Expected Lifetime of Ultrasonic Transducers in Gamma Fields

Transcription

1 Expected Lifetime of Ultrasonic Transducers in Gamma Fields Tony Sinclair Mechanical & Industrial Engineering University of Toronto Andriy M. Chertov Institute for Diagnostic Imaging Research University of Windsor

2 Outline Objective and Motivation Assumptions Gamma radiation susceptibility of individual transducer components: Failure modes of transducer assemblies Guidelines to manufacture radiation resistant transducers Neutron damage Conclusion

3 Objective & Motivation Inspection of reactor components, spent fuel: Gamma radiation is dominant concern Unexpected transducer failure can present major operational difficulties Very frequent replacement of transducers is expensive, time consuming Objective: Determine cumulative gamma dose lifetime for conventional ultrasonic transducer

4 Assumptions Dose rate is immaterial, damage depends only on cumulative dose (Grays) Gamma dose is measured with conventional dosimeters used for assessing biological damage. Conventional transducer design is used (with minor accommodation for radiation fields) Gamma ray spectrum is typical for irradiated reactor fuel, within first few months following reactor shutdown

5 Sources of Data National laboratory reports (Harwell, Idaho Natl Lab, ORNL, SCK CEN, ) Reactor operational experience (OPG, AECL, Kinectrics, ) Transducer manufacturers and their advertising (UTX, PCB Piezotronics, Meggit Endevco ) University studies Conferences & workshops

6 Transducer Components Piezoelectric element Plastics: ¼ wave layer; delay line; lens; wear plate Backing element Housing Cabling Bonding agent

7 Piezoelectric Element Correlate temperature resistance radiation resistance. Much piezomaterial radiation damage data are published. Avoid PZT above ~200 o C: Low Curie temperature and moderate radiation resistance lead to early depolarization Go to lead metaniobate at moderate temperatures and higher gamma fields ( o C) At even higher temperatures and fields: lithium niobate, aluminum nitride, or composites (but poor piezoelectric efficiency and higher cost) Piezoelement failure will NOT be a lifetime liming factor up to several MGy, but may necessitate recalibration

8 Gamma Damage Data: Lithium Niobate: T C 1200 o C No significant degradation at 100 MGy [R.W. Smith, Proc IEEE (Apr 1971)] Some decrease in d 33 at 40 MGy [S.L. Halverson et al, IEEE Trans on Nucl Sci. 17 (1970)] No effect from 4 MGy on accelerometers [R.L. Thomas, Intl Symp on Vibe prob in Ind., Kewick (1973)] Degradation at MGy [K.E. Holbert et al, IEEE Trans Nucl Sci, 52(2005)] Good resistance to 22 MGy [R. Kažys et al, IEEE Trans UFFC, 52 (2005)] No effect at 0.4 MGy [T.K. Bierney, Endevco Meggit report TP272, (1977)]

9 Plastic Components Plastics can be used in delay line, lens, wear plate, backing element composite, ¼ wave matching layer Some plastics (e.g. Teflon) are very radiation sensitive; some epoxies are very hardy. Extensive test data exists for plastics: e.g., ORNL TM 1175; CERN 72 7; Handbook of Radiation Effects by Holmes Siedle & Adams Radiation can cause cross linking, gas production, swelling, embrittlement, decomposition of plastics Build up of interfacial thermal stress, some efficiency loss, water ingress at ~1 2 MGy (conventional plastics)

10 Housing and Cabling/Connectors Radiation damage to SS or aluminum housing will not be a direct threat, but may imperil adjacent plastics Avoid electrical circuitry (e.g., impedance matching system) inside the transducer Radiation hardened cables are essential, due to susceptibility of plastics in conventional cables. Use mineral cable, SiO2 based cable (commercially available) These factors will not limit transducer lifetime

11 Inter layer Bonding Agent Inter layer stresses must be minimized by choice of thermally compatible materials Conventional epoxies weaken at high temperature & radiation damage. They can also swell, break down Alternatives (pressure bonding, brazing) bring technical challenges but are considered at extreme conditions Interfacial adhesive failure is the primary cause of catastrophic unexpected failure of commercial transducers. This is encountered at doses > 2 MGy

12 Neutron Fluence Effects Far less damage data is available on effects of neutron fluence on transducers Data are complex due to mixed presence of gamma and data radiation The dependence on neutron energy is not known. (for radiation, only absorbed energy/kg is considered relevant) Neutron effects are of limited interest to commercial nuclear power reactors for inspections conducting during shutdown

13 Conclusion Little published data on radiation endurance of ultrasonic transducers, or design of radiation hardened transducers Radiation intensity is immaterial for use up to 10 4 Gy/hr gamma, and 10 9 cm 2 s 1 neutron flux Radiation resistance is linked to temperature endurance Use radiation resistant materials, and thermally compatible, stable components relatively low cost modifications to traditional transducer design for high radiation doses Re calibrate for minor piezo element and plastic degradation Safe lifetime is ~2 MGy (with low neutron fluence)

14 Private Communication Sources Ken Chaplin: Atomic Energy of Canada (Chalk River, ON) Audrey Gardahaut: Commissariat à l'energie Atomique Ed Ginzel: Materials Research Institute (Waterloo, ON) Robert Ginzel: Eclipse Scientific (Waterloo, ON) Bill Hatcher: UTX Inc. (Holmes, NY) Keith Holbert: Arizona State University (Tempe, AZ) Alex Karpelson: Kinectrics Inc. (Toronto, ON) Shaddy Shokralla: Inspection Mtce Services, OPG (ON) Mike Trelinski: UT Fuel Channels, OPG (ON) Ed Waller: UOIT (Oshawa, ON)

15 Acknowledgement This project was funded by contract A TDS from the Canadian Nuclear Waste Management Organization (NWMO)