Effects of chemistry and microstructure on corrosion performance of Zircaloy-2 based BWR cladding

Transcription

1 Effects of chemistry and microstructure on corrosion performance of Zircaloy-2 based BWR cladding Yang-Pi Lin, David White, Dan Lutz, Global Nuclear Fuel Americas ASTM 18th International Symposium on Zirconium in the Nuclear Industry, Hilton Head, South Carolina, May, 2016

2 Introduction Cladding for Boiling Water Reactors (BWR) Zircaloy-2 in use since early days Improvements made over the years, e.g. to address corrosion performance Based on laboratory testing and in-reactor performance feedback, e.g. Crud Induced Localized Corrosion (CILC) reactor water chemistry effect Some plants never had corrosion issues corrosion environment is not the same for all plants and times Cladding Improvements Optimizing size of Second Phase Particles (SPP) Optimizing cladding chemistry specification Within ASTM UNS R60802 (Zircaloy-2) Outside of ASTM limits 2

3 Present Work Integrated look at cladding chemistry and SPP size Aimed at enhanced corrosion resistance across the BWR fleet Combined effects of chemistry and SPP size through cladding process Cladding Chemistry Microstructure Laboratory Testing In-reactor Performance 3

4 Chemistry Sampling Hot working Beta-quench Extrusion Cold Reductions Ingot Billets Tubeshell Tubes Certification Sampling Additional Sampling Sampling at tubeshellstage more representative of final tested tubes Fe ~ wt% -extended range Cr ~ wt% Ni ~ wt% Sn ~ wt% 4

5 Cladding Types Type A -no special outer surface heat treatment (4 recrystallization anneals subsequent to beta-quench) With outer surface treatment (inside cooling) after first cold reduction (2 recrystallization anneals subsequently) Type B -temperature in α + βrange Type C -temperature in βrange 5

6 Laboratory Testing ASTM G2 400 C and 10.3 MPa for 24 hrs Two-stage Commonly referred to as uniform corrosion test 410 C 4 hrsfollowed by 520 C 16 hrsboth at 12 MPa Developed for assessing nodular corrosion susceptibility, with link to in-reactor performance Modern cladding generally do not show nodules Used here as a more aggressive environment for forming uniform oxide 6

7 In-reactor Performance Focus on Type B and C cladding Type B Type C US reactor to discharge 52 GWd/MTU bundle average European reactor to 68 GWd/MTU US reactor to discharge 52 GWd/MTU plus extended exposure to 62 GWd/MTU 7

8 Cladding Characterization Zircaloy-2 Type A Zircaloy-2 Type B Zircaloy-2 Type C GNF-Ziron Type C Type A has coarsest SPP; Type C has finest 8

9 Cladding Characterization SPP distribution Log-normal SPP distribution sensitive to process types Not sensitive to chemistry (GNF-Ziron has higher Fe than Zry2) 9

10 Corrosion Testing Weight Gain Isolated nodules in some Type A samples ASTM G2 400 C 10.3 MPa 24 hours 12 MPa 4 hrsat 410 C plus 16 hrsat 520 C No sensitivity to Fe or process type in 400 o C test Two-stage 410/520 o C test reveals higher weight gain for low Fe, and for Type A. Generally no nodules except in some Type A samples with low Fe 10

11 Corrosion Testing - Nodules 12 MPa 4 hrsat 410 C plus 16 hrsat 520 C Two-stage test typically used to assess presence of nodules Modern cladding generally do not develop nodules in this test, Except Type A with low Fe 11

12 In-reactor Performance - Poolside Eddy-current liftoff measurements and visual examinations Eddy-current liftoff measurements Bundle Average Exposure Visual examinations 12

13 In-reactor Performance - Hotcell 20 µm Epoxy Zircaloy-2 Cladding cross-section from 1.5 2m axial elevation Type B > Type 52GWd/MTU Type C 62 > 52 GWd/MTU Extended exposure Typical end of life Type B Cladding crosssection from ~2.6m axial elevation 13

14 TEM of Type C cladding at End-of-Life 65 GWd/MTU local exposure (52 GWd/MTU bundle average) 80 GWd/MTU local exposure (62 GWd/MTU bundle average) ~2.8 m axial elevation Extensive SPP dissolution Amorphous Cr-rich remnants Ni-Fe remnants Near absence of SPPs or remnants Occasional Zr 3 Si and Ni-Fe remnants Change associated with increased corrosion [Takagawa ASTM STP 1467 ] with increased corrosion with Cr remnants reported Valizadeh ASTM STP 1529 ] 14

15 Summary Integrated look at composition and microstructure in Zircaloy-2 based BWR cladding, with Fe content exceeding ASTM limit and three different cladding microstructures (SPP size distribution) Modern cladding has adequate corrosion resistance in laboratory tests and in-reactor to end-of-life A more aggressive corrosion test condition is needed to reveal higher corrosion weight gains for cladding with lower Fe and coarser SPPs Increased corrosion in extended exposure likely related to advanced SPP dissolution dispersal of Cr-remnants 15

16 Ackowledgement EPRI Fuel Reliability Program for co-sponsoring some of the hot cell work. GNF ChemLabteam for conducting corrosion tests and inspection team for poolside inspections The Vallecitosand Studsvikteams for the hot cell work Y. Takagawaof NFD for conducting the TEM work. 16

17 Effects of chemistry and microstructure on corrosion performance of Zircaloy-2 based BWR cladding Yang-Pi Lin, David White, Dan Lutz, Global Nuclear Fuel Americas ASTM 18th International Symposium on Zirconium in the Nuclear Industry, Hilton Head, South Carolina, May, 2016