1. Pb and LBE: a technological comparison Alessandro Gessi, Mariano Tarantino, Pietro Agostini ENEA Cr Brasimone 40032 Camugnano, BO, Italy Matgen IV School, Santa Teresa, 21/9/2011

2. Introduction The goal of this work is to compare critically LBE (Lead-Bismuth Eutectic) and Pb, as coolants for GenIV fast reactors. The choice of Heavy Liquid Metals for a nuclear fast reactors, comes from several known advantages, both technological and nuclear. Hystorically, LBE was the first choice, due to its very low melting point (125°) compared with Pb (327°C). However, several esperimental evidences, gained in recent years, suggest the need of a deep analysis and comparison between LBE and Pb as coolants, expecially as far as technological issues are concerned. This work is a comparison of the two, starting from basic properties and going through non metallic elements behaviours, (i.e. Oxygen), corrosion, of structural materials and related technologies.

3. Part 1: thermophysical properties (rif. OECD –NEA HLM Handbook, Chapter 2, A.Gessi, V. Sobolev)

4. Part 1: thermophysical properties (rif. OECD –NEA HLM Handbook, Chapter 2, A.Gessi, V. Sobolev)

5. Part 1: thermophysical properties (rif. OECD –NEA HLM Handbook, Chapter 2, A.Gessi, V. Sobolev) Volume change at melting and solidification: A detailed knowledge of volume changes in metals and alloys at their melting points is of critical importance in the understanding of solidification processes. Solid lead. Similar to the majority of metals with the FCC crystal structure, lead exhibits a volume increase upon melting. At normal conditions a volume increase of 3.81 % has been observed in pure lead [Iida, 1988]. The situation is more complicated for LBE freezing and melting accompanied by rapid temperature change. In the handbook of Lyon [Lyon, 1954] a 1.43 vol. % contraction of LBE on freezing with a subsequent expansion of the solid of 0.77 vol.% at an arbitrary temperature of 65°C has been reported. P. Agostini et al. [P. Agostini, 2004] and Zucchini et al. [Zucchini, 2005] showed that the consequences of LBE volume expansion by recrystallization could lead to severe damages to pipeworks. The numerical and experimental studies described show that over-stressing due to LBE recrystallization and expansion in containment vessels such as in the MEGAPIE target must be considered during the design phase of the containment structures and can be managed by means of engineering rules. To avoid over-stressing of structures it is proposed to redouce: the height of each solid LBE layer, the presence of internal structures, the LBE yield strength.

6. Part 2: Oxygen The solubility and diffusivity of Oxygen in Molten Pb and LBE are very similar. The goal of controlling and monitoring Oxygen is a common need. Solubility and diffusivity of Oxygen in LBE and Pb, cfr. T. Gnanasekaran, Liquid Metals and Structural Chemistry Division Chemistry Group, IGCAR

7. Part 2: Oxygen sensors Oxygen sensors for LBE and Pb are based on the same principles: galvanic cells using YZR as solid electrolyte. Recent experiments have shown commonalities between LBE and Pb behaviours Basic components  Solid electrolyte  Yttria stabilized zirconia (YSZ)  Tubes with 4.5–4.8 mole% Y 2 O 3  "Thimble" with 3 mole% Y 2 O 3  Reference electrode  Metal/metal-oxide like Bi/Bi 2 O 3 and In/In 2 O 3 with Mo wire as electric lead  Pt/air using steel wire with platinised tip as electric lead  Second (working) electrode  The liquid Pb alloy  Auxiliary wire or the steel housing of the sensor serves as part of the electric lead Sensor output  Voltmeter reading, E  Measure of the chemical potential of oxygen in the liquid metal  May in general depend on the specific combination of the sensor with a high- impedance voltmeter  Ideal sensor/voltmeter system  Ideal zero-current potential:  Calculated oxygen concentration, c O :  C 1 and C 2 are constants specific for the reference electrode

8. Part 2: Oxygen sensors

9.  Metallic sheath (austenitic steel) with Pt mesh  Electric contact by pressing the electrolyte against the Pt mesh  The contact with the mesh is established at the highest testing temperature  Disadvantages are the different thermal expansion of YSZ tube and steel sheath (rupture of the mesh during cooling) and oxidation of the steel sheath at high temperature  Pt wire fixed with Pt paste  Allows for producing different thermoelectric voltages using different materials (wires) for connecting the Pt wire at the closed end of the electrolyte tube with the sensor housing  Electric contact with the electrolyte may degrade during thermal cycling  Comparatively small area of electric contact gives rise to high electrolyte resistance Configuration of the working electrode Part 2: Oxygen sensors

10. Work area Part 2: Oxygen sensors

11. Characteristics  Electrolyte thimble  Seal between electrolyte and housing immersed in the liquid metal  Glass ceramic sealant developed for compatibility with YSZ and steel (thermal), and with liquid Pb alloys (chemical)  Reference electrodes:  Bi/Bi 2 O 3  3-YSZ with optimized mechanical properties  Prototype for oxygen measurement in a depth of ~5 m below the surface of a liquid-metal pool (based on R&D by IPPE) Part 2: Oxygen sensors

15. Sensor 1, 6 m Sensor 2, 2 m Sensor 3, 4 m Thermocouples Part 2: Oxygen sensors

17. Design and Testing of Electrochemical Oxygen Sensors for Service in Liquid Lead Alloys L, м Т, °СV, m/sЕ ref, mVa ref Е6Е6 а6а6 14700,2511711201 24700,251191113- 34800,251201102- 44800,251320,891- 54800,251400,583- 14800,251480,41410,4 Immersion depth Output of reference sensor Output of the sensor under investigation as a function of the immersion depth  Sensor design scaled-up from experience in smaller experimental facilities  Output significantly decreases for immersion depth > 1 m  Improvements of signal transmission required for oxygen measurements in pool-type reactors Part 2: Oxygen sensors

18. Two-shell electric of the reference electrode with guarding potential Part 2: Oxygen sensors

19. The issue of solid impurities, “black dust” and macroscopic slags, has been one of the most important topics in the frame of HLM activities and experiments. In fact, during the operation (with LBE) of the CHEOPE III, LECOR and CIRCE facilities at ENEA several problems (filters and pipes occlusions, loops’ malfunctions, gas piping's blocks) have been encountered. Formed impurities have been sampled and analyzed: the presence of a relatively high amount of  and  phases together with the 40wt% ca. Of Massicot and Litharge (PbO) suggests a complex formation mechanism. Also, a sampling method problem exist: analytical methods can determine the composition of the samples, but not quantitatively determine a possible “formation rate”. The use of adsorption filters in the liquid phase gave good results. The filtered part appeared to be enriched in PbO, confirming the selectivity of the filters. A deeper sealing's control coupled with gas inlet filtration minimized the phenomena in LBE. NO solid impurity has been observed in flowing Pb (CHEOPEIII last campaign), even after 10.000 hours of operation, nor any operational problem. A fibreglass filter has been used also in Pb, where a small amount of PbO has been measured. Outgas systems appear clean. Part 3: solid slags and black dust

20. “ Black dust ” SEM image, CHEOPE III outgas pipe Solid slags over CIRCE free level Part 3: solid slags and black dust Examples of microscopic “black dust” and macroscopic slags (1m ca.)

21. CompoundConcentration PbO40 wt% ca. LBE (  phases) 50 wt% ca. Fe, Al, Cr10 wt % Ca. CompoundConcentration PbO60 wt% ca. LBE (  phases) 30 wt% ca. Fe, Al, Cr10 wt % Ca. CompoundConcentration PbO15 wt% ca. Pb80 wt% ca. Fe, Al, Cr5 wt % Ca. Table 1 Composition of a slag in the CHEOPE loop, LBE, 400°C, outgas filter. Table 2. Composition of the filtered particles, fiberglass adsorption filter in the liquid phase, LBE, CHEOPE III Table 3. Composition of few filtered particles, fiberglass adsorption filter, liquid phase CHEOPEIII, Pb, 500°C. Part 3: solid slags and black dust

22. Experiments performed in the frame of the TRASCO program: evaporation rates vs temperature. (* P. Turroni et Al., J.Vac. Sc. Tech. A 22(4)). Part 3: solid slags and black dust

23. The observed mechanism of solid impurities (gas and liquid phase) can be summarized as follows: Uncontrolled cold area on the facility Air pollution (ingas pollution) LBE recrystallization-phase separation Particle formation-macroscopic slags Loop draining-cooling downsamples. (2Pb+O 2 2PbO) (In the cold leg of LBE loops, T=350°C) (reducing gas mixture bubbling is not effective) (samples are taken at room temperature in air) In the CHEOPE III loop Pb operated, where T=500°C and the maximum  T with the cold leg is 80°C, no slags or black dust has been observed. An indirect confirmation of this speculative mechanism is the recrystallized LBE found in the filters: it is not Pb+Bi but Gamma and Beta phases (Pb 7 Bi 3 and Bi99,9Pb), suggesting a rapid cold point freezing. The formation of “black dust” happens ONLY with LBE. Part 3: solid slags and black dust

24. The need for data on reference structural materials in contact with HLM is a crucial issue in the development of GenIV technologies. Lead and LBE are two highly corrosive media. The possibility to protect them by means of in situ passivation or artificila protections are widely studied in the frame of european programmes Corrosion mechanisms are driven by the same principles, both in LBE and in Pb. Elemental solubilities can generally be considered similar. However, given the higher temperatures of a Pb cooled reactor, corrosion phenomena are generally worse. Protecting steels from corrosion by means of in situ passivation is quite straighforward in LBE at 400°C, extremely tricky and less effective in pure Pb, at 500°C. in the latter, corrosion happens by means of mass transfer more than elemental straight dissolution. Part 4: corrosion

25. T91 exposed to LBE, 3.000 hours of experiments, 500°C, Oxygen 10 -6 wt%. Thick protective oxide scales.

26. T91 exposed to Pb, 10.000 hours of experiments, 500°C, Oxygen 10 -6 wt%. Weak, thick, quickly formed oxide scales, easily eroded by HLM flux. FPN FIS ING Part 4: corrosion

27. 20  m scale micrography: oxide layers with corresponding EDS spots Fe: 89.5 wt% Cr: 8.3 wt% Fe: 71.4 wt% Cr: 8.4 wt% O: 18.5 wt% Fe: 41.5 wt% Cr: 12.5 wt% O: 42.9 wt% Fe: 57.0 wt% Cr: 0.4 wt% O: 41.3 wt% FPN FIS ING Part 4: corrosion

28. 4000 h 10.3 µm The coating scale have a very good continuity; Oxygen precipitation is observed below the FeAl coating; Small damages are observed in the coating maybe due to the post examination analysis; Part 4: corrosion

29. 16 µm 33 µm Part 4: corrosion

30. Old experiment at 400°C and latest experiment at 500°C.  Is the corrosion depth in microns FPN FIS ING Part 4: corrosion

31. Corrosion curves for old and new experiments. Few points do not allow a critical comparison. FPN FIS ING Part 4: corrosion

32. The choice between LBE and Pb as coolants for GenIV fast reactor is connected to several open points: 1.Technological advantages and disadvantages (i.e. melting point, volume expansion, solid impuririties production, higher temperatures for structural materials) 2.Commercial issues, expecially Bi cost and natural abubdance 3.Nuclear safety issues, expecially Po210 aerosols production by irradiated Bi. The global amount of Polonium is produced only by Bi. With pure Pb, only the Bi traces are responsible of the eventual Polonium aerosol. The protection of structural materials from high temperature corrosion is thus the critical open point for Pb LFR technologies. Once solved, Pb could be the winning choice over LBE. Conclusion