Measurement of in-reactor stress relaxation in pre- irradiated ... - … · 2019. 6. 26. · moisture free environment with hydrogen getter – Thermocouple attached to dummy samples

© 2019 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m

Measurement of in-reactor

stress relaxation in pre-

irradiated zirconium alloys by four-point bend technique

19th INTERNATIONAL SYMPOSIUM ON ZIRCONIUM IN THE NUCLEAR INDUSTRY

May 21, 2019

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© 2019 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m2

Acknowledgments

▪ Work performed during 2003-16 in the NFIR Program

▪ Coauthors:

Yagnik, Suresh (EPRI, USA)

Arimescu, Ioan (Framatome, USA)

Adamson, Ronald (Zircology Plus, USA)

Kobylyansky, Gennady (RIAR, Russian Federation)

Seryodkin, Sergey (RIAR, Russian Federation)

Obukhov, Alexander (RIAR, Russian Federation)

▪ Valuable discussions:

Ramasubramanian, Natesan (Eccatec, Inc)

NFIR Steering Committee members

http://www.epri.com/


Presentation Outline

▪ Objective and Approach

– Principle of four-point bend (4 PB) technique

▪ Experimental

– Loading fixture (‘Case’), ‘Relaxometer’, ‘Movable element’

– Materials investigated

▪ Results and Discussions

– In-reactor stress relaxation (SR) tests

– Out-reactor SR tests

– Post-test examinations

▪ Data Trends and Analyses

– Application of a creep law

▪ Key Conclusions



Creep and Stress Relaxation

▪ Objective:

– Elucidate irradiation creep behavior in Zr-alloys:

▪ What factors contribute to it?

▪ Does it increase at high fluence as does irradiation growth?

▪ What is the effect of pre-irradiation dose?

▪ Application of a creep law to stress-relaxation data

▪ Approach:

– Application of 4 PB technique to pre-irradiated specimens in and out of irradiation flux

▪ Ttest : 315 ± 5°C

▪ Pre-irradiation dose ranged from 0 (unirradiated) to 34 dpa

▪ Negligible incremental dose (~ 0.26 dpa) during maximum of 2600 hr exposure in RBT-6 reactor (Flux: 2 x 1017 n/m2/s; Dose rate: 3 x 10-8 dpa/s)



Principle of 4PB Technique For rectangular sample with 4PB loading, stress s is compressive at

inner fiber and tensile at outer fiber:h – sample thickness

L – length between inner supports

a – lengths between inner and

Outer supports

b – sample width

P – applied load

Maximum stress

ǀ𝝈ǀ=𝟔𝒂𝑷

𝒃𝒉𝟐

Deflection ya is related to sample

geometry and elastic modulus by

classical elastic beam theory

𝒚𝒂= 𝟐𝑷𝒂𝟐 𝟐𝒂+𝟑𝑳

𝑬𝒃𝒉𝟑

Eliminating P between the two Eqs:

𝝈 =𝟑𝒚𝒂𝑬𝒉

𝒂 𝟐𝒂 + 𝟑𝑳

The sample is bent to get desired range of 𝝈 ..and as the initial elastic strain is gradually converted to

permanent creep:

𝜖𝑐 =𝜎0 − 𝜎

𝐸



Experimental

Loading fixture (‘Case’):• Can apply 4 PB on up to 6

samples (size 35 x 6.5 x 0.8

mm)

• Is tested in-flux (RBT-6

reactor) and out flux

Movable element:• Applies load on a pair of

samples at a time at room

temp

• Load is applied through

‘Relaxometer’ (next slide)



Experimental (cont’d)

▪ Relaxometer:

– Applies a load measured by a load cell

– The load imparts 4 PB on the samples

▪ Schematic of load application on samples:

– 3 regions with downward motion of stressor toward sample loader



Experimental (cont’d)

▪ Typical Relaxometer data:

▪ Test rig:– Can accommodate up to 4 ‘Cases’ in a leak-tight

moisture free environment with hydrogen getter

– Thermocouple attached to dummy samples

– Controllable heating unit to maintain temp (Ttest : 315 ± 5°C)

– Similar setup for in- and out-reactor testing



Materials Investigated

▪ Most SR tests performed on pre-irradiated samples (4-34 dpa) – From a previous irradiation program in BOR-60 (See Yagnik et.al. STP-1597)

– Test matrix also included unirradiated archives (0 dpa)

– A total of 24 Zr-alloy variants tested: 18 in-reactor; 6 out-reactor

▪ RXA Zircaloy-2 variant served as a reference material with 10, 119, and 339 ppm uniform [H]

▪ High Fe Zircaloy-2 (Ziron) samples with 2 different textures (longitudinal and transverse)

▪ Also included Nb-containing alloys ‘Zirlo-like’ and ‘M5-like’:– One ‘M5-like’ sample had 119 ppm [H]

▪ Out-reactor tests included Nb-containing samples only



Results (SR data): Load

Change vs. Test Duration:

In-reactor

Out-reactor



Results: In-reactor (Load ratio vs. time plots)

Zr-2 RXA (w/o H charging)—various exposures▪ Pre-irradiation dose has very little

effect on the shape of SR curve

▪ More than half of the relaxation occurs in first 400 h

▪ A quasi steady state seems to have reached at longer time– Rate of decline in stress is considerably

lower (and almost linear)

▪ Note: Irradiation hardening is almost saturated at 4 dpa– Incremental dpa in RBT-6 (~ 0.26 dpa) is

small compared to the lowest pre-irradiation dose



Results: In-reactor (Load ratio vs. time plots);

Zr-2 RXA (with [H])—approx. same exposure▪ Hydrides have no effect on SR

▪ 70% cold work appear to have marginally higher load drop at short times

▪ The hardness of these variants should be about the same due to similar range of exposures

– Literature (Yagnik et. al.): hardness values for Zr-2 at 34 dpa: 277-294 Hv



Results: In-reactor (Load ratio vs. time plots)— contd.

‘M5 like’—different exposures and [H]

▪ Unirradiated (0 dpa) ‘M5-like’ variant has very high initial SR rate

▪ For pre-irradiated ‘M5-like’ variant with 119 ppm [H], initial SR rate increases by a small amount, in comparison with no pre-hydriding




‘Zirlo like’—different exposures ▪ Unirradiated (0 dpa) ‘Zirlo-like’ variant has very low initial SR rate

– Different behavior compared to ‘M5-like’ variant at 0 dpa(previous slide)

▪ The reasons behind this difference compared to ‘M5-like’ is not clear

▪ RXA Ziracloy-2 (prior slide) shows that SR rate didn’t depend on pre-irradiation dose



Results: In-reactor (Load ratio vs. time plots)

Comparison of SR rates:

Zircaloy-2, ‘Zirlo like’, and ‘M5-like’ at 23 dpa▪ Zircaloy-2 and ‘M5 like’ have rather

similar SR behaviors

▪ But ‘Zirlo-like’ material has much higher primary (i.e., initial) SR

▪ Both are RXA




‘Ziron’—different texture at 23 dpa▪ Texture has a significant effect

– Longitudinal Ziron (f=0.07) relaxes much slower than transverse sample (f=0.25)

– This unexpected effect of texture in sheet samples (compared to tube material) is not well understood



Results: Out-reactor (Load ratio vs. time plots)

‘M5 like’—0 and 17 dpa ‘Zirlo like’—different exposures

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 500 1000 1500 2000

Pt/

P0, re

l. u

nit

Duration, h

A10B 0 dpa A10L 17 dpa A10F 17 dpa

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 500 1000 1500 2000

Pt/

P0, re

l. u

nit

Duration, h

A20O 0 dpa A20N 12 dpa A20F 23 dpa

▪ For comparable materials and dpa, out-reactor SR rates are much lower than in-reactor ones



Post-test TEM: Dislocation characteristics



Post-test TEM: Dislocation images (examples)

Initial dislocations in unirradiated RXA Zr-2

loops at 23 dpa RXA Zr-2

(no H-charged)

loops at 23 dpa RXA Zr-2

(116 ppm [H])



Application of a phenomenological creep model

During SR tests, the total strain remains constant, but its elastic stain component 𝜺𝒆 decreases while creep strain component 𝜺𝒄 increases with time

A time-hardening equation for creep

strain rate can be described as

K and m are model parameters

𝒏 ≈ 𝟏 for low strain regimes

Further mathematical treatment yields

Parameters KE and m were fitted to experimental stress (or load) ratios of SR data and then

by using 𝜖𝑐 =𝜎0 − 𝜎

𝐸

Experimentally derived creep strain 𝜺𝒄 were calculated. They are plotted on the next slide.



Application of a phenomenological creep model— Cont’d

In-reactor data for Zircaloy-2 Out-reactor data: Nb-containing alloys



Key conclusions (SR behavior)

▪ An initial high SR rate is normally observed (primary transient stress decrease during the first 400 hours)

– It is in this regime that most differences between material variants are observed.

▪ Differences in longer-term quasi steady state SR rates are relatively small, with a few outliers.

▪ Generally, in-reactor SR rates are significantly higher than out-reactor ones for specimens of similar pre-irradiation fluences

– For both primary-transient, as well as for the long-term steady-state SR regimes.

▪ Among the four alloy types studied, ‘M5-like’, Zircaloy-2 and Ziron have similar SR rates, while ‘Zirlo-like’ has a higher initial transient rate.

▪ The pre-irradiation dose of specific materials does not significantly affect the SR rates

– Since -component dislocation loops exist at high dpa but not at low dpa, this indicates that loops do not influence the mechanism or magnitude of SR.



Key conclusions—(cont’d)

▪ For Zircaloy-2, 70% cold work has only a small effect on SR. Note that at > 4dpa the strength of RXA and CW Zircaloy-2 are much closer due to irradiation hardening effect.

▪ For Zircaloy-2, neither 116 nor 339 ppm hydrogen contents have any noticeable effect on SR rates. For ‘M5-like’, 119 ppm hydrogen content has only a small effect on the initial transient rate.

▪ Among pre-irradiated materials, a relatively high initial transient SR rate is observed for the ‘Zirlo-like’ alloy.

▪ Among unirradiated materials, a relatively high initial transient SR rate is observed for the ‘M5-like’ alloy.



Key conclusions—(cont’d)

▪ The creep strain was calculated from the SR data using a specific phenomenological model.

– For Zircaloy-2 samples the creep rate during the quasi steady state stage is in a relatively narrow range of (7–9) x 10-7 s-1

– The unirradiated Zircalo-2 sample shows the highest creep, in agreement with its largest magnitude of SR and known creep strengthening impact of fast fluence

▪ Based on relatively low applied stress (64-102 MPa), the use of stress exponent of 1 is justified, which implies that diffusion creep is likely most dominant creep mechanism in this study.



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Measurement of in-reactor stress relaxation in pre- irradiated ... - … · 2019. 6. 26. · moisture free environment with hydrogen getter – Thermocouple attached to dummy samples