Nuclear Safety Research

5-6

Estimating Fracture Resistance of Reactor Pressure Vessel Steels–Estimating Toughness against Combination of Cleavage andIntergranular Fracture–(MPa√m)Ͱn (Ͱn(1/(1-Pf))

Pf 㧦fracture probability
Modified MC
methodStandard MC
method for cleavage
fracture only

Low fracture toughness
due to grain-boundary
decohesion

Fracture toughness (KJc)

Intergranular fractureIntergranular fracture

Mean curve:Standard MC
Lower tolerance
bound: Standard MCMean curve:
Modified MC
Lower tolerance
bound: Modified MC

Test temp.㧦 㧙10͠Ͱn (KJc-20)

Test temperature (͠)

Fig.5-12 A Weibull plot of fracture toughness ofintergranularly brittle material
The distribution of fracture toughness (KJc) values indicating
grain boundary decohesion does not follow the linear
regression defined in the standard method for cleavage
fracture. A modified method is needed to estimate lower
KJc values.

Fig.5-13 Temperature dependence of KJc values for theintergranularly brittle material together with the MasterCurve
The KJc values are beyond the lower tolerance bound. The
modified method enabled us to estimate lower KJc values
due to intergranular fracture.

The reactor pressure vessel (RPV) is one of the mostimportant safety-related structural components, because it has
no redundancy and its failure is not considered in the designof the reactor. There is concern is that because the portionof the RPV surrounding the core region called “beltline”
is exposed to neutron irradiation, the RPV material will bedegraded (irradiation embrittlement). The effect is manifested
by an increase in yield strength, a decrease in ductility, anddegradation of toughness which are monitored by surveillance
tests during plant operation. In the technical evaluation of every
nuclear power plant whose operating period reaches 30 years,
the irradiation embrittlement upon 60-years operation at highneutron fluence is estimated to ensure the structural integrityof RPV. One of possible types of embrittlement by such highneutron fluence is intergranular embrittlement caused bygrain-boundary phosphorus (P) segregation. The P segregation
is promoted by neutron irradiation and the presence of P
weakens the cohesive strength of grain-boundaries, leading toembrittlement through intergranular fracture. How to estimate
fracture toughness for the materials exhibiting intergranularfracture is one of the issues.
For fracture toughness evaluation of RPV steels, extensive
efforts have been made to achieve direct determination of
fracture toughness (KJc) in the ductile-to-brittle transition
temperature region, using the so-called the “Master Curve”(MC) method. This estimates the fracture toughness versus
temperature curve from a test at one temperature. Since

fracture initiation is assumed to be cleavage in the MC method,
it is questionable whether the method is applicable to materials
subject to intergranular fracture.
The KJc values of the material exhibiting a mixed mode fractureof intergranular and cleavage due to intergranular embrittlement
exhibit a large scatter. A Weibull plot of KJc values is shown inFig.5-12, together with the data fitting denoted as a blue line by
the standard MC method. The figure clearly indicates that thedistribution of KJc values does not follow the linear regression
defined in the standard method. It seems that the distributionof KJc values is from an inhomogeneous material creating
two distributions of fracture toughness. This feature becomes
more pronounced with increase in P concentration at the grainboundaries. Since fracture toughness depends on the type ofbrittle fracture which initiates at the crack front, the obtained
KJc values may follow the dual fracture toughness distributionsof cleavage and intergranular fracture. We applied a modified
MC analysis to the estimation of intergranular fracture bycensoring the upper KJc data in Fig.5-12. The test temperaturedependence of KJc is shown in Fig.5-13. The solid and dashedblue lines represent the mean curve and lower bound forfracture, respectively. The dependence of material subject tointergranular fracture as well exhibits low KJc values below thelower tolerance bound of brittle fracture. The modified method
enables a more conservative fracture toughness estimate, as
shown by the solid and dashed red lines in Fig.5-13.

Reference
Nishiyama, Y., Onizawa, K. et al., Phosphorus Segregation and Intergranular Embrittlement in Thermally Aged and Neutron Irradiated Reactor Pressure
Vessel Steels, Journal of ASTM International, vol.4, issue 8, 2007, p.12, Paper ID JAI100690.

62

JAEA R&D Review 2008