Corresponding author: Sergey E. Kharchuk ( skharchuk@ippe.ru ) Academic editor: Yury Kazansky
© 2021 Vladimir V. Ulyanov, Mikhail M. Koshelev, Vladlena S. Kremlyova, Sergey E. Kharchuk.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Ulyanov VV, Koshelev MM, Kremlyova VS, Kharchuk SE (2021) Investigations of regularities in the accumulation of hydrogen-reduced slags in circulation circuits with lead-containing coolants. Nuclear Energy and Technology 7(3): 245-252. https://doi.org/10.3897/nucet.7.74154
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The paper presents a computational analysis of regularities in the accumulation of slags during the interaction of lead and lead-bismuth coolants with oxygen gas. Oxidation of lead-containing coolants will cause the formation of lead oxide, while the formation of bismuth oxide is unlikely. Dosed supply of oxidizing gas to lead-containing coolants makes it possible to oxidize, selectively, chromium and nickel to their oxides without the slag formation from solid lead oxide. Regularities were studied which are involved in the lead oxide formation during the interaction of lead-containing coolants with oxygen gas. It has been found that, in the process of interacting with oxygen gas, a lead-bismuth alloy is oxidized 1.7 times as intensively as lead, this being explained by the presence of bismuth in the alloy. Bismuth is oxidized more intensively than both lead and the lead-bismuth alloy. The inert gas overpressure during depressurization does not prevent air oxygen from entering the circuit, and the dependence of the nitrogen and oxygen flow into the circuit on the argon flow out of the loop is close to linear regardless of the circuit state (cold, without coolant; heated, without coolant; heated, with circulating coolant). Oxygen is a chemically active impurity and is absorbed by the circuit; it is therefore important to control nitrogen in the gas spaces of the reactor and research plant circuits with lead-containing coolants. This will make it possible to signal, in a timely manner, the ingress of oxygen into the circuit and to take measures required to avoid or reduce the scale of the slag formation from lead oxides.
Lead-containing coolant, slags based on lead oxides, lead, lead-bismuth eutectic, oxygen gas, lead oxide activity, Gibbs energy, hydrogen-based cleaning from slags, oxidation pattern, backpressure ingress
The choice of liquid metals as nuclear reactor coolants made by A.I. Leypunsky in the 1940s is explained by their unique properties. With a turbulent flow of liquids in tubes, heat is transferred due to the turbulent mixing of the flow and by the molecular thermal conductivity of the coolant. Liquid metals have better molecular thermal conductivity as compared with other coolants. This leads to an increased share of heat transported by means of thermal conductivity and provides better heat-transfer properties, which defines the advantages of liquid metals as coolants. Mercury was the first to be tested as coolant (the Clementine reactors in the USA and the BR-2 reactor in the USSR). Further on, sodium was selected as the coolant for civilian fast neutron reactors because of a shorter fuel doubling time, and lead-bismuth eutectic was selected as the coolant to be used in reactor plants for small-displacement ships and high-power plants, which has made it possible to achieve such specific weight and dimension parameters as are not possible in water-cooled plants (
Lack of sufficient information on lead-bismuth coolants and the initially predominant concept that the cleanliness of the entire circulation circuit can be ensured through the coolant purification by removing lead oxides from free surfaces, led in 1968 to an accident at the K-27 nuclear submarine’s portside reactor plant and a pre-emergency situation at the 27/VT ground prototype test facility (
By the time the reactor plants for project 705 and 705K nuclear submarines started to operate, an extensive experience had already been gained in handling of lead-bismuth coolants, so none of the nine reactor plants was decommissioned due to the excessive slag accumulation as the result of operation (
Lead and lead-bismuth coolants represent a complex thermodynamic system with respect to oxygen gas. With a small content of oxygen in the coolant, its properties are similar to strong scavengers (chromium or iron), and, in the saturation condition, the reaction with oxygen gas leads to the formation of solid lead oxide. Such property can be used for oxidizing, selectively, the coolant-contained impurities with a high affinity to oxygen which change from a dissolved state to a suspended state. However, uncontrolled interaction of lead-containing coolants with oxygen gas can lead to the formation and accumulation of slags based on the coolant oxides. This is confirmed by analyzing thermodynamically the reaction of the interaction of the coolant components with oxygen gas:
{Pb} + 1/2(O2) ↔ 〈PbO〉, (1)
2/3{Bi} + 1/2(O2) ↔ 1/3〈Bi2O3〉, (2)
1/3〈Bi2O3〉 + {Pb} ↔ 〈PbO〉 + 2/3{Bi}, (3)
where {Pb}, {Bi} are liquid lead and bismuth; (O2) is oxygen gas; and 〈PbO〉, 〈Bi2O3〉 are solid lead and bismuth oxides. Equations (2), (3) are practically meaningful for lead-bismuth eutectic, and only reaction (1) can take place in lead coolant (1). The driving force for the reactions is defined by the actual change of Gibbs energy, ΔG, for particular conditions of interaction also referred to as Gibbs free enthalpy (
ΔG = ΔG0 + R×T×ln K, (4)
where ΔG0 is the standard Gibbs energy change (with the activities of solid and liquid substances equal to unity, and the oxygen gas pressure equal to 1·105 Pa), J/mol; T is temperature, K; R = 8.314 J/(mol×K); and K is the constant of equilibrium defined by the ratio of the product of the activities of the resultants from reactions (1) – (3) to the product of the activities (pressures) of the initial reagents for the analyzed reactions. Value ΔG0 for lead and bismuth oxides is determined by equations (
ΔG0form.PbO = – 219003 + 100.4×T, (5)
ΔG0form.Bi2O3 = – 622411 + 313.8×T, (6)
For reaction (3), value ΔG0 is determined from the equation
ΔG0р.3 = ΔG0form.PbO – 1/3ΔG0form.Bi2O3. (7)
The activities of lead and bismuth in lead-bismuth eutectic (
lg aPb = – 98/T – 0.32, (8)
lg aBi = – 64/T – 0.23. (9)
With equilibrium ΔG = 0, so, for reaction (3), the value of the standard Gibbs energy change will be expressed by the equation
ΔG0 = – R×T×ln [aPbO×aBi2/3/(aBi2O31/3×aPb)]. (10)
By converting (10), using (7) – (9), one can obtain the relationship between aPbO and aBi2O3 as a function of temperature in the equilibrium state (see Fig.
Therefore, there are no thermodynamic grounds for the formation of solid Bi2O3 in lead-bismuth eutectic, and reaction (1) is of prime importance in analyzing processes of oxidation by oxygen and slag accumulation not only for lead but also for lead-bismuth coolant. The driving force for the lead oxide formation reaction (1), which fits a negative value of ΔG, increases as the lead oxide activity decreases. Equation (4) for reaction (1) is transformed as
ΔG = ΔG0form.PbO + R×T×ln [aPbO/(aPb×PO21/2)], (11)
where PO2 is the partial pressure of oxygen in the reaction area, atm; aPb is the activity of lead (for lead coolant, aPb = 1); and aPbO is the activity of lead oxide often referred to as thermodynamic activity of dissolved oxygen.
Fig.
It can be seen in the figure that, in certain conditions, the Gibbs energy changes during oxidation of lead-containing coolants and standard Gibbs energy changes during iron, nickel or chromium oxidation coincide. This means that oxidation of lead-containing coolants and oxidation of a metal contained in structural steel are equally possible at coincidence points. For instance, oxidation of lead coolant with aPbO ≥ 1×10–6 and oxidation of iron with the formation of solid iron oxide (aFe ≈ aFe3O4 ≈ 1) have similar thermodynamic grounds. A major increase in the activity of dissolved lead oxide in lead-containing coolants will cause deeper oxidation of an iron impurity to iron oxide. In this case, achieving a thermodynamic equilibrium will require the following conditions to be fulfilled: with aPbO >> 1×10–6, aFe3O4 ≈ 1, but aFe << 1. The formation of solid nickel oxide can occur with the lead oxide activity being aPbO ≥ 1×10-2.
Metal impurities in the circulating coolant originate largely from structural materials and most of such impurities (Fe, Cr, Ni, etc.) can be oxidized with the formation of respective solid oxides in conditions of oxygen-undersaturated lead-containing coolant, that is, without the formation of solid lead oxide. Therefore, it can be expected that, by providing a moderate amount of dissolved lead oxide, it is possible to oxidize, selectively, impurities of metals to their oxides in a solid phase through removing impurities from the circuit without the formation of slag based on lead oxide.
It follows from equation (11) that oxidation of lead-containing coolants is thermodynamically grounded at up to very small partial pressures of oxygen. For instance, the formation of solid lead oxide in lead coolant at a temperature of 420 °C is possible with an oxygen pressure of not less than 3.02×10–18 P. Therefore, in the event of depressurization of a circulation circuit with lead-containing coolant, atmospheric oxygen gas will interact with the coolant through reaction (1) and will lead to the saturation state with the subsequent formation of solid PbO. The composition of the slags formed during operation of circulation circuits with lead-bismuth coolant is shown in Table
Place of slag formation | Content of elements in slag, wt. % | |||||
---|---|---|---|---|---|---|
O | Pb | Bi | Fe | Cr | Ni | |
TT-2M test facility, pipeline dead-end | 2.4 | 51.6 | 46.4 | 4×10–4 | 3×10–4 | 3×10–4 |
SVT-3M test facility, internal wall of flow-through pipeline | 3.5 | 55.9 | 39.5 | 4.2×10–1 | 1.5×10–1 | 9.5×10–2 |
VNIIAM’s pump testing facility, internal wall of pump tank | 3.9 | 51.9 | 42.1 | 2.6×10–2 | 2.4×10–2 | 1.2×10–3 |
VNIIAM’s pump testing facility, internal wall of pressure pipeline | 6.5 | 76.0 | 14.2 | 2.6×10–2 | 1×10–2 | 1.2×10–2 |
27/VT prototype test facility, core inlet header | 3.6 | 63.4 | 24.8 | 1.8 | 8.6×10–2 | 7.0×10–2 |
The authors happened to deal with studies in chemistry of lead-containing coolants, in which experiments were conducted in lead-bismuth eutectic and, believing its properties to be close to those of lead, the obtained results are generalized for both coolants (
The regularities of the lead and lead-bismuth eutectic oxidation were investigated on a facility the layout of which is shown in Fig.
The experiments were conducted in two phases: oxidation of lead and an alloy with equal mass fractions of lead and bismuth was compared at stage 1, and oxidation of lead, bismuth and an alloy with equal mass fractions of lead and bismuth was compared at stage 2. Lead S0 (GOST 3778-98) and bismuth Vi1 (GOST 10928-90) were used.
Experimental procedure at stage 1:
The obtained regularity is presented in Fig.
It can be seen that the lead and bismuth alloy, as applied to the experiment conditions, is oxidized in air approximately 1.7 times as fast as lead. A bismuth sample was also tested to identify the reason for the detected regularity at the experiment’s stage 2.
Experimental procedure at stage 2:
After the experiment is over, all tested metal samples were reduced by hydrogen in the same manner (
Table
Regularity of oxidation for lead, bismuth and an alloy composed of equal mass fractions of lead and bismuth at temperatures of 420 and 535 °C for 19 h
Sample type | Specific weight of absorbed oxygen at 420 °C, g/cm2 | Specific weight of absorbed oxygen at 535 °C, g/cm2 |
---|---|---|
Lead | 1.09×10–2 | 3.64×10–2 |
Alloy with equal mass fractions of Pb and Bi | 1.95×10–2 | 4.87×10–2 |
Bismuth | 2.27×10–2 | 6.17×10–2 |
Given the fact that the circuit depressurization is inevitable for certain operations, e.g., during the core refueling, there are proposals to reduce the ingress of air into the circuit by keeping an inert gas overpressure in the circuit, that is, by organizing a leakage from the circuit and into the atmosphere. Solutions of the kind have been implemented in a number of applications, e.g., in control of the polyatomic liquid evaporation front (
The ingress due to backpressure was experimentally estimated at the TT-2M test facility (Fig.
Layout of the TT-2M test facility circulation circuit: 1 – circulation pump; 2 – recuperative heat exchangers; 3 – core simulating heater; 4 – steam generator simulating chiller; 5 – ejector for gas mixture injection into coolant; 6 – water evaporator; 7 – coolant flow meter; 8 – coolant volume change compensator; 9 – gas circuit connection to gas supply and vacuum system; 10 – gas composition monitoring system; 11 – liquid metal valves; 12 – gas valves; 13 – oxygen activity sensors of zirconium dioxide; 14 – coolant storage and melting tank; 15 – gas flow meter; 16 – coolant flow direction.
Temperature was variable during the experiment, with or without the coolant in the circuit, the argon overpressure in the gas circuit was 1×104 to 5×104 Pa, and the gas phase composition change was observed in all modes. A Kristall-5000 chromatograph was used to monitor the composition of the gas mixture in the circuit. The nitrogen flow into the circuit was determined from the change of the nitrogen concentration in the circuit and the total gas pressure and with regard for the fact that the total volume of the circuit was 430 l of gas during normal conditions. The obtained results are summarized in Table
Dependence of the nitrogen flow into the circuit on the argon flow out of the circuit, temperature and coolant presence
Nitrogen flow into circuit, l/h | Argon flow out of circuit, l/h | Temperature, °C | Coolant in circuit |
---|---|---|---|
0.013 | 0.4 | 19 | Absent |
0.051 | 1.7 | 240 | Absent |
0.085 | 2.5 | 320 | Present |
0.175 | 5.5 | 19 | Absent |
The dependence of the nitrogen flow into the circuit on the argon flow out of the circuit is close to direct proportionality, and the nature of the argon outflow from the circuit and the ingress of nitrogen into the circuit did not practically change independent of the test facility operation mode (cold, without coolant; heated, without coolant; heated, with circulating coolant):
Q N2 = 0.032 QAr, (12)
where QN2, QAr are the nitrogen flow into the circuit and the argon flow out of the circuit respectively, l/h. At a temperature of 19 °C, the content of oxygen grew with the increase in the nitrogen content in the gas circuit in ratios close to the nitrogen and oxygen ratio in air. At the same time, the volume fraction of oxygen decreased and stabilized at a level of 0.02%vol. at a temperature of 240 °C following a short-term increase. The nature of the oxygen content change in the circuit with the coolant circulation in it and a temperature of 320 °C did not change as compared with the circuit without coolant at a temperature of 240 °C. Its volume fraction did not exceed 0.02%vol., which indicates that oxygen is absorbed by the circuit materials and the coolant at elevated temperatures.
Therefore, the ingress of air into the circuit due to backpressure has been confirmed. And the estimated argon ingress is more reliable than for oxygen, due to the oxygen absorption by the coolant and the circuit surfaces in the heated condition. A comparison of the argon and oxygen ingress into the cooled circuit has confirmed that these gases are capable to penetrate leaky joints and enter the circuit by backpressure. This allows estimating the ingress of oxygen gas into the circuit from air if the nitrogen ingress is known:
Q O2 = (QN2×CO2)/CN2 = (QN2×20.95)/78.08 = 0.27×QN2, (13)
where QN2, QO2 are the nitrogen and oxygen flows into the circuit, l/h; and CO2, CN2 are the content of oxygen and nitrogen in air, %.
It follows from relation (13) that the increase in the content of nitrogen in the gas circuit proves reliably that atmospheric oxygen enters the circuit, including in the event of its absorption by the coolant and the circuit surfaces.
Regularities have been investigated which are involved in the accumulation of hydrogen-reduced slags in circulation circuits with lead-containing coolants. Lead oxide will form in lead-containing coolants in contact with oxygen gas, and the bismuth oxide formation has no thermodynamic grounds. While providing a moderate amount of dissolved lead oxide in a controlled manner, it is possible to selectively oxidize impurities of metals to their oxides in a solid phase by removing impurities from the circuit without solid lead oxide formation.
In the event of depressurization, the oxidation of a lead and bismuth alloy is 1.7 times as intensive as that of lead which is explained by the presence of bismuth in the alloy. Bismuth is oxidized more intensively than both lead and lead-bismuth alloy. As applied to the conditions of the BREST reactor plant during the core refueling, the formation of 185 kg of solid lead oxide can be predicted.
An inert gas overpressure during depressurization does not prevent atmospheric oxygen from entering the circuit, in which case nitrogen monitoring in the circuit will make it possible to signal the ingress of oxygen into the circuit and, accordingly, to take measures required to avoid or reduce the scale of slag formation. It is therefore important to control nitrogen in the gas spaces of the reactor and research plant circuits with lead-containing coolants.