Modeling Irradiation Damage 2.5Nb and its Effects on Delayed Hydride Cracking Growth Rate

Transcription

1 Modeling Irradiation Damage in Zr Zr-2.5Nb 2.5Nb and its Effects on Delayed Hydride Cracking Growth Rate Grant A. Bickel, M. Griffiths, H. Chaput, A. Buyers, and C.E. Coleman 2012 February th International Symposium on Zirconium in the Nuclear Industry

2 Outline Zr-2.5 Nb pressure tubes are exposed to widely varying irradiation conditions while in service within a CANDU reactor. The state of the microstructure evolves in response to the irradiation conditions and can be monitored by X-ray diffraction (XRD). Pressure tube properties depend on the microstructure, e.g., Delayed hydride crack (DHC) growth rate. Models to predict the state of the microstructure based on irradiation conditions are required. 2

3 CANDU reactor fuel channel Feeder pipe Pressure tube Calandria tube 3

4 CANDU reactor fuel channel Fast neutron flux Temperature Flow Feeder pipe Pressure tube Calandria tube 4

5 Fast neutron flux near end-fitting of a CANDU reactor pressure tube End fitting Rolled joint Pressure tube 5

6 Max. strain ~5.1% (flow by-pass) Diametral expansion as reactor ages T decreases, decreases Crept PT Original PT Fuel Bundle T increases, static 6

7 Microstructure of Zr-2.5Nb pressure tubing Beta-phase Alpha-phase Platelet hcp alpha-grains containing dislocations surrounded by a network of bcc beta-phase filaments Dislocation structure in the alpha phase is a mixture of a- and c-type, of mixed edge/screw character 7

8 Slip planes and dislocation Burgers vectors a-type dislocations distort prism planes primarily c- and c+a-type dislocations distort basal planes primarily il X-ray diffraction line- broadening used to measure relative dislocation densities 8

9 Radiation damage affects dislocation structure Beta phase Dislocation Loop 50 nm 9

10 Thermal decomposition of β-phase during pressure tube fabrication 0.1 m As-extruded: Single bcc -phase containing 20 wt% Nb. Finished autoclaved tube: Nb depleted, hcp -phase embedded in a Nb enriched -phase phase (20-50 (20 50 wt% Nb) 10

11 Beta-phase state in Zr-2.5Nb pressure tubes after irradiation Un-irradiated Irradiated 0 1 m 0.1 Neutron induced dissolution of the -phase competes with the thermal induced growth of the -phase phase and leads to steady state composition within beta filaments. State of the filaments monitored by XRD: shift of -phase phase diffraction peaks due to Nb enrichment change on volume fraction of -phase 11

12 Delayed hydride crack growth rates Comparison of XRD results with measured DHC growth rates (in longitudinal direction of tube) for specimens from various locations on a pressure tube removed after a service life of 15 years. Dislocations harden alphaphase matrix and increase DHC growth rate. State of beta-phase affects hydrogen diffusion to crack tip. 12

13 Delayed hydride crack growth rates Comparison of XRD results with measured DHC growth rates in radial (through wall direction of tube) State of the microstructure can be used to predict the DHC growth rate Destruction of pressure tube required to measure microstructure Need models to predict state of the microstructure 13

14 Modelling framework Mechanical properties are modelled as a response to the microstructure The microstructure is modelled as a response to the manufacturing process and the operating conditions 14

15 Developing model dependencies for body of tube ERABLE/OSIRIS: fast flux (> 1MeV) = 10-18x10 17 n/m 2 /s CANDU ex-service tubes: fast flux (> 1MeV) = 0.1-4x10 17 n/m 2 /s Temperature = 250 CC Dislocation density is fluence (dose) dependent and independent of flux (dose rate) down to 0.1x10 17 n/m 2 /s 15

16 Developing model dependencies for body of tube Irradiation increases dislocation density. Temperature is a co-variate. Increasing the temperature t reduces the dislocation density for the same neutron dose. 16

17 Developing model dependencies for body of tube A simple quadratic surface fits the integral breadth as a function of fluence and temperature. 17

18 Developing model dependencies for body of tube ERABLE/OSIRIS data at constant temperature. Nb concentration of beta-phase approaches steady state more slowly than integral breadth of alpha-phase. 18

19 Developing model dependencies for body of tube Nb concentration in the beta phase does depend on flux (dose rate). Increased neutron dose rate reconstitutes -phase (decreases Nb concentration). 19

20 Developing model dependencies for body of tube Irradiation decreases Nb concentration in beta-phase (reconstitution). Temperature is a co-variate. Increasing the temperature causes decomposition of the beta-phase for same neutron dose rate. 20

21 Developing model dependencies at end of tube At inlet, integral breadth and Nb concentration in -phase do not follow trends established for body of tube. 21

22 Developing model dependencies at end of tube At outlet, integral breadth does not follow trend established for body of tube. 22

23 Developing model dependencies at end of tube Inlet: Nb and volume fraction of -phase do not correlate between 100 and 300 mm from inlet. Nb concentration is below initial value. Suggests that other factors are affecting bcc lattice parameter. Outlet: Nb is correlated with volume fraction of -phase. Suggests Nb concentration is an appropriate metric of -phase decomposition. 23

24 Summary The properties of Zr-2.5Nb are dictated by the microstructure. Microstructure is controlled by manufacturing and is further modified by the in-service conditions High temperature Neutron irradiation Modelling/predicting evolution of microstructure is necessary for assessing the condition of operating pressure tubes Delayed hydride crack growth rates, for example, are affected by the microstructure accordingly (see paper for more details) 24

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