Electrochemical Sensors for Controlling Oxygen Content and Corrosion Processes in Lead-Bismuth Eutectic Coolant—State of the Art

Abstract

Controlling oxygen content in the primary circuit of nuclear reactors is one of the key tasks needed to ensure the safe operation of nuclear power plants where lead-bismuth eutectic alloy (LBE) is used as a coolant. If the oxygen concentration is low, active corrosion of structural materials takes place; upon increase in oxygen content, slag accumulates due to the formation of lead oxide. The generally accepted method of measuring the oxygen content in LBE is currently potentiometry. The sensors for measuring oxygen activity (electrochemical oxygen sensors) are galvanic cells with two electrodes (lead-bismuth coolant serves as working electrode) separated by a solid electrolyte. Control of corrosion and slag accumulation processes in circuits exploring LBE as a coolant is also based on data obtained by electrochemical oxygen sensors. The disadvantages of this approach are the low efficiency and low sensitivity of control. The alternative, Impedance Spectroscopy (EIS) Sensors, are proposed for Real-Time Corrosion Monitoring in LBE system. Currently their applicability in static LBE at temperatures up to 600 °C is shown.

A Review of Corrosion Behavior of Structural Steel in Liquid Lead–Bismuth Eutectic

“…If the oxygen concentration in LBE is more than 10 −6 wt%, the elemental O in liquid LBE will react chemically with the metal elements (Fe, Cr, etc.) in the steel material, thus causing oxidation corrosion on the surface of the structural steel [8,11,[13][14][15][16][52][53][54][55][56]. If a continuous and dense oxide film is produced on the surface of the structural steel, then oxidative corrosion will be effectively blocked.…”

“…At high concentrations (C O > 10 −6 wt%) [56], the structural steel undergoes an oxidation reaction (Figure 1b). If multiple layers of poorly adherent oxide films are formed, these films can delaminate and oxidize the underlying surface.…”

“…At low levels of oxygen in the LBE (C O < 5 × 10 −7 wt%), dissolution corrosion occurs mainly in structural steel. On the contrary, if the oxygen content in LBE is high (C O > 10 −6 wt%), oxidative corrosion will increase and Pb-O compounds will be formed, contaminating the liquid LBE and clogging the pipeline [56]. When the LBE's oxygen content is in this range, which can produce a stable Fe 3 O 4 that is not formed between Pb-O compounds, then a double-structured oxide layer will be formed.…”

“…Studies have shown that oxygen is a reliable corrosion inhibitor in the LBE environment, and oxygen control techniques are used to slow corrosion by forming a protective film on the material surface [95]. When the oxygen content in the liquid LBE is low, it cannot form an effective protective film and actively corrodes the structural material; when the oxygen content increases, the slag accumulation will precipitate and block the loop tube [56,89]. Therefore, it is necessary to adjust the oxygen content in liquid LBE in order to mitigate the dissolution corrosion of structural materials.…”

“…At moderate oxygen concentrations (10 −7 wt% < C O < 10 −6 wt%), Ni dissolution and Pb-Bi penetration caused slight iron seepage. In high oxygen concentrations (C O > 10 −6 wt%), only oxidation occurs [56,100].…”