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.

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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.

Copper lid & shell

Spent fuel

Cast iron insert

Figure 2 - Philips MCN 451 Figure 3 - Varian Linear Acc. M3

2.2 Results and discussion

The results from a 50 mm slice examined in axial direction are shown in Figure 4 and Figure 5. The

defects include small darker scattered indications in the bulk material (Figure 4a/c), indications and

their agglomerates near the channel surfaces Figure 4b, big casting blowholes and/or their clusters

adjacent to the steel channel bridging strips and delaminated areas between the matrix and steel

channels (Figure 5). The comparison of the indications found along the inner surfaces (Figure 4b) of

the channels with microstructure observations suggests that these indications correspond to inclusions

and inclusion agglomerates.

a b

c

Figure 4 - a - small inclusions scattered in the bulk of the cast iron

b - the inclusions along the interface with the steel channel and

c - cavities at the strips used fix mutual position of steel tubes during casting

The maximum size of blowhole located between the channels in Figure 5 is almost 17 mm. It

seems to be one hole rather than the agglomerate of several holes formed near the insert during

casting. Debonding between the matrix and steel channels is relatively often visible around the corners

of the channels. The results of radiographic examination of a 200 mm thick slice revealed in principle

the same defects – inclusions, debonding and blowholes, however, the contrast was considerably

lower. The largest blowholes observed were around 10 mm in diameter.

Figure 5 - Large blowhole adjacent to the bridging strip fixing neighboring steel channels and partial

debonding between the tubes and cast iron (see arrows) in 50 mm slice.

Radiographic testing of cut out segments after pressure loading is examined in such a way that

the source (Varian Linear Accelerator M3) is 2 m away from the outer surface and the radiographic

film is placed at the inner surface of the steel channel insert. Five branched cracks with axial

orientation and two cracks with radial orientation are identified in the segment shown in Figure 6.

Their length is up to 40 mm and two of them are surface breaking cracks.

Figure7 shows this piece in axial direction. Compared to Figure 6, optical density of the films is

in this direction approxiately constant due to constant thickness of the slice. Two relatively large

indications are found - clusters of inclusions, indicated as A and B, as well as at least two cracks near

the inner surface of the insert wall (see arrows). The detailed image in Figure 7b reveals also the

presence of numerous smaller inclusions under the inner surface (see dotted box) and another crack in

the corner of the channel (see arrow). The cracks present in this area of the insert wall seem to result

from the pressure test, because such cracks were never observed in the walls of the inserts, which were

in as-produced state. The crack and debonding around the corner could result from the residual

stresses due to different thermal contraction during cooling and scabs at the corners of the insert

channel, respectively.

Figure 6 - Radiographic image of the side wall of cut out segment with several large cracks. The zone

with the largest cracks in dotted box is cut for further studies. Large variations in optical density

resulted from the variation in wall thickness.

a

Radial direction

cracks

A B

b

Figure 7 - Image of cut out segment in axial direction.

a - two clusters of inclusions and two cracks (see arrows) are visible

b - magnified image of this piece after cutting and image enhancement reveals multiple

inclusions along the inner surface (insert) and several cracks at the inner surface and at the

channel corner (arrows)

3. ULTRASONIC LONGITUDINAL WAVE ATTENUATION MEASUREMENTS

3.1 Description of the sample

The sample used in this work is shown in Figure 8. The grey spot indicates the location where it was

cut from the ductile cast iron insert. The dimensions of the sample are 77.4 x 77.3 x 77.4 mm3. The

mass density of this material is ρ = 7059.8 Kg/m3. The ultrasonic phase velocities are determined by

measuring the time-of-flight between the first two consecutive echoes in pulse-echo configuration

yielding respectively 5651 m/s and 3105 m/s for the longitudinal and shear wave velocity. The

velocities for longitudinal and shear waves differ by less than 0.3% and 0.6% respectively in the

directions x, y, z, and, therefore, the macroscopic structure is characterised by isotropic symmetry. The

two independent elements of the stiffness matrix are thus c11 = 2.2545x1011

N/m2 and

c44 = 0.6806x1011

N/m2, which are required to calculate the reflection and transmission coefficients

that account for the energy losses caused by scattering at the boundaries of the specimen.

Figure 8 - Sample used for ultrasonic attenuation measurements and cut out location.

3.2 Ultrasonic data acquisition

A schematic representation of the experimental set-up used in this work is shown in Figure 9. Similar

experimental set-ups have been used to characterise the ultrasonic fields generated by immersion

transducers3-5

and to measure the scattering-induced attenuation of an ultrasonic beam in austenitic

steel6. In this set-up a flat, 12.7 mm diameter transducer is used to generate an ultrasonic beam in

water. A hydrophone of a needle-type design with a 0.635 mm diameter is used as a receiver. The

A B

x

y

z

Sample

location

position of the hydrophone can be changed in a plane that is parallel to the transmitter face and in the

direction normal to it by using a 3-axial motorised positioning stage. Each axis of the stage can cover a

maximum distance of 200 mm, by steps of 1 µm, and with repeatability of 1 µm. The stage is

connected to a computer-controlled motor controller.

Figure 9 - Experimental set-up for the ultrasonic data acquisition.

The experimental approach used in this work maps the ultrasonic field of the flat transmitter

twice by recording the pressure field first at the location where the front face of the sample is placed,

and then in the proximity of the sample’s back face. Each map of the beam consists of a matrix of

111x111 observation points. At each point the A-scan is averaged 50 times, sampled at 50 MS/s, and

digitised with 12 bits using a National Instruments PXI-5124 digitiser. Each digitised signal consists

of 1024 time samples. First neighbouring points are separated by a distance equal to the linear

dimension of the hydrophone, which is 0.635 mm. The scanned area is approximately 39 times larger

than the transducer’s surface area. By placing the sample at a distance of about 0.85 near-field lengths

calculated at a frequency of 2.25 MHz, this configuration allows the energy carried by the beam to be

recovered almost entirely. This feature of the scanning technique makes corrections for beam

diffraction unnecessary.

The automated data acquisition process is controlled by software which was developed with

National Instruments LabWindows/CVI. The software takes care of the initialisation and configuration

of the motor controller and digitiser, the triggering and synchronisation of operations as well as the

data transfer and storage.

3.3 Results and discussion

The approach used to evaluate longitudinal wave attenuation in ductile cast iron is called energy

approach. The energy approach accounts only for the energy loss due to ultrasonic scattering occurring

within the sample. Each A-scan acquired by the hydrophone is Fourier-transformed into the angular

frequency domain to obtain the spectrum of the incident and transmitted signals at each observation

point. The transformation yields a frequency-dependent, complex field, which can be further

represented in terms of plane waves to obtain the corresponding two-dimensional spectra in the

Fourier space domain. More details regarding the energy approach are given in reference 6.

Figure 10 shows the values of the longitudinal wave attenuation as a function of the frequency.

The attenuation exhibits a fairly linear behaviour in all 3 directions in the frequency range between

1 MHz and 4.5 MHz. The longitudinal wave attenuation in x-, y- and z-direction at 1 MHz is

21.1 dB/m, 21.8 dB/m and 18.5 dB/m, respectively, and at 4.5 MHz 123.6 dB/m, 111.5 dB/m and

83.2 dB/m. The measurements show that the attenuation in x- and y-direction is similar in behaviour

Water Sample

Transmitter Hydrophone

Pulser

5052 PR

XYZ-

Motor

Motor

Controller

Amplifier

Oscilloscope

IEEE-488

PXI

over the entire frequency range, but significantly lower in z-direction at higher frequencies.

Investigations are still ongoing to explain these variations.

Figure 10 - Longitudinal wave attenuation in ductile cast iron obtained in 3 different directions.

4. CONCLUSIONS

Radiographic inspections of as-produced cast iron insert mock-ups of nuclear waste storage canisters

were 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. Indications in the bulk material, indications

and their agglomerates near the channel surfaces, big casting blowholes and/or their clusters adjacent

to the steel channel bridging strips and delaminated areas between the matrix and steel channels were

detected. The comparison of the indications found along the inner surfaces of the channels with

microstructure observations suggests that these indications correspond to inclusions and inclusion

agglomerates. Several surface breaking cracks resulting from the pressure test were found in the zone

of high tensile stresses.

The longitudinal wave attenuation was measured by means of the so-called energy approach in

the frequency range between 1 MHz and 4.5 MHz in three different directions. The attenuation

measured in one of the directions was significantly lower at higher frequencies. Investigations to

understand and explain these observations are still ongoing.

REFERENCES

1. The Green paper on Energy “Towards a European Strategy for the security of energy supply",

Final report, June 2005, DG Energy and Transport, EC, Bruxelles, 2005. ISBN 92-894-8419-.

2. EUR Report “Ultrasonic and Radiography Investigations of Defects in Ductile Cast Iron Casks

for Disposal of Spent Nuclear Fuel”, DG JRC, EC, Luxembourg, 2006, EUR 22145 EN.

3. F.P. Higgins, S.J. Norton, M. Linzer, “Optical Interferometric Visualization and Computerized

Reconstruction of Ultrasonic Fields”, J. Acout. Soc. Am., 68 (4), 1980, pp. 1169-1176.

4. M.E. Schafer, P.A. Lewin, “A Computerized System for Measuring the Acoustic Output from

Diagnostic Ultrasound Equipment”, IEEE Trans. Ultrason. Ferroelec. Freq. Contr., 35 (2),

1988, pp. 102-109.

5. M.E. Schafer, P.A. Lewin, “Transducer Characterization using the Angular Spectrum Method”,

J. Acout. Soc. Am., 85 (5), 1989, pp. 2202-2214.

6. T. Seldis, C. Pecorari, “Scattering-Induced Attenuation of an Ultrasonic Beam in Austenitic

Steel”, J. Acout. Soc. Am., 108 (2), 2000, pp. 580-587.

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