STUDY ON THE APPLICABILITY OF CAST STEEL OVERPACK-EVALUATION OF CASTING DEFECTS AND CORROSION RESISTANCEUSING FULL-SCALE PROTOTYPEYusuke Ogawa, Satoru Suzuki, Kazuhisa Yamashina, Shigeru Kubota
Nuclear Waste Management Organization of Japan (NUMO)
2F, Mita NN Bldg, 4-1-23, Shiba, Minato-ku, Tokyo, Japan
Key words; Geological disposal, Overpack, Cast steel, Non-destructive test, Corrosion, Polarization tests
Abstract
The corrosion resistance of cast steel overpack, which may be effective to reduce manufacturing cost ascompared with forged steel overpack, was examined. The specimens were taken from a cast steel overpack
manufactured on a full-scale to reproduce realistic distributions of microstructure, chemical composition andcasting defects. Consequently, there were only negligible heterogeneities of distributions of microstructure andchemical composition and no difference of corrosion behavior between specimens. On the other hand, thecasting defects showed the heterogeneous distribution and concentrated to the lower half of overpack. Thesecasting defects were distributed in the region including the corrosion allowance layer. Thus, the influences ofthese defects on the corrosion allowance and the treatment in the design of thickness of overpack were discussed.
1. Introduction
In Japanese geological disposal concept, high-level
radioactive waste (HLW) is contained in a metal
container, referred to as overpack. Overpack isrequired to have containment for at least 1,000 years,
and the reference material is carbon steel [1].
Carbon steel materials are classified such as forgedsteel, cast steel, cast iron and rolled steel. The
promising material will be selected by evaluatingsuch as corrosion resistance, structural integrity andmanufacturability. Cast iron has difficulty of welding,and the rolled steel has difficulty in the application tothe overpack whose thickness is 190 mm [1]. Cast
steel is seemed to be better than the above twocandidates, however there are concerns about thepresence of casting defects and the heterogeneity ofmetallographic structure and chemical composition,
which may adversely affect corrosion resistance andstructural integrity. On the other hand, forged steel
has few defects, and has been used as a pressure
vessel material such as a light water reactor, and has
high reliability. Therefore, research and developmentof overpack so far has been carried out mainly onforged steel as the primary candidate material.
However, for reducing manufacturing cost, caststeel overpack may be effective as compared with
forged steel overpack. This is because in the
manufacturing of forged steel products, an ingot is
produced from molten steel and then formed into a
product shape by pressing or hammering, in contrast,
cast steel products are manufactured with fewer steps
as molten steel is cast in a mold near a product shape.
Hence, we are now examining the applicability ofcast steel to overpack by investigating the technicalconcern as mentioned above.
First of all, the relationship between corrosion
resistance and metallographic microstructure andchemical composition of cast steel which may
distribute heterogeneously in the products has been
investigated. Whether casting defects occurred in the
corrosion allowance has also been confirmed. To
reproduce realistic distributions of metallographic
structure, chemical composition and casting defects, afull-scale cast steel overpack was fabricated. Then,microstructure observation, chemical composition
analysis and corrosion tests using specimens takenfrom various positions in the cast steel overpack werecarried out to evaluate the heterogeneity of corrosion
resistance depending on the position of the overpack.
Furthermore, it was confirmed by non-destructive
testing whether casting defects occurred in the
corrosion allowance, and the required additional
corrosion allowance was considered if casting defects
occurred.
2. Experimental method
2.1 Fabrication of full-scale cast steel overpack
Fig. 1 shows the dimensions of the manufactured
overpack. The dimensions were based on thespecification indicated by JNC [1], and the container
part excluding the lid part was manufactured. The
steel grade was the Japanese Industrial Standard, JIS
G 5102 SCW 410. The reason for selection of thiswas that JIS G 5102 is cast steel with consideration
for weldability and SCW 410 is a steel grade withlow yield strength (the specification is 175 N/mm2 ormore) which is advantageous for resistance to

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanhydrogen-induced cracking (HIC) and weld cracking.
As shown in Fig. 2, the overpack was cast lager thanthe designed dimensions in the inverted state and thenmachined. The heat treatment was conducted at
920 °C for 11 hours then furnace cooled as annealing
for the purposes of removing internal stress andimproving mechanical properties.

Fig. 1 Dimensions of fabricated overpack (mm)Feeder head partBase plate part

Product part

Molded shape
Product shape
Microstructure
Chemical composition
Corrosion test

500 mmCylindrical partFig. 2 Casting configuration and sampling
positionsformicrostructuralobservation,
chemical composition analysis and corrosion test
2.2 Non-destructive testing of casting defect
Magnetic particle testing (MT) and ultrasonic testing(UT) were conducted as non-destructive testing of
casting defects. The test conditions are shown in
Table 1.
Table 1 Test condition of MT and UT
Magnetic particle testing

Ultrasonic testing

(MT)

(UT)

Testing area

Surface

InsideVertical beam method

MethodYoke method

Procedure

JIS Z 2320

JIS Z 2344

Detection limit≥ 1 mm≥ 2 mm diameter

2.3 Microstructural examination and chemicalcomposition analysis
As shown in Fig. 2, the specimens for microstructuralobservation (25 × 25 × 25 mm3) and the specimens
for chemical composition analysis (25 × 25 × 25mm3) were taken from six locations, that is, the outersurface, the center, and the inner surface of the baseplate part and the cylindrical part. In the
microstructure observation, after finishing thespecimen to mirror, the surface etched in a nital
solution was observed with an optical microscope. In
thechemicalcomposition
analysis,thecombustion-infrared absorption method was used forcarbon (C) and sulphur (S), and the emission
spectrometric analysis method was used for silicon(Si), manganese (Mn) and phosphorus (P).
2.4 Corrosion experiment
From six sampling positions where the microstructure
observation and the chemical composition analysis
were carried out, three points shown in Fig. 2 wereselected as the sampling parts of the specimen for
corrosion test (25 × 25 × 3 mm3). Two positions at
the outer surface are selected because these positions
are in the corrosion allowance layer, and one positionat the center of the bottom plate part is selected
because the bottom plate portion is the final solidified
region so that the segregation and the peculiarity ofmetallographic structure may be occurred. The
anodic / cathodic polarization tests were performed
with a platinum sheet as the counter electrode and astandard calomel electrode (SCE) as the referenceelectrode. The polarization curves were measured at ascan rate of 20 mV/min. The exposure area of thespecimens was 1 cm2. The test solution was thesynthetic sea water (pH 8.4) according to ASTM
D1141 to simulate the disposal environment. Table 2
shows the composition of the synthetic sea water.
Nitrogen gas was blown into the solution during theexperiment and the experiment temperature was
80 °C. The number of specimens tested for eachcondition was two.
Two different corrosion conditions were assumed
for the corrosion test. Just after the emplacement of
overpack, the oxidizing environment can be expected.
After the consumption of the residual oxygen by
reducing substances in the back filled tunnels, thereducing conditions can be expected. The corrosion
behavior for the former conditions was investigatedwith the anodic polarization test, while the latter one
was done with the cathodic polarization test.

Table 2 Composition of the synthetic sea water (g/L)
NaCl
MgCl2
Na2SO4CaCl2KCl
NaHCO3
24.53
5.20
4.09
1.16
0.69
0.201

KBr
0.101

H3BO3
0.027

SrCl2
0.025

NaF
0.003

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan3. Results
3.1 Fabrication of full-scale cast steel overpack
Fig. 3 shows the photograph of the fabricatedfull-scale cast steel overpack. The chemicalcomposition acquired by ladle analysis and themechanical properties satisfied JIS G 5102.

Bottom surface

A

B
B

D

0 mm

A
D

1750 mmCzθr

Side surface

C

B

C

A

10 mm

D

A

1750 mmFig. 3 Photograph of the fabricated full-scale caststeel overpack with 1750 mm in length and 820
mm in diameter
0 mm

Fig. 4 Results of MT (surface defect testing)16
14

Number of casting defects

3.2
Non-destructive testing of casting defectFig. 4 shows the distribution of casting defects
obtained by MT performed as detection of surface
defects. The type of casting defects was not a crack
but a cavity. 82 defects were detected, almost all ofthem were detected at the bottom surface, and only
one was detected at the side surface. Fig. 5 shows thenumber of casting defects as a function of distancefrom the periphery to the center of the bottom surface,with information of the size of casting defects. As canbe seen from this figure, casting defects were
distributed in the outer edge region of the overpackbottom surface with a peak at 20-30 mm from theperiphery. The defect size was 1-5 mm, and there was
the tendency to include larger defects as the
distribution of the number of defects is close to thepeak. On the other hand, the largest size of thecasting defect found at the corner of bottom surface
was 10 mm.
Fig. 6 shows the distribution of casting defects
obtained by UT performed as detection of internal
defects. 62 defects were detected and distributed in
the lower half of the overpack. No casting defect was
detected in the part of 440 mm radius from the centerof the bottom plate part. In addition, Fig. 7 shows the
distribution of the number of casting defects from theouter surface to the inner surface of the cylindricalbody with information of the size of casting defects.
Casting defects were distributed from the outersurface to the center of the cylindrical body thickness
with a peak at the depth of 50-60 mm from the outersurface. The defect size was 2.0-3.6 mm, and there
was the tendency to include larger defects as the
distribution of the number of defects is close to thepeak. Casting defects were found in the corrosion
allowance layer whose thickness is 17 mm [2] fromthe outer surface.

5 mm
4 mm

12

3 mm
2 mm

10

1 mm

8
6
4
2
0

0

50

100 150 200 250 300 350

410

Distance from periphery [mm]Fig. 5 Distribution of casting defects detected by
MT from the periphery to the center of the bottom
surface

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

A

A
θ
B

r

C

D

θ

B

D

410 mmΦ820 mm

Viewpoint： r - z plane
A

C

D

D

A

1750 mm1045 mmCz

Cylindrical part

400 μm

400 μm

400 μm

400 μm

400 μm

400 μmNote: projection drawing
C

B

A

B

1750 mm

190 mmr

0-410 mm

Outer surface

Base plate partCenterViewpoint： r - θ plane

θ
z

θ

0 mm

Note: projection drawing

Fig. 6 Results of UT (inside defect testing)Inner surface

r(White: Ferrite, Black: Pearlite)

Fig. 8 Microstructures at six sampling positions

Number of casting defects

20
3.6 mm
3.2 mm
2.8 mm
2.5 mm
2.2 mm
2.0 mm15

Table 2 Chemical compositions at each sampling
position (wt-%)

10

5

Corrosion
allowance
17 mm
0
0

Thickness of overpack
190 mm
50

100

150

190

Distance from cylindrical outer surface [mm]Fig. 7 Distribution of casting defects detected with
UT from the outer surface to the inner surface ofthe cylindrical body
3.3 Microstructural observation and chemicalcomposition analysis
Fig. 8 shows the microstructures at each position of
specimen indicated in Fig. 2. The difference of
microstructures was negligible between sampling
positions.
Table 2 shows the analysis results of chemicalcomposition for the specimens at each positionindicated in Fig. 2. No significant difference wasobserved in the chemical compositions depending on
sampling positions.

Specification
Ladle analysis
Base plate Outerpartsurface
CenterInner
surface
Cylindrical Outerpartsurface
CenterInner
surface

C
Si
Mn
P
S
<0.22 <0.80 <1.50 <0.040 <0.040
0.16 0.42 0.80 0.009 0.001
0.16

0.42

0.80

0.006

0.002

0.15

0.42

0.80

0.008

0.001

0.19

0.43

0.83

0.010

0.006

0.17

0.44

0.84

0.011

0.002

0.15

0.42

0.80

0.008

0.001

0.16

0.42

0.81

0.009

0.001

3.4 Corrosion experimentFig. 9 shows the anodic / cathodic polarization curvein synthetic sea water. There was no significant
difference in any of the polarization curves between
the different sampling positions.
Table 3 shows the corrosion current density
obtained by extrapolating the tangent of the cathodic
polarization curve in the Tafel region up to the
corrosion potential. From this result, the behavior ofhydrogen evolution reaction in synthetic sea water
was similar between the sampling positions.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

10-6

Base plate part, outer surface - 1
Base plate part, outer surface - 2
Base plate part, center - 1
Base plate part, center - 2
Cylindrical part, outer suface - 1
Cylindrical part, outer suface - 2

10-7-1400-1300-1200-1100-1000 -900 -800 -700 -600 -500 -400

Potential (mV vs.SCE)

Fig. 9 Anodic / cathodic polarization curves in thesynthetic sea water
Table 3 Corrosion current density of hydrogen
evolution reaction in synthetic sea water
Base plate part, outer surface-1
Base plate part, outer surface-2
Base plate part, center-1
Base plate part, center-2
Cylindrical part, outer surface-1
Cylindrical part, outer surface-2

Corrosion current
density(μA/cm2)
15
12
8
15
10
10

4. Discussion
4.1 Effect of heterogeneity of microstructure andchemical composition on corrosion resistance
The heterogeneity of the microstructure and chemicalcomposition of the cast steel was negligibly small.
This may be a cause of homogeneous corrosion
resistance. This is also indicative of the little
possibility of macro cell corrosion induced byheterogeneity of microstructure and chemicalcomposition.
4.2 Effect of casting defect on corrosion resistance
The large steel ingot has a characteristic segregation
zone called inversed V-segregation zone, the inversed
V-segregation zone appears from the outside to thecenter of the steel ingot close to the feeder head [3].
In this inversed V-segregation zone, micro shrinkage
cavities are generated.
The size of casting defects found in this work was
small. They distributed from the outer surface to thecenter of the cylindrical body close to the feeder headshown in Fig. 2. These observations suggested thatthe casting defects in this work were caused byinversed V-segregation peculiar to large cast steel
products.
In order to reduce the occurrence of casting defects
in the corrosion allowance, countermeasures are
considered such as making the mold additionallylarger than the product shape and thereby shifting thedefect generation zone to the out of the product shape.

Remained corrosion allowancerθ

2 mm diametercasting def ect

(a)

(b)

(c)

13 mm

10-5

15 mm

10-4

15 mm

10-3

17 mm

10-2

However, it is a trade-off with cost because themachining margin becomes larger, and it isconsidered difficult to completely eliminate casting
defects, so that a method of providing the addition tocorrosion allowance was examined as compensation
for permitting the occurrence of casting defects in the
corrosion allowance.
The examination of the additional corrosion
allowance was carried out by using results of UT
having information of three dimensional coordinates
including thickness direction. As shown in Fig. 10, if
casting defects exist in the corrosion allowance layer,
it is assumed that the thickness corresponding to thesize of the defects is not provided as corrosion
allowance at the position where the defect exists. It isalso necessary to consider the possibility that defects
overlap in the thickness direction as shown in Fig. 10(d). Therefore, in case the probability that defects
overlap in the thickness direction was more than one
out of 40,000 overpacks (i.e. more than 3×10-5), thetotal dimension of overlapped defects was added tothe corrosion allowance, otherwise, the dimension of
one defect was added.

Corrosion allowance layer

Current Density (A/cm2)

10-1

(d)

Fig. 10 Schematic diagram showing a state inwhich the required corrosion allowance is not
ensured due to casting defects (Note: the axes
corresponds to Fig. 6)
According to Fig. 6, casting defects existed in theregion from the bottom surface to the height of 1045
mm. In addition, according to Fig. 7, the size ofcasting defects included in the region from thesurface to 17 mm depth (which is the corrosion
allowance) were all 2 mm. Therefore, it is supposedthat 2 mm casting defects occur in random in thisregion. As shown in Fig. 11, the probability that twocircles with a diameter of 2 mm overlap in the
thickness direction is the probability that the center
point of one circle is within 2 mm radius from thecenter point of the other circle. This probability is thesquare value of the value obtained by dividing thearea of a circle with a radius of 2 mm by the area ofthe cylindrical outer surface from the bottom to theheight of 1045 mm. This was calculated as 2×10-11. In
addition, since there were five casting defects in the

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanregion from the surface to 17 mm depth, thecombination that two out of the five casting defects
overlap is 5C2 = 10. Consequently, the probability
that two out of the five casting defects of 2 mmdiameter overlap is 2×10-10. The probability of threeor more overlapping is further reduced. Since it wasmuch lower than the criterion of the probability(more than 3×10-5), according to the result of thiswork, it was not necessary to consider the possibilityof casting defects overlapping in the thickness
direction, and it was considered appropriate toprovide 2 mm as the additional corrosion allowance.
820π mm1045 mm

2 mm diametercasting defectheterogeneity of corrosion resistance was examinedbyelectrochemicalcorrosion
test.
These
heterogeneities were negligible.
Non-destructive testing of the cast steel overpack
was conducted, and the distribution and dimensionsof casting defects were investigated. As a result, since
the casting defect occurred in the corrosion allowance,a method of ensuring corrosion allowance by
providing the additional thickness was examined.
Consequently, the number of casting defects
generated in the corrosion allowance layer was small
and the dimensions were also small, so that theprospect of ensuring the corrosion allowance by
providing an additional thickness of a few mm wasindicated.
References[1] JNC (2000): H12: project to establish thescientific and technical basis for HLW disposal in
Japan, Supporting Report 2, JNC TN1410 2000-003.

zθFig. 11 Schematic diagram of overlapping of
casting defects in the thickness direction (Note:the axes corresponds to Fig. 6)
Although the current reference thickness of the
carbon steel overpack is set to 190 mm based on theevaluation by JNC [1], in the evaluation based on thelatest knowledge, the required thickness of theoverpack is 122 mm [3]. Thus, since the current
specification of the carbon steel overpack has 68 mm
as a margin, the additional corrosion allowance of 2
mm can be sufficiently ensured.
However, because the above evaluation was
conducted within the current small amount of data,
more detailed examination is necessary for accurate
assessment of the additional corrosion allowance.
Specifically, it is necessary to perform casting defects
detection by X-ray computed tomography so as todetect casting defects below the detection limit of UT,
to grasp the shape of casting defects and to obtain theposition coordinates of casting defects moreaccurately. In addition, it is also considered necessary
to observe the shape and the dimension of casting
defects visually by cutting the cross section, andconfirm the consistency with non-destructive testing.
Furthermore, studies for confirming conditions ofoccurrence of casting defects and examinations basedon a larger number of casting defect data arenecessary.

[2] Ogawa, Y., Suzuki, S., Kubota, S., Deguchi, A.
(2017): Re-evaluation of the required thickness of the
carbon steel overpack for high-level radioactive
waste disposal in Japan based on the latest scientificand engineering knowledge, Corros Eng Sci Technol,
Vol.52, pp.204–209.
[3] Ohno, A. (1973): The Solidification of Metals,
Chijin Shokan Co.

5. Conclusion
By fabricating the cast steel overpack on a full-scale,the heterogeneities of the microstructure andchemical compositions were investigated and the

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan