Creep life simulations of EB welded copper overpack

Stefan Holmström, Anssi Laukkanen, Tom Andersson

Research output: Book/ReportReportProfessional

Abstract

The long term life predictions of copper overpack (sealed by EB welding in Finland) have previously been based on stress estimations that vary over a wide range, typically between 40-100 MPa. These values are usually not based on structural calculation including the EB-weld that increases the complexity of the stress state in the copper overpack. This report will attempt to pinpoint and simulate the stresses and strains developing in the copper overpack during its first decennia of repository service by advanced FEA simulations including the impact of the EB-weld. The main challenge of this work is the extrapolation of the creep strain response of OFP copper to the service relevant loads and temperatures. The uniaxial creep model is translated to a multiaxial constitutive equation form with adequate computational efficiency. The copper overpack strain and stress evolution has been simulated at up to 100 000 years at a conservative constant temperature of 80 °C with 14 MPa of external pressure. The results indicate rapid creep relaxation in the initial stages after the load has been applied followed by limited creep strain accumulation thereafter. Local elastic-plastic and creep deformation is predicted at the EB weld root with a total strain of below 12 %. The predicted stresses after external loading and short term relaxation are moderate and the impact of weld residual stresses and the lower creep rupture properties of the EB seem not to be detrimental to the predicted long term creep response. The simulation results imply that the most crucial impact on the creep strain accumulation of the copper overpack is related to the OFP copper primary creep properties. The present study predicts sufficiently low creep strains for a 100 000 years canister life with the conservative assumption at a constant temperature of 80 °C. However a sensitivity study on the impact of primary creep is strongly recommended due to contradicting analysis results from earlier FEA studies.
Original languageEnglish
PublisherPosiva
Number of pages30
Publication statusPublished - 2013
MoE publication typeD4 Published development or research report or study

Publication series

NameWorking Report
PublisherPosiva Oy
No.2012-96

Fingerprint

Creep
Copper
Welds
Finite element method
Computational efficiency
Constitutive equations
Extrapolation
Temperature
Loads (forces)
Residual stresses
Welding
Plastics

Keywords

  • EB weld
  • creep
  • FEA simulations
  • canister
  • copper
  • life prediction

Cite this

Holmström, S., Laukkanen, A., & Andersson, T. (2013). Creep life simulations of EB welded copper overpack. Posiva . Working Report, No. 2012-96
Holmström, Stefan ; Laukkanen, Anssi ; Andersson, Tom. / Creep life simulations of EB welded copper overpack. Posiva , 2013. 30 p. (Working Report; No. 2012-96).
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Holmström, S, Laukkanen, A & Andersson, T 2013, Creep life simulations of EB welded copper overpack. Working Report, no. 2012-96, Posiva .

Creep life simulations of EB welded copper overpack. / Holmström, Stefan; Laukkanen, Anssi; Andersson, Tom.

Posiva , 2013. 30 p. (Working Report; No. 2012-96).

Research output: Book/ReportReportProfessional

TY - BOOK

T1 - Creep life simulations of EB welded copper overpack

AU - Holmström, Stefan

AU - Laukkanen, Anssi

AU - Andersson, Tom

PY - 2013

Y1 - 2013

N2 - The long term life predictions of copper overpack (sealed by EB welding in Finland) have previously been based on stress estimations that vary over a wide range, typically between 40-100 MPa. These values are usually not based on structural calculation including the EB-weld that increases the complexity of the stress state in the copper overpack. This report will attempt to pinpoint and simulate the stresses and strains developing in the copper overpack during its first decennia of repository service by advanced FEA simulations including the impact of the EB-weld. The main challenge of this work is the extrapolation of the creep strain response of OFP copper to the service relevant loads and temperatures. The uniaxial creep model is translated to a multiaxial constitutive equation form with adequate computational efficiency. The copper overpack strain and stress evolution has been simulated at up to 100 000 years at a conservative constant temperature of 80 °C with 14 MPa of external pressure. The results indicate rapid creep relaxation in the initial stages after the load has been applied followed by limited creep strain accumulation thereafter. Local elastic-plastic and creep deformation is predicted at the EB weld root with a total strain of below 12 %. The predicted stresses after external loading and short term relaxation are moderate and the impact of weld residual stresses and the lower creep rupture properties of the EB seem not to be detrimental to the predicted long term creep response. The simulation results imply that the most crucial impact on the creep strain accumulation of the copper overpack is related to the OFP copper primary creep properties. The present study predicts sufficiently low creep strains for a 100 000 years canister life with the conservative assumption at a constant temperature of 80 °C. However a sensitivity study on the impact of primary creep is strongly recommended due to contradicting analysis results from earlier FEA studies.

AB - The long term life predictions of copper overpack (sealed by EB welding in Finland) have previously been based on stress estimations that vary over a wide range, typically between 40-100 MPa. These values are usually not based on structural calculation including the EB-weld that increases the complexity of the stress state in the copper overpack. This report will attempt to pinpoint and simulate the stresses and strains developing in the copper overpack during its first decennia of repository service by advanced FEA simulations including the impact of the EB-weld. The main challenge of this work is the extrapolation of the creep strain response of OFP copper to the service relevant loads and temperatures. The uniaxial creep model is translated to a multiaxial constitutive equation form with adequate computational efficiency. The copper overpack strain and stress evolution has been simulated at up to 100 000 years at a conservative constant temperature of 80 °C with 14 MPa of external pressure. The results indicate rapid creep relaxation in the initial stages after the load has been applied followed by limited creep strain accumulation thereafter. Local elastic-plastic and creep deformation is predicted at the EB weld root with a total strain of below 12 %. The predicted stresses after external loading and short term relaxation are moderate and the impact of weld residual stresses and the lower creep rupture properties of the EB seem not to be detrimental to the predicted long term creep response. The simulation results imply that the most crucial impact on the creep strain accumulation of the copper overpack is related to the OFP copper primary creep properties. The present study predicts sufficiently low creep strains for a 100 000 years canister life with the conservative assumption at a constant temperature of 80 °C. However a sensitivity study on the impact of primary creep is strongly recommended due to contradicting analysis results from earlier FEA studies.

KW - EB weld

KW - creep

KW - FEA simulations

KW - canister

KW - copper

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PB - Posiva

ER -

Holmström S, Laukkanen A, Andersson T. Creep life simulations of EB welded copper overpack. Posiva , 2013. 30 p. (Working Report; No. 2012-96).