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					                             Universities Nuclear Technology Forum
                                  Univ. of Salford, Manchester
                                      14th – 16th April 2010

                      CORROSION OF SPENT ADVANCED GAS REACTOR
                        (AGR) FUEL CLADDING IN TRACE AQUEOUS
                              ELECTROLYTE ENVIRONMENTS

                                                                PhD Student:
                                                                Sunny Phuah


                                           Supervisor:
                                     M. P. Ryan, W. E. Lee
                    Department of Materials, Imperial College London, SW7 2AZ



DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   THE AGR FUEL CLADDING                                                                                                            2

    − 15% of UK’s electricity is nuclear, mostly
      from the Advanced Gas Reactors (AGRs)
    − An AGR fuel assembly contains 36 fuel pins
    − Fuel pin is fuel pellets that are encapsulated
      in the fuel cladding




                                                                                                Fuel Cladding          Stainsby, AMEC (‘09)




                                                                 Fuel                  AGR Power Stations in the UK
                                                                 Pin                          British Energy website



                          AGR Fuel assembly
                                Springfields website
                                                                                                 Fuel Pellets

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   RADIATION EFFECTS ON FUEL CLADDING                                                                                   3

      − AGR fuel cladding made of austenitic
        20Cr/25Ni/Nb stainless steel
                                                                                         Cr depletion
      − Subject to neutron fluence:
      + ubiquitous point defects

                                                                                         Fe depletion



      + alloying elements have different
        diffusivity (DNi> DCr> DFe) cause
                                                                                         Ni enriched
        preferential segregation



  Okada et al (‘99)



                                                                                       STEM analysis on spent fuel cladding
            Radiation-Induced Segregation (RIS)                                                    Norris et al (‘87)


DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   CORROSION IN WET-STORAGE PONDS                                                                                                  4

  − Spent fuels stored in wet ponds                                                    1. Potential for metal dissolution
    demineralised water
  − gravitational deposition of sea-salt particles;
    trace 0.2ppm chloride (Cl-) and 0.3ppm
    thiosulphate (S2O32-)

                                                                                       2. Sustained hydrolysis :
                                                                                          M+Cl‾+H2O→
                                                                                          MOH+H++Cl‾
                                                                                                         Cl-       O2+2H2O+4e- → 4OH-


                                                                                                               M+         e-
                                                                                                                      M(s)
                                                                                       3. Intergranular Stress Corr. Cracking




               Spent fuel wet-storage ponds
                            Cook, Sellafield (‘08)
                                                                                               Corrosion Mechanism

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   OBJECTIVES                                                                          5



         − Quantify grain boundary chromium depletion in
           20Cr/25Ni/Nb austenitic stainless steel and its potential for
           corrosion in trace aqueous electrolyte environments.
         − Develop a thermal sensitisation protocol to produce
           discrete levels of chromium depletion (e.g: 18Cr, 15Cr, 12Cr,
           9Cr, 5Cr) from as-received AGR fuel cladding.
         − Compare experimental results to selected test performed at
           the National Nuclear Laboratory (NNL) active facilities.




DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   THERMALLY-INDUCED CHROMIUM DEPLETION                                                                                                  6

     − Thermal Sensitisation:                                                                                    Heat Duration (hour)
     + formation chromium-carbide precipitates                                                                  1 2 4 12 24 48
       (Fe,Cr)23C6 at elevated temperatures
     + Level of chromium-depletion a function of                                                          200
       heating temperature and time
                                                                                                          300




                                                                                       Temperature (°C)
                           Sourmail Mat. Sci. Tech. (2001)
                                                                                                          400
                                                                           α
                                                                                                                     Quantify chrom-
                                                                                                          500        depletion levels?
                                                                                                          600
            (Fe,Cr)23C6
                 sample
                                                                                                          700

                                                                                                          800
                                           furnace
                                                                                       Correlate discrete chromium-depletion
                                                                                       levels as a function of heating time and
                     Thermal Sensitisation
                DTA Instrument: Tfurnace = Tsample
                                                                                            temperature for AGR fuel clad

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   SPECIMEN PREPARATION                                                                                                        7

                 Scanning Electron Microscopy                                          Transmission Electron Microscopy

                                                electrolytic
                                                etching 6V for                  1.5mm
                                                10s in
                                                saturated
                                                oxalic acid
                                                                                         Focus Ion Beam (FIB) specimens by
                                                                                                 Gallium sputtering

                                                    Anodic Polarization Method

           Crevice free                                                                                     Three-electrode
               working                                                                                      cell and test
            electrodes                                                                                      solution that
                                                                                                            tailored Sellafield’s
                                                                                                            water chemistry




DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
GENERAL APPEARANCE                                                        8


Fixed Heating Time: 2 hours            Fixed Temperature: 900C
Variable Temperatures                  Variable Time
                                         (becomes
                  600°C                  magnetic)    24h          12h
                                           48h
          700°C

800°C



                               500°C
                       400°C                                             4h
                                                            2h
              300°C                              1h

Color change with increased            Decreased gloss over time
temperature
SEM ANALYSIS                                   9

                 as-received


− Highly strained
  matrix
− Small grains



                               50mm         20mm

                 1000°C, 2h


− Larger grains about                 NbC
  20μm across
− Occurrences of NbC
  precipitates.


                               50mm         20mm
TEM ANALYSIS                                                 10

          as-received             grain B




                                     grain A

− 10% grain boundary
  chrom-depletion
                                                            0.2mm


         1000°C, 2h                            grain A



                        grain B
                                                   gb A-C




                                            grain C

− Homogenous
                                                            0.2mm
   CORROSION POTENTIAL FOR AS-RECEIVED CLADDING                                                       11




                                                       0.7V
                                                 (less noble)

                                                                                          1.6V
                                                                                       (more noble)




DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal
   CONCLUSIONS & FUTURE WORK                                                           12



                − Conclusions:
                + Priority is correlate between grain-boundary chromium
                  depletion and thermal sensitisation for AGR fuel
                  cladding
                + experimental method developed enables timely
                  research objectives

                − Future Work:
                + compare with active samples to account for actual
                  radiation effects




DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal

				
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