E-Journal of Advanced Maintenance Vol.1 (2009) 87-98
Japan Society of Maintenology

Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels
Mikio KUSUHASHI1, Yasuhiko TANAKA1,∗, Tsuyoshi NAKAMURA2,
Tomoharu SASAKI2, Yoichi KOYAMA2, and Hisashi TSUKADA1
1

The Japan Steel Works Ltd., 1-11-1 Osaki, Shinagawa, Tokyo 141-0032, JapanThe Japan Steel Works Ltd., Muroran Plant, 4 Chatsu, Muroran, Hokkaido 051-8505, Japan

2(Received; February 3rd, 2009)
Abstract. Neutron irradiation embrittlement of nuclear reactor pressure vessels (RPV), which occurs in the coreregion material during service, is a critical issue for the safety and reliability of nuclear power plants. The
copper (Cu) and Phosphorus (P) that occur as impurity elements in RPV steel may not only reduce its fracture
toughness but significantly increase its susceptibility to irradiation embrittlement. In order to manufacturematerials with low susceptibility to neutron irradiation embrittlement and high fracture toughness, technologies
to reduce these harmful impurity elements have been established at the Japan Steel Works, Ltd. (JSW). These
technologies employ a double degassing process with the use of high purity raw materials. In this way, thecontent level of these impurity elements has been reduced as low as can possibly be achieved in an industrialrefining process. This technology has been applied to the manufacture of large components made from the world
largest ingots, which weigh up to 600 tons. It has also been confirmed that these high purity steels develop agood degree of toughness and strength even in heavy section forgings for RPV. A neutron irradiation test was
performed in order to investigate the irradiation embrittlement of forging grade steels. The test revealed a small
degradation of toughness, at a level that is consistent with the degree of neutron irradiation embrittlement
accepted by authorized standards. Furthermore, the advantage that a low Si version of steel offers in terms ofsusceptibility to neutron irradiation embrittlement is also suggested.

KEYWORDS: core region shell, irradiation embrittlement, chemical elements, impurity,
manufacturing technology, forging
1.

Introduction

The demand for the construction of large capacity, high performance nuclear power plants is
growing, not only for the generation of sufficient electric power, but also as a measure for reducing
CO2 gas emissions. The evolution of large unit capacity nuclear power plants depends on therealization of high quality, high performance large nuclear reactor pressure vessels (RPVs) and theirrelated components. One of the important considerations for these RPV components is neutron
irradiation embrittlement, which may degrade the fracture toughness of the material.
Nuclear RPVs, more specifically, the steel in the core region, suffer from neutron irradiationfrom nuclear fuel, which causes a progressive embrittlement of material during service. The degree of
embrittlement depends on many factors such as neutron flux, fluence, irradiation temperature, specificchemical elements in the materials, the microstructure of the steel. From the viewpoint of material
chemistry, phosphorus (P) and copper (Cu) have long been recognized as the most influential
elements, which is why there has been a continuing significant effort to determine the mechanismsthat cause the degradation of fracture toughness [1][2][3][4][5]. Based on such investigation andirradiation tests, a procedure for the prediction of neutron irradiation embrittlement has beenstandardized; it appears in NRC regulatory guide 1.99, and is based on P and Cu content, together
with the fluence of fast neutrons [6]. The effect of Ni, which is the major alloying element of
MnMoNi reactor pressure vessel steels such as SA508 Gr3.Cl.1 and JIS SFVQ1A, has also become∗

Corresponding author, Email: yasuhiko_tanaka@jsw.co.jp
ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

the object of keen attention. The regulatory guide was revised to include the effect of Ni content in its
prediction [7]. The investigation that was conducted using state-of-the-art analysis equipment
disclosed a complicated damage mechanism that takes place through atom clustering in steel, due toboth neutron irradiation and the role of impurities with alloying elements [3][4][5]. This investigationconsequently leads to the establishment of a more reliable prediction method based on Cu and Ni
content as a dominant influence of chemical elements.
The Japan Steel Works, Ltd. has continued to make efforts to decrease the harmful tramp
elements in the material used in the production of high quality large forgings. The reduction of tramp
elements improves not only the material properties such as ductility, toughness, and creep strength,
but also the resistance to corrosion, aging embrittlement, and neutron irradiation embrittlement.
Advanced refining technologies have made possible the removal of tramp elements from steel, so thatquite a low content has become attainable. The installation of advanced refining facilities also makes
possible the casting of high purity large ingots as heavy as 600 tons. An advanced forging technology,
which is used to make large ring forgings and heads, and which contributes to the reduction of thewelding line, has also been established [8]. The application of these technologies has led to improved
reliability, serviceability, inspectability, and economy in recently constructed RPVs.
This paper describes the history of the technology used in the production of nuclear RPV
components with a focus on the chemistry of core region shell steels. The results of neutron
irradiation embrittlement tests for heavy section ASME SA508 Gr.3 Cl.1 steels are also introduced.
2.

Development of Production Technology for Forged Shells for RPVs

JSW supplied the components for the Japan Power Demonstration Reactor (JPDR) in 1961, andhas been supplying components for RPVs ever since. In the area of ring forging, JSW was the first to
supply a forged flange for the RPV of the Tsuruga #1 nuclear power plant of The Japan Atomic Power
Company in 1969. Since then, JSW has supplied many types of forging components for nuclear RPVs
to manufacturers worldwide [8]. A ring forging was first used for a core region shell in 1974.
Furthermore, the integration of components has advanced through the development of production
technologies and the installation of advanced facilities. The distinct advantage realized through theuse of forging components is reduction of the welding line, as compared to plate construction typevessels. This reduction helps to significantly reduce the manufacturing cost of a RPV, as well as
shorten the construction time. It also increases its reliability and reduces the time required forin-service inspection(ISI). Fig. 1 shows a photograph of a core region shell ring for a domestic 1200
MW PWRPV constructed from a design that has no welding line in the core region. Relization of theshell with no welding line in the core region is quite an improvement in the safety and reliability of a
RPV because it eliminates the neutron irradiation embrittlement of the weld seam.

Fig.1. Appearance of core region shell for 1200MW PWRPV

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Core Region Shells for Nuclear Reactor Pressure Vessels
80

RPV

70

Number

60
50
40
30
20
10
0
1970

1975

1980

1985

1990

1995

2000

2005

2

Year

Fig.2 Manufacturing experience of RPV components in JSW

Fig. 2 shows the quantities of nuclear RPV components manufactured at JSW, with a total of 550
RPV components shipped as of 2008. Due to the enlargement of RPVs to keep pace with increases inplant capacity, the maximum weight of ingot used has reached 600 tons.
The reduction of impurity elements is one of the most important processes for the production ofhigh quality steels. Although each chemical element has a different effect on the property of steels,
tramp elements such as P, S, Cu, Arsenic(As), Tin(Sn), Antimony(Sb), Oxygen(O) and Hydrogen(H)definitely need to be decreased to eliminate their harmful effects on toughness, ductility, and aging
degradation during service. Fig.3 shows a brief history of the steelmaking process at JSW’s Muroranplant since 1950 [8].
Year

1950

55

60

65

70

75

80

85

90

95

Acid and Basic OpenHearth Furnace
Basic Open Hearth Furnace
Electric Furnace

MeltingandRefining

Electric Furnace
Ladle Refining Furnace
Holding Furnace

150ton130ton

30ton
x2

20ton ESR

150tonx2
100ton ESR
5ton VIM

Bochumer-Verein Type Mold Stream Degassing
Ingot
MakingMaximum
Ingot
WeightAir Casting
Mechanical Pump

140ton220ton250
ton

Steam Ejector (High Vacuum)
Multi
Pouring

Low Si-VCD

Prediction ofSegregation

DoubleDegassing400
ton

500ton570ton600tonFig. 3. Development of manufacturing technology in JSW

In the early 1950s, steels were refined in an open hearth furnace (OHF) and were cast in air. At
that time, the absorption of hydrogen in steel was one of the most serious problems in the process,since hydrogen causes defects such as flaking. In 1961, many cracks were found in the steel plates
that were imported from the UK for the pressure vessel of the first commercial nuclear power plant inJapan. It was proved that hydrogen was responsible for these cracks. At JSW, the installation of
vacuum degassing equipment made it possible to significantly reduce the hydrogen to less than 1 ppm[9][10]. This also made possible the use of a basic OHF and basic electric arc furnace (EAF), in which
the molten steel tends to absorb hydrogen, but which also offer superior refining ability. The
efficiency of the vacuum degassing equipment was further improved by the installation of a steam
ejector instead of a mechanical pump, which made it possible for a higher vacuum to be realized. With
the introduction of vacuum casting equipment, the vacuum carbon deoxidization (VCD) process wasestablished. This process enhances the reaction between C and O in low Si content molten steel during
casting, and removes O as CO [11]. The application of the VCD process made it possible to reduce the89

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

Si used for deoxidization. With a decrease in Si content, the formation of macro-segregation in an
ingot can be reduced [12]. Fig.4 demonstrates the effect of the VCD process on the solidification
structure of a large rotor forging made from 3.5 NiCrMoV steel.

Si:0.02 S:0.008wt%(a) Turbine Rotor Shaft from 140 ton VCD Ingot

Si:0.38 S:0.008wt%(b) Si deoxidized 75 ton Ingot
Fig. 4. Sulfur prints of rotor shafts made by the VCD process and Si deoxidized process

The sulfur print of the cross section clearly shows the reduction in indications of sulfur, whichcorrespond to the area of macro segregation, that was achieved through application of the VCD
process. Major alloying elements such as C, Ni, Cr, Mo, Si, and Mn, and impurity elements such as P,
S, and Cu enrich segregation, which enhances hardenability, increases hardness, yet also increases thesusceptibility to several types of embrittlement.
In order to increase the capacity of an energy plant, larger forgings are needed for components
such as rotor forgings and pressure vessels. The casting technology used to produce large ingots is the
key to the realization of large components, and it was developed through the use of a ladle refining
furnace (LRF) [10]. Ingots weighing up to 600 tons can be cast, at present, by fully ladle refined melt[13]. Fig.5 shows the multi pouring process.
Electric Arc Furnace
1st Charge

Vacuum Pouring②
Ladle Refining Furnace (LRF)
Pony Ladle

2nd Charge

VacuumTank③

Manufacturedby an electricarc furnace

Mold

3rd Charge④①～④ : Pouring
sequence
4th Charge①

Fig. 5. Schematic of multi pouring process

After melting and oxidizing refining by an electric furnace, the molten steels poured into a ladle.
Further refining takes place in the ladle, and the molten steel is kept in optimum condition until thetime of casting. Multiple ladles are prepared to hold the molten steel, and are then poured successively
into a large mold. On the other hand, special melting equipment such as vacuum induction melting
furnaces (VIM) and electro slag remelting furnaces (ESR) have been installed for the production of90

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

clean, homogeneous, high quality materials. Through the development of these facilities andproduction technologies, forgings have been manufactured that exhibit both high performance andhigh reliability. Fig.6 shows a typical refining process, the double degassing process, which takes
place through the use of an EAF and LRFs.

Fig. 6. Schematic of the double degassing process

Table 1 Examples of the chemistry of raw materials
(wt%)
C

Si

Mn

P

S

Ni

Cr

0.01

0.01

0.13

0.012

0.004

0.02

0.01

Cu

Mo

V

As

Sn

Sb

Co

0.02

0.01

<0.01

<0.003<0.0030.0011

0.002

Formerly, the refining was done only in an EAF. In the advanced process, after melting the raw
materials in an EAF, oxidizing refining is performed in the EAF by adding a basic oxidizing slag to
reduce the P. The molten steel is then poured into the LRF through a pony ladle, and the oxidizing
slag is completely removed to prevent oxidized elements in the slag from returning to the molten steel.
In the ladle, reducing slag is added and the molten steel is stirred under a vacuum to remove the S.
After adjustment of its chemical composition, the molten steel is poured into a mould in a vacuum
chamber. In order to produce a low Cu, As, Sn and Sb content, the use of high purity raw materials isessential, since these elements cannot be removed through the refining process. Table 1 shows
examples of the chemical composition of raw materials.
In response to the nuclear power industry’s need for larger components, advanced free forging
technologies and facilities were developed. Since it became impossible to forge the extremely large
components that were required even by a 10,000 ton free forging press, equipment and technologies
that could treating the material outside the press were developed. The resulting outside pressing
technology made possible the forging of large shells that were more than 4 m high and had an outer
diameter up to 10m. Fig.7 shows a photograph of outside pressing process. This technology made it
possible to design a large diameter RPV that does not have a welding line in the core region.
As regards heat treatment, a water tank with a 9-m outer diameter and a depth of 6 m wasinstalled for the large ring forging components of RPVs. Quenching heat treatment of the shell isachieved by immersing the forging in a water tank equipped with apparatus that applies strong stirringto the quenching media.
Through the production technologies mentioned above, high quality RPV components have been
manufactured and shipped worldwide. Table 2 shows specifications of the chemistry of ASME SA508
Gr.3 Cl.1., the average of the chemical elements refined by EAF and cast in a vacuum, with theaverage of the chemical composition refined by the double degassing process for typical RPV
components.

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

Fig. 7. Appearance of outside pressing

The table suggests that a significant decrease in impurity content is realized through applicationof the double degassing process, which also produces an improvement in material properties. The
impact test data show that the 50% fracture appearance transition temperature (FATT) shifts to a lower
temperature at around 25 °C in the double degassing process for PWR shells with a thickness ofaround 300 mm, compared to those produced in late 1970s by EAF [14]. Since the strength level iscontrolled at almost the same level by heat treatment, the results indicate a considerable improvementin the balance between strength and toughness.
Table 2 Effect of double degassing on chemistry of materials

3.
3.1.

Historical Change in Chemistry of Core Region Shell Steels
Impurity content

As mentioned previously, the content of impurity elements such as P and S was reduced throughthe application of advanced steelmaking and casting technology. Fig.8 and Fig.9 show the history of
the P and S content, respectively, of core region shell material produced since 1974. As can be seen,
both the P and S content decrease noticeably. P content decreases until 1986, then decreases less than
50 ppm after that. S content decreases less than 20 ppm, contributing to an improvement in fracture
toughness, strength, and homogeneity. The P content also decreases, with P content recently ranging
between 0.003 and 0.005 wt% (the 2006-2007 average is 0.004 wt%).
Fig.10 shows the history of Cu content. Cu content was relatively higher until the early 1980s,
then was reduced and controlled at less than 0.05 wt%. The reduction of P and S content directly
corresponds to the period in which the double degassing process was established. On the other hand,
Cu content is quite difficult to reduce through the refining process, and as is shown in Table 1, highpurity raw material needs to be used. The Cu content of recent core region shells ranges between 0.02
and 0.05 wt%, which is the lowest currently attainable level.

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

0.010

P

P Contents, wt%

0.008

0.006

0.004

0.002

0.000
1970

1975

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 8. History of P content of core region shells
0.010
●

S

S , wt%

0.008

0.006

0.004

0.002

0.000
1970

1975

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 9. History of S content of core region shells

0.10

Cu

Cu Content, wt%

0.08

0.06

0.04

0.02

0.00

1975

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 10. History of Cu content of core region shells

3.2.

Major alloying elements

The typical material designation for the forging component of a light water reactor pressure
vessel (LWRPV) is ASME SA508 Gr.3 Cl.1(JIS SFVQ1A), which is alloyed with Si, Ni, Mn, Mo toachieve the design strength and toughness. Silicon, too, is generally added to facilitate deoxidization
during the refining process. In early LWRPVs, steels such as ASTM A201 and A212 were used for thesteam boiler, since these materials offer good weldability and have an excellent history of application.
Then, Mo was added to enhance the high temperature strength, and the resulting material was
designated SA302B. Ni has also been used as an alloy to develop a good freezing microstructure thathas a positive balance between good strength and fracture toughness in heavy section components.
Mn is also an effective element, contributing both better hardenability and deoxidization.

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels
2.0
1.8
1.6

Mn

Content, wt%

1.4
1.2
1.0

Ni

0.8
0.6

Mo

0.4

Si

0.2
0.0
1970

1975

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 11. History of major alloying elements in core region shells

Fig. 11 shows the history of the content of Si, Mn, Mo, and Ni. As can be seen, the content ofeach alloying element has remained at almost the same level. In the case of Ni, content levels ofaround 0.8 wt% and 0.9 wt% stand out, with this depending on the material specification applied forthe prediction. State-of-the-art analysis technology has determined the effect of Ni and other alloying
elements on neutron irradiation embrittlement [3][4][5]. The results suggest that a beneficial effect isobtained through the reduction of several major alloying components, such as Ni. However, thereduction of the content of these elements may have an opposite effect on the development of therequisite strength and fracture toughness of the steel. Among the alloying elements, Si still remains at
a higher level of content, though the specification of a low-Si version of SA508 type steel has beenstandardized. A decrease in Si content through the VCD process is reported to be effective for the
reduction of the macro segregation, which tends to appear in large ingots [12]. In macro segregation,
generally, the content of the alloying and impurity elements is significantly enriched, which canincrease a steel’s susceptibility to neutron irradiation embrittlement. A decrease in Si content may
therefore prove beneficial by reducing this embrittlement.
4.

Neutron Irradiation Embrittlement of Forging SteelThe prediction of embrittlement caused by neutron irradiation is one of the most important issues
not only in assessing the reliability and safety of RPVs, but in evaluating the possibility of life
extension. So far, several correlations have been presented between the degradation of toughness andthe principal factors that influence this, such as chemistry, P and Cu content, Ni content, neutron flux,
and fast neutron fluence. Some of these relationships have been used in the assessment of toughness
degradation in design codes or standards, such as the NRC regulatory guide, ASTM standards, and theJapan Electric Association Code (JEAC). The effect of Ni was shown to be pronounced, and was also
considered a very important contributory factor that should be considered in the estimation ofembrittlement. Recent prediction methods such as those in the NRC regulatory guide 1.99 rev.2,
JEAC4201-2007 [15], also consider Ni and Cu content as contributory elements.
Table 3 Chemistry of the materials tested
Steel

S A508
G r.3 Cl.1

wt%

C

Si

Mn

P

S

Ni

Cr

Cu

Mo

V

A

0.18

0.27

1.38

0.006

0.005

0.79

0.09

0.04

0.48

＜0.01

B
C

0.20
0.18

0.24
0.27

1.47
1.35

0.006
0.007

0.005
0.005

0.80
0.76

0.09
0.12

0.04
0.04

0.50
0.49

＜0.01
＜0.01

D
E

0.18
0.20

0.27
0.02

1.35
1.29

0.005
0.005

0.005
0.006

0.76
0.79

0.12
0.07

0.04
0.03

0.49
0.49

＜0.01
＜0.01

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

In the manufacture of RPV components, the most important factor in minimizing neutron
irradiation embrittlement is the reduction of tramp elements such as P and Cu. Increasing the initial
toughness is also important, and this is attributed to the decrease of tramp elements such as S.
Although Ni affects neutron irradiation susceptibility, it may be difficult to reduce the Ni content,
since material toughness decreases with decreasing Ni content. A reduction in the content of tramp
elements is therefore important from the standpoint of RPV reliability, and the quality of maintenancethat can be expected.
Table 4 Mechanical properties and RTNDT of the materials tested

Material

0.2% Offset
StrengthMPa

TensileStrength
MPaElongation％

Reductionof Area
％

RTNDT
℃

A

444

582

24

77

-37

B

464

612

22

72

-37

C

468

597

26

71

-52

D

468

597

26

71

-52

E

410

543

26

74

-35

Table 5 Location and orientation of the specimens
Thickness/
Outer dia.
mm
300
5,552

Ingot
LocationThickness
Location

Specimen
Direction

Bottom

1/4t

Axial

300
5,552

Bottom1/4t

Tangential

Shell

300
5,552

Bottom

1/4t

Axial

D

Shell

300
5,552

Bottom

1/4t

Axial

E

Forged Plate

300

Bottom

1/4t

Trensverse

Steel

Components

A

Shell

B

Shell

C

Since the use of forging material for nuclear RPVs was introduced later than the use of rolled
plate, in the early stage of nuclear power generation in Japan, the forging steel data base was small as
regards the behavior of neutron irradiation embrittlement. JSW took the opportunity to performneutron irradiation embrittlement tests of ASME SA508 type forging grade materials, together withheavy section plate steels such as ASME SA302B and SA533 steels [16][17]. As for the SA508 Cl.3
(presently SA508 Gr.3, Cl.1) steels, five heats of heavy section forging materials were tested using amaterial testing reactor [17]. Among them, one heat of a low-Si version steel made by using the VCD
process was subjected to the test. Its mechanical and impact properties were investigated after
irradiation. Table 3 shows the chemistry of the steels tested.
Since the materials were produced in the late 1970s, the P content is rather higher than ispresently the case. The Cu content, on the other hand, is similar to that of currently produced steels.
Table 4 presents the mechanical properties and RTNDT of the steels tested. The materials were removed
from prolongation of actual shells or heavy section plates for a PWRPV. Table 5 shows thedimensions of the components from which the specimen was removed, and the orientation of thespecimen.
Irradiation was performed at the Japan Material Testing Reactor (JMTR) for steels A, B, D, and E,
and at the University of New York at Buffalo reactor for steel C. The irradiation period was a
maximum of 1100 hrs. After irradiation, a Charpy impact test was performed for each material, andthe results were compared to those before irradiation. Table 6 summarizes the actual irradiation
conditions and the change in vTr41J. Differences in irradiation temperature, fast neutron fluence (E > 1
MeV), and other contributory factors make the results appear complicated. From the test results it cangenerally be said, however, that the increase in neutron fluence causes an increase in the Charpy index

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Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

temperature, although the hardening data is not available. Among the materials tested, the behavior ofsteel E is distinct, since its increase in Charpy 41 J absorbed energy (ΔvTr41 J) was comparatively
small. Steel E was made using the VCD process, hence the Si content is 0.02%, which is muchsmaller than that in conventional material. Taking into account the fact that the irradiation temperatureof steel E was 10 to 30 °C higher than that of the other steels, this effect seems meaningful. As
mentioned earlier, the low- Si version steel offers the advantage of decreasing the sensitivity to form
macro segregation, from which it can be presumed that the reduction of impurities in macro
segregation may lead to reduced embrittlement.
In addition, test results are compared to the several estimations that have been developed. An
advanced method proposed by Soneda et al. [5] is compared to the results of tests on forged RPVsteels that were adopted in JEAC4206-2007 [15]. Table 7 summarizes the irradiation test results andthe estimation in accordance with NRC regulatory guide 1.99 rev.1 [6], JEAC 4201-2004, and JEAC
4201-2007. Where NRC regulatory guide rev.1 stipulates 0.08 wt% Cu and 0.008 wt% P,
JEAC-4201-2004 [18] and -2007 stipulates 0.05 wt% Cu as the lower limit, and this estimation
includes the margin.
A recent JEAC prediction method gives a conservative estimation of the degradation of thetransition temperature. Although the JEAC estimation includes a safety margin that is twice thestandard deviation, the results suggest that the present test data fall in the scattering of the data used toestablish the JEAC correlation. On the other hand, NRC regulatory guide 1.99 rev.1 includes no safetymargin, and the test results strongly agree with the NRC prediction, except in the case of steel E, thelow-Si version steels. The effect of a reduction in Si content is important in order to reduce macro
segregation [12]. In the macro segregation of RPV steel, the enrichment of Ni is around 20% and thatof Mo is around 50%, as compared to the bulk chemistry are revealed and also the enrichment of
impurity elements are confirmed. Because the impurity and alloying elements condense during macro
segregation, neutron irradiation embrittlement must be enhanced. The impact of a local increase inchemical elements in the segregation zone upon the degradation of toughness remains an open
question. However, since segregation is essentially difficult to completely eliminate in large ingots,
the effect needs to be investigated.
Table 6 Actual irradiation conditions and shift in vTr41J

Steel

Irradiation

Temperature℃vTr41J , ℃

Fluence2
n/cm >1MeV

InitialIrradiated

ΔT

19

-53

-18

35

A

JMTR

300

2.3ｘ10

B

JMTR

270

2.0ｘ1019

-49

-20

25

290

1.5ｘ10

19

-63

-32

31

3.2ｘ10

19

-32

8

40

2.9ｘ10

19

-57

-37

20

C

UBR

D

JMTR

E

290

JMTR

300

Table 7 Test results and predictions by standards
Δ ｖTr41J
Steel

Irradiation
test

NRC 1.99
Rev.1

,℃

Note

JEAC4201-2004 JEAC4201-2007

A

SA508 Gr.3 Cl.1

35

34

46

43

B

SA508 Gr.3 Cl.1

25

31

(45)

46

C

SA508 Gr.3 Cl.1

31

27

44

39

D

SA508 Gr.3 Cl.1

40

40

49

49

E

SA508 Gr.3 Cl.1

20
38
46
＊ ) Irradiation Temperature is lower than 274 ℃

96

46

*

Low Si

M. KUSUHASHI et al
Manufacturing of Low Neutron Irradiation Embrittlement Sensitivity
Core Region Shells for Nuclear Reactor Pressure Vessels

5.

Conclusion

By applying advanced steelmaking and casting technologies, and using raw materials of highpurity, high reliability core region shells have been manufactured at JSW. Steel with a low impurity
content undoubtedly develops a low susceptibility to neutron irradiation embrittlement. We wouldemphasize that the state-of-the art estimation method gives a conservative prediction of the transition
temperature degradation due to the neutron irradiation of these forging steels. On the other hand,alloying elements have been optimally controlled to develop the strength and toughness required of
RPV steel. In order to further reduce the steel’s susceptibility to neutron irradiation embrittlement,
modification of the major alloy contents must be investigated, together with the means of achieving afurther decrease in impurity elements. The effect of alloying elements, however, should be addressed
not only in terms of susceptibility to neutron irradiation embrittlement but also in terms of othercharacteristics such as hardenability, toughness, and strength. Considering the effect of Si in a neutron
irradiation embrittlement test, a decrease in Si content may be one measure that can realize lower
susceptibility through the reduction of macro segregation. The process technologies for controllingand optimizing alloy contents have already been established and are ready to be applied if necessary.
Careful attention, however, should be paid to any change in material properties by the needed alloy
modification. Any new modifications should be developed on the basis of accumulation and analysisof laboratory and field test data, especially with regard to neutron irradiation embrittlement.

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