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Equivalent Radiation Damage in Zr-Alloys Irradiated in Various Reactorsby L. Walters, S.R. Douglas and M. Griffiths

18th International Symposium on Zirconium in the Nuclear Industry

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• Over the past 40 years, CNL has conducted numerous material irradiation experimental programs in a variety of test reactors to obtain data on CANDU®1

reactor materials, including Zr-2.5Nb.

• Initially, experiments performed in the NRX and NRU reactors were used to develop and validate predictive models for deformation, fracture, corrosion, and hydrogen ingress.

• More recently, offshore reactors which have higher total neutron fluxes than a power reactor have been used to characterize reactor material behaviour long before the end of the reactor design life.

1CANDU is a registered trademark of Atomic Energy of Canada Limited.

Introduction

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Experiments to predict deformation of CANDU reactor core components:

• OSIRIS – accelerated tests in a high neutron flux

~2 x 1014 n/cm2/s E > 1 MeV

• NRU – tests at CANDUequivalent flux ~2 x 1013 n/cm2/s E > 1 MeV

Introduction - Irradiation Tests in OSIRIS and NRU

• To use test reactor data to determine the effect of radiation damage on power reactor materials – must convert the accumulated dose into a unit that is common in its effect on the material properties.

• This presentation summarizes a comparison of radiation damage in Zr irradiated in various test reactors and power reactors.

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Neutron Spectra in Power Reactors• Each nuclear reactor has a specific neutron flux spectrum.

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• Neutron flux spectrum is different depending on location in the core.

Neutron Spectra in CANDU Reactor

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Neutron Spectra in BWR and PWR

J.E. White et al., “Production and Testing of the Revised VITAMIN-B6 Fine-Group and the BUGLE-93 Broad-Group Neutron/Photon Cross-Section Libraries Derived from ENDF/B-VI.3 Nuclear Data”, NUREG/CR-6214, Rev.1, ORNL/TM-6795/R1, February 2000.

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Neutron Spectra in Various Test Reactors• Each reactor has its own total flux that corresponds to its

power level.

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Radiation Damage - PKA• Radiation fields induce displacement of the atoms within the crystal lattices of

all the metallic structural components.

• The principle behind displacement damage created by direct collisions of atoms with neutrons of given energies is momentum transfer.

• If the energy of the incoming neutron is above the displacement threshold energy Ed, this collision will set the target atom (known as the primary knock-on atom or PKA) in motion.

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• The recoil energy of the PKA is transferred to nearby nuclei which, in turn, recoil and are displaced from their lattice positions. Each is able to participate in secondary collisions.

• A collision cascade develops involving hundreds of displaced atoms, the number depending statistically on the initial PKA recoil energy.

Radiation Damage - Cascade

• Most of the mobile interstitials recombine at nearby lattice vacancies.

• Thought that 1 to 10% are available as freely migrating point defects.

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The displacement function ν(T) depends on T which is the energy transferred to the target atom and Ed (~40eV for Zr) and takes into account energy lost Q by the PKAby processes other than displacements

ν(T)= 0 when T<Ed

ν(T)= 1 when Ed<T<2Ed

ν(T)= κ(T-Q)/2Ed when T>2Ed

Where κ is the displacement efficiency equal to approximately 0.8 derived byNorgett, Robinson, Torrens (NRT) by means of a simple binary collision model. It was also assumed to be independent of target and temperature for all materials.

Radiation Damage – Displacement Function

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• To determine radiation damage in a reactors we obtain calculated neutron spectrum data and corresponding damage cross sections (t2.lanl.gov; www-nds.iaea.org).

• Damage cross sections describe the relative probability of causing an atomic displacement.

• Damage energy rate in eV/s is: R= Σg φg σg.

• Multiplying R by 0.8/2Ed gives the displacement rate dpa/s.

Radiation Damage – Displacement per Atom

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Radiation Damage - DPA

Damage energy cross sections for Zr from ENDF/B-V

dpa calculated using SPECTER code (www-nds.iaea.org)L.R. Greenwood and R.K. Smither, “SPECTER: Neutron Damage Calculations for Materials Irradiations”, ANL/FPP/TM-197, 1985 January

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DPA and Fast Flux

• Best to describe accumulated damage as the material senses it, i.e., displacements per atom.

• Often fast neutron flux E > 1 MeV is used as the basic input for empirical models.

CANDU pressure tube deformation equation where φ = flux E > 1 MeVHolt, R.A., “In-Reactor Deformation of Cold-Worked Zr-2.5Nb Pressure Tubes”, J. Nucl. Mater., Vol. 372, 2008, pp. 182-214.

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Relative Damage in Various Reactors

• A conversion of the relative damage per unit flux above 1 MeV is needed to compare irradiation damage in one reactor to another.

E > 1 MeV E > 0.1 MeV E > 1 keV

CANDU PT 59% 95% 100%

EBRII 29% 94% 100%

HFIR PTP 67% 97% 100%

The table shows the percentage of dpa calculated using only part of the spectrum compared to the dpacalculated using the spectrum from 1x10-9 to 10 MeV.

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Comparison of Radiation Damage for Zirconium

in Different Reactors for E > 1 MeV

dpa/fluenceE > 1 MeV

n·cm-2 1x1022

relative to CANDU

pressure tube

relative to

PWR core

CANDU Pressure Tube 16.87 1.000 1.093BWR core (Boiling Water Reactor) 16.06 0.952 1.040PWR core (Pressurized Water Reactor) 15.43 0.915 1.000RBMK site 33 H=1.9 m 19.52 1.157 1.265ATR A09 (Advanced Test Reactor) 11.43 0.677 0.741ATR A10 16.24 0.963 1.053DIDO B5 16.81 0.996 1.089BOR-60 6th row 20.49 1.215 1.328SM-2 C4W 17.18 1.018 1.113OSIRIS 15.29 0.906 0.991EBRII 8D5 (Experimental Breeder Reactor) 30.88 1.830 2.001HFIR PTP (Peripheral Target Position) 15.06 0.892 0.976HFIR RB (Removable Beryllium) 16.74 0.992 1.084Halden Boiling Water Reactor 13wt% booster fuel 15.06 0.893 0.976NRU Mk4 Fast Neutron Rod 16.35 0.969 1.059

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Comparison of Radiation Damage for Zirconium

in Different Reactors for E > 0.1 MeV

dpa/fluence E > 0.1 MeV

n·cm-2 1x1022

relative to CANDU

pressure tube

relative to

PWR core

CANDU Pressure Tube 6.81 1.000 0.910BWR core (Boiling Water Reactor) 7.20 1.057 0.962PWR core (Pressurized Water Reactor) 7.48 1.099 1.000RBMK site 33 H=1.9 m 6.38 0.937 0.852ATR A09 (Advanced Test Reactor) 8.51 1.249 1.137ATR A10 6.93 1.017 0.925DIDO B5 6.98 1.025 0.933BOR-60 6th row 6.05 0.889 0.809SM-2 C4W 6.89 1.011 0.920OSIRIS 7.31 1.073 0.977EBRII 8D5 (Experimental Breeder Reactor) 5.14 0.755 0.687HFIR PTP (Peripheral Target Position) 7.52 1.104 1.004HFIR RB (Removable Beryllium) 6.40 0.940 0.856Halden Boiling Water Reactor 13wt% booster fuel 7.50 1.102 1.002NRU Mk4 Fast Neutron Rod 7.05 1.035 0.942

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Point Defect Fluxes to Sinks as a Function

of Temperature and Flux• At low temperatures

(<200 °C) there will be an effect for Zr in an accelerated test if the vacancy migration energy is ~1 eV.

• Linear dependence on point defect production rate (neutron flux) for Zrat 300 °C

sink strength (dislocations)=2E14, 30% dislocation bias, 10% damage efficiency, recombination parameter = 12Di, HFIR = 10-6 dpa.s-1 NRT, CANDU = 10-7 dpa.s-1 NRT.

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• To use test reactor data to determine the effect of radiation damage on power reactor materials, one must convert the accumulated dose into a unit that is common in its effect on the material properties.

• For many property changes in nuclear reactor cores, this unit is displacements per atom (dpa).

• If experimental data is provided as a function of fast fluence E > 1 MeV or E > 0.1 MeV then the data must be converted to values consistent with the full spectrum of the power reactor for use in radiation damage models.

Summary

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