quantity begins to decrease and reaches a value of 0 mol. % At 1500 K. The increase in the number of condensed Eu2O3 observed in the temperature range 900–1300 K, and the reduction 1800–2500 K. The growth of gaseous EuO happening to 2000 K at 2500 K. On further heating to 3300 K, the reduction in the amount of vapour EuO (with 99 mol.% to 89 mol.%) with a simultaneous increase in the amount of ions Eu + (8 mol.%) and vaporous Eu (2 mol.%).


Introduction
In connection with the completion of the operation and as a result of accidents and man-made emergencies, there are a large amount of radioactive graphite waste. Experts estimate that the total number of irradiated graphite in the world is about 250 thousand tonnes. Exactly reactor graphite is the most of the accumulated solid waste [1,2].
In the Russian Federation there are about seventeen units. The weight of the graphite stack one reactor, depending on its type, is 1.5-2.5 thousand tonnes. The total mass of irradiated graphite in Russia is approximately equal to 60 thousand tonnes [3].
Technologies for waste disposal of nuclear energy, in the majority, based on the principle of isolation of radioactive graphite from the environment. Considered the most appropriate construction of underground storage in solid granite or basalt rocks. Such a method, by far, the safest, cheapest, and is used by many countries in the fifties of the twentieth century. However, this method also has a number of disadvantages, the main of which is the fact that the area for disposal not unlimited, and this disposal method does not require a reduction of waste [4].
Become a promising high-temperature treatment of radioactive waste. Today, there are several ways to burn radioactive graphite, oxidation air, oxygen, oxidation in molten alkali chlorides, carbonates [2,5,6]. The criterion for the successful development of the combustion process is acceptable emissions of radioactive elements to preserve the environment [7]. Radioactive elements cannot be incinerated. Either they are part of a non-combustible waste, or evaporate, depending on their volatility. The gaseous components condense on the larger particles in the flue gases are removed scrub system [7].

Objective
The main objective of this study is the behaviour of some radioactive elements in combustion of the reactor graphite under oxygen.

Computer experiment
The behaviour of radioactive elements is not different from that of their non-radioactive isotopes. Studies conducted by thermodynamic modelling.
Thermodynamic modelling is a thermodynamic analysis of the equilibrium state of the system as a whole (complete thermodynamic analysis) [8,9]. Calculation methods are developed based on variation principles of thermodynamics.
One of the most advanced and efficient programs that implement such thermodynamic calculations is a software package TERRA, which is a step further development of the software package ASTRA [10,11].
Calculations of the equilibrium phases and characteristics are carried out using the reference database on the properties of individual substances [12,13].

Results and discussion
Fused EuOCl is predominant compound in the temperature range from 375 to 800 K, then the quantity begins to decrease and reaches a value of 0 mol. % At 1500 K. The increase in the number of condensed Eu 2 O 3 observed in the temperature range 900-1300 K, and the reduction 1800-2500 K. The growth of gaseous EuO happening to 2000 K at 2500 K. On further heating to 3300 K, the reduction in the amount of vapour EuO (with 99 mol.% to 89 mol.%) with a simultaneous increase in the amount of ions Eu + (8 mol.%) and vaporous Eu (2 mol.%).

01013-p.2
Thermophysical Basis of Energy Technologies  Plutonium is distributed in the following way: to a temperature of 1800 K prevails in the fused PuO 2 (98-99 mol.%). With increasing temperature, the content of the condensed PuO 2 begins to decrease, at the same time increasing the content of gaseous PuO 2 . At a temperature 2400 K PuO 2 vapor content is ≈ 99 mol. %, The content of the condensed PuO 2 reduced almost to 0 mole. %.
At temperatures from 375 to 2300 K in the system prevails fused AmO 2 (99 mol.%). Increasing the temperature to 2700 K causes a rise in the content of the condensed Am 2 O 3 to 90 mol. %, As well as vapor Am to 9 mol. % And a decrease in the amount of condensed AmO 2 to 0 mole. %. At 3300 K decreases the amount of condensed Am2O3 to 0 mole. % and amount of vapor Am increases to 100 mol. %.
As a result of thermodynamic modeling revealed that the combustion of radioactive graphite in an atmosphere of O 2 , americium, plutonium and europium are present in the form of oxygen compounds. Increasing the temperature above 2000 K lead to evaporation of these compounds.
In accordance with the above set of phase transitions and guided by numerical simulation results, the minimum set of basic reaction (equilibrium) within the individual phases and at the interface is written below.
The dependence of the equilibrium constants of reactions (1)-(6) of the temperature is described by the equation of the form: The numerical values of the coefficients A and B of Eq. (7) shown in Table 1.
The values of the equilibrium constants indicate that the reaction shifted towards the formation of products.

Findings
The combustion of radioactive graphite oxygen europium is in the form of condensed EuOCl (to a temperature of 1200 K), the condensed Eu 2 O 3 (in the temperature range 1200-2200 C) and vapor EuO (at temperatures above 2200 K).
The combustion of radioactive graphite oxygen plutonium is in the form of condensed PuO 2 (to a temperature of 2100 K) and vapor PuO 2 (above 2100 K).
The combustion of radioactive graphite oxygen americium is in the form of condensed AmO 2 (to a temperature of 2500 K), the condensed Am 2 O 3 (in the temperature range 2500-2900 C) and vapor Am (at temperatures above 2900 K).
The findings should be considered when disposing of radioactive graphite by incineration.