Pb and LBE: a technological comparison

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-BismuthEutectic) and Pb, ascoolants for GenIV fast reactors. • The choice of Heavy Liquid Metals for a nuclear fast reactors, comes from severalknownadvantages, bothtechnological and nuclear. • Hystorically, LBE was the first choice, due to itsverylowmeltingpoint (125°) compared with Pb (327°C). • However, severalesperimentalevidences, gained in recentyears, suggest the need of a deepanalysis and comparisonbetween LBE and Pb ascoolants, expeciallyas far astechnologicalissues are concerned. • This work is a comparison of the two, starting from basicproperties and goingthrough non metallicelementsbehaviours, (i.e. Oxygen), corrosion, of structuralmaterials and relatedtechnologies.

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

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

5. Part 1: thermophysicalproperties (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 meltingaccompanied by rapid temperature change. In the handbook of Lyon [Lyon, 1954] a 1.43 vol. % contraction of LBE on freezing with a subsequentexpansion of the solid of 0.77 vol.% at an arbitrary temperature of 65°C hasbeenreported. P. Agostiniet 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 verysimilar. The goal of controlling and monitoringOxygenis a common need. Solubility and diffusivity of Oxygen in LBE and Pb, cfr. T. Gnanasekaran, Liquid Metals and StructuralChemistryDivisionChemistry Group, IGCAR

7. Part 2: Oxygensensors • 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, cO: • C1 and C2 are constants specific for the reference electrode • Basic components • Solid electrolyte • Yttria stabilized zirconia (YSZ) • Tubes with 4.5–4.8 mole% Y2O3 • "Thimble" with 3 mole% Y2O3 • Reference electrode • Metal/metal-oxide like Bi/Bi2O3 and In/In2O3 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 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 Pbbehaviours

8. Part 2: Oxygensensors

9. Part 2: Oxygensensors Configuration of the working electrode • 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

10. Work area Part 2: Oxygensensors

11. Part 2: Oxygensensors • 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/Bi2O3 • 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)

12. Part 2: Oxygensensors

13. Part 2: Oxygensensors

14. Part 2: Oxygensensors

15. Part 2: Oxygensensors Sensor1, 6m Sensor2, 2m Thermocouples Sensor3, 4m

16. Part 2: Oxygensensors

17. Design and Testing of Electrochemical Oxygen Sensors for Service in Liquid Lead Alloys Part 2: Oxygensensors • 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 Output of the sensor under investigation as a function of the immersion depth Output of reference sensor Immersion depth

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

19. Part 3: solidslags and blackdust • 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 G and B 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.

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

21. Part 3: solidslags and blackdust 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.

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

23. Part 3: solidslags and blackdust The observed mechanism of solid impurities (gas and liquid phase) can be summarized as follows: Uncontrolledcold area on the facility Air pollution (ingaspollution) (2Pb+O2 2PbO) LBE recrystallization-phaseseparation (In the cold leg of LBE loops, T=350°C) Particleformation-macroscopicslags (reducing gas mixture bubbling is not effective) samples. Loopdraining-cooling down (samples are taken at room temperature in air) In the CHEOPE III loop Pb operated, where T=500°C and the maximum DT with the coldlegis 80°C, no slags or blackdusthasbeenobserved. An indirectconfirmation of this speculative mechanismis the recrystallized LBE found in the filters: itisnotPb+Bibut Gamma and Beta phases (Pb7Bi3 and Bi99,9Pb), suggesting a rapidcoldpointfreezing. The formation of “blackdust” happens ONLY with LBE.

24. Part 4: corrosion • The need for data on referencestructuralmaterials in contact with HLM is a crucialissue in the development of GenIVtechnologies. • Lead and LBE are twohighly corrosive media. The possibility to protectthem by means of in situ passivation or artificilaprotections are widelystudied in the frame of europeanprogrammes • Corrosionmechanisms are driven by the sameprinciples, both in LBE and in Pb. Elementalsolubilities can generally be consideredsimilar. • However, given the highertemperatures of a Pb cooledreactor, corrosionphenomena are generallyworse. • Protectingsteels from corrosion by means of in situ passivationisquitestraighforward in LBE at 400°C, extremelytricky and lesseffective in pure Pb, at 500°C. in the latter, corrosionhappens by means of mass transfer more thanelementalstraightdissolution.

25. Part 4: corrosion T91 exposed to LBE, 3.000 hours of experiments, 500°C, Oxygen 10-6wt%. Thick protective oxide scales.

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

27. FPN FIS ING Part 4: corrosion 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% 20 mm scale micrography: oxidelayers with corresponding EDS spots

28. Part 4: corrosion 10.3 µm 4000 h • 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;

29. Part 4: corrosion 16 µm 33 µm

30. FPN FIS ING Part 4: corrosion Oldexperiment at 400°C and latestexperiment at 500°C. sIs the corrosiondepth in microns

31. FPN FIS ING Part 4: corrosion Corrosioncurves for old and new experiments. Fewpoints do notallow a criticalcomparison.

32. Conclusion • The choicebetween LBE and Pb ascoolants for GenIV fast reactorisconnected to several open points: • Technologicaladvantages and disadvantages (i.e. meltingpoint, volume expansion, solidimpuririties production, highertemperatures for structuralmaterials) • Commercial issues, expecially Bi cost and naturalabubdance • Nuclearsafetyissues, expecially Po210 aerosols production by irradiated Bi. The global amount of Poloniumisproducedonly by Bi. With pure Pb, only the Bi traces are responsible of the eventualPolonium aerosol. • The protection of structuralmaterials from high temperature corrosionisthus the critical open point for Pb LFR technologies. Once solved, Pb could be the winningchoice over LBE.