Other Major Component Inspection I NDT of Ductile Cast Iron from Nuclear Waste Storage Canisters T. Seldis, EC-JRC-Institute for Energy, The Netherlands; F. Lofaj, Institute of Materials Research of the Slovak Academy of Science, Slovak Republic ABSTRACT The concept for geological disposal of nuclear waste and spent nuclear fuel relies on a multi-barrier system with the copper/cast iron canister as the first barrier. The canister is designed to retain its integrity for at least 100000 years, which means that future glaciations need to be considered. A thick ice block together with hydrostatic pressure from groundwater would produce hydrostatic compressive stresses of maximum 44 MPa. A critical issue for the acceptance of the canister is to guarantee that it does not contain defects that may cause loss of integrity during design life time. Radiographic inspections of as-produced cast iron insert mock-ups of the canister were carried out to check the presence of manufacturing defects. Numerous indications were found both inside and outside the critical zone of tensile stresses. Mock- ups subjected to a hydrostatic compressive stress up to 130 MPa were inspected as well, and several cracks were detected in the deformed canister walls. The largest defects located in the zone of tensile stresses – slag inclusions and their agglomerates – were deemed to be critical for crack initiation. The inspection of the canisters by means of ultrasound is another useful test to assure the compliance with predefined acceptance criteria for critical defect sizes. Reliable ultrasonic inspections, however, require a good understanding of the beam’s behaviour within the inspected material. Among the physical parameters characterising the interaction of the beam with its supporting medium, ultrasonic attenuation is important because it limits the volume of the system that should be inspected, and is an input parameter for mathematical models, which play an increasingly role in non- destructive testing by allowing computer simulations. Measurements of the intrinsic longitudinal wave attenuation in as-produced cast iron were carried out in 3 different directions and first results are reported in this paper. 1. INTRODUCTION Limited reserves of fossil fuels, global warming and insufficient output of renewable energy resources coupled with soaring demand for energy and rapid increase of energy price result, after more than two decades of stagnation, into renewed interest in nuclear energy 1 . One of the major challenges for this technology is the safe and cost effective geological disposal of radioactive waste and spent fuel. Despite long-term research and technological activities in this area, only USA, Sweden and Finland are close to applying for a license for repositories 2 . The Scandinavian concept for deep disposal, known as KBS-3, is based on copper shielded canisters with a ductile cast iron insert with channels for the radioactive waste and spent fuel assemblies. The diameter of the canister is about 1 m, its length is almost 5 m and the total weight is up to 27 tons. The copper shell is corrosion resistant and the cast iron insert shall provide mechanical strength for a safe containment of radionuclides for at least 100000 years. In the repository the canisters will be loaded in compression by both hydrostatic and swelling pressure from the surrounding bentonite, giving a total pressure of 14 MPa. Several ice ages are expected with a maximum ice sheet of 3 km, which results in an additional pressure of 30 MPa. The maximum design pressure for the KBS-3 canisters is therefore 44 MPa. Figure 1 shows a schematic illustration of the KBS-3 canister. It consists of an outer shell and an insert with twelve quadratic channels for the radioactive waste and spent fuel assemblies. The shielding of the canister is a 50 mm thick copper tube with inner diameter of 952 mm. Top and bottom openings are closed by 48 mm thick steel plates and the copper lids are welded to the tube to ensure leak-tightness. The material for the insert is ductile cast iron grade EN-GJS-400-15U. For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components October 2007, Budapest, Hungary
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NDT of Ductile Cast Iron from Nuclear Waste Storage Canisters
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Other Major Component Inspection I
NDT of Ductile Cast Iron from Nuclear Waste Storage Canisters T. Seldis, EC-JRC-Institute for Energy, The Netherlands; F. Lofaj, Institute of Materials Research of
the Slovak Academy of Science, Slovak Republic
ABSTRACT
The concept for geological disposal of nuclear waste and spent nuclear fuel relies on a multi-barrier
system with the copper/cast iron canister as the first barrier. The canister is designed to retain its
integrity for at least 100000 years, which means that future glaciations need to be considered. A thick
ice block together with hydrostatic pressure from groundwater would produce hydrostatic compressive
stresses of maximum 44 MPa.
A critical issue for the acceptance of the canister is to guarantee that it does not contain defects
that may cause loss of integrity during design life time. Radiographic inspections of as-produced cast
iron insert mock-ups of the canister were carried out to check the presence of manufacturing defects.
Numerous indications were found both inside and outside the critical zone of tensile stresses. Mock-
ups subjected to a hydrostatic compressive stress up to 130 MPa were inspected as well, and several
cracks were detected in the deformed canister walls. The largest defects located in the zone of tensile
stresses – slag inclusions and their agglomerates – were deemed to be critical for crack initiation.
The inspection of the canisters by means of ultrasound is another useful test to assure the
compliance with predefined acceptance criteria for critical defect sizes. Reliable ultrasonic
inspections, however, require a good understanding of the beam’s behaviour within the inspected
material. Among the physical parameters characterising the interaction of the beam with its supporting
medium, ultrasonic attenuation is important because it limits the volume of the system that should be
inspected, and is an input parameter for mathematical models, which play an increasingly role in non-
destructive testing by allowing computer simulations. Measurements of the intrinsic longitudinal wave
attenuation in as-produced cast iron were carried out in 3 different directions and first results are
reported in this paper.
1. INTRODUCTION
Limited reserves of fossil fuels, global warming and insufficient output of renewable energy resources
coupled with soaring demand for energy and rapid increase of energy price result, after more than two
decades of stagnation, into renewed interest in nuclear energy1. One of the major challenges for this
technology is the safe and cost effective geological disposal of radioactive waste and spent fuel.
Despite long-term research and technological activities in this area, only USA, Sweden and Finland
are close to applying for a license for repositories2.
The Scandinavian concept for deep disposal, known as KBS-3, is based on copper shielded
canisters with a ductile cast iron insert with channels for the radioactive waste and spent fuel
assemblies. The diameter of the canister is about 1 m, its length is almost 5 m and the total weight is
up to 27 tons. The copper shell is corrosion resistant and the cast iron insert shall provide mechanical
strength for a safe containment of radionuclides for at least 100000 years. In the repository the
canisters will be loaded in compression by both hydrostatic and swelling pressure from the
surrounding bentonite, giving a total pressure of 14 MPa. Several ice ages are expected with a
maximum ice sheet of 3 km, which results in an additional pressure of 30 MPa. The maximum design
pressure for the KBS-3 canisters is therefore 44 MPa.
Figure 1 shows a schematic illustration of the KBS-3 canister. It consists of an outer shell and
an insert with twelve quadratic channels for the radioactive waste and spent fuel assemblies. The
shielding of the canister is a 50 mm thick copper tube with inner diameter of 952 mm. Top and bottom
openings are closed by 48 mm thick steel plates and the copper lids are welded to the tube to ensure
leak-tightness. The material for the insert is ductile cast iron grade EN-GJS-400-15U.
For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70
6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized ComponentsOctober 2007, Budapest, Hungary
Figure 1 - Schematic illustration of KBS-3 canister.
The integrity of the canisters requires that defects, inherently present in the ductile cast iron
inserts, are smaller than critical defects that could cause canister failure. Radiographic inspections of
as-produced cast iron insert mock-ups of the canister are carried out to check the presence of
manufacturing defects. Mock-ups subjected to a hydrostatic compressive stress up to 130 MPa are
inspected as well.
Furthermore, reliable inspections by means of ultrasound require a good understanding of the
beam’s behaviour within the inspected material. Among the physical parameters characterising the
interaction of the beam with its supporting medium, ultrasonic attenuation is important because it
limits the volume of the system that should be inspected, and is an input parameter for mathematical
models, which play an increasingly role in non-destructive testing by allowing computer simulations.
Measurements of the intrinsic longitudinal wave attenuation in as-produced cast iron are carried out in
3 different directions and first results are reported here.
2. RADIOGRAPHIC INSPECTIONS
2.1 X-ray sources and sensitivity
Radiographic inspections utilise two different X-ray sources:
• In radial direction a Philips MCN 451 covering the range of energies up to 450 kV (Figure 2).
• In axial direction a Varian Linear Accelerator M3 with energies of 1 MeV and 3 MeV
(Figure 3).
Radiographic films are Agfa Structurix D2, D4, and D7, with the size of 150 mm x 240 mm, the
useful optical film density range after exposure was 2.5 – 3.5 units of the optical density.