Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

growth into the LAS heat-affected zone was

observed at ≥ 65 MPa∙m 1/2 , but this has to be

verified with larger specimens.

Sub-project-IV – SCC Initiation in Ni-base

alloys and LAS: A PhD thesis proposal on the

effect of hydrogen on SCC initiation and subsequent

short crack growth in Ni-alloy weldments

in BWR environment is in preparation

and the thesis shall be started in 2013. For that

purpose a multiple specimen SCC initiation

set-up with on-line crack initiation monitoring

and a servo-pneumatic loading system was designed

and is currently fabricated.

1. Introduction

With regard to the new nuclear legislation and the

increased age of the Swiss nuclear power plant

fleet (27 to 43 years), the current focus of materialrelated

regulatory safety research funded by the

Swiss Federal Nuclear Safety Inspectorate (ENSI)

is placed to the assessment and assurance of the

integrity of the primary coolant circuit and containment

in the context of material ageing [1].

Pressure boundary components in the primary

coolant circuit (PPBC) of light water reactors (LWR)

are made of low-alloy (LAS) and stainless steels

(SS) and are very critical components with regard

to safety and lifetime (with the reactor pressure

vessel (RPV) being the most critical one). Assurance

of structural integrity of these components in the

context of material ageing is thus a key task in any

ageing and lifetime management program. During

service, toughness and ductility of these materials

can decrease with time, due to irradiation

induced embrittlement (RPV and reactor internals

only), thermal ageing or potential environmental

(hydrogen) effects. Under simultaneous effect of

the reactor coolant, thermo-mechanical operational

loads and irradiation, cracks can initiate and

grow by environmentally-assisted cracking (EAC)

and thermo-mechanical fatigue (TMF), which finally

could lead to a large leak or component failure.

A plenty of EAC cracking incidents occurred

in both boiling water (BWR) and pressurised water

reactors (PWR) in a wide range of stainless steel,

nickel-base alloy, carbon and low-alloy steel PPBC

in the last three decades. Critical components are

thus periodically inspected by non-destructive examination

to detect defects before they reach a

critical size necessary for rapid fracture. [2–4]

An accurate knowledge on the degradation of the

toughness and fracture properties of these materials

during service and of the system conditions

which may lead to EAC initiation and growth is

thus evidently indispensable to ensure the safe

and economic long-term operation in this context.

Reliable quantitative experimental data on these

phenomena and a basic knowledge on the underlying

mechanisms are essential to evaluate their

possible effects on structural integrity/safety and

lifetime of components, to identify critical component

locations/operating conditions and to define

and qualify possible mitigation, repair and maintenance

actions.

2. Structure and Goals of the

SAFE Project

The SAFE project (2012–2014) aims to fill selected

important knowledge gaps in the field of EAC

and environmental effects on fatigue and rapid

fracture in safety-relevant PPBC [3]. It consists of

four sub-projects (Table 1) and deals with environmental

effects on fracture and fatigue, stress

corrosion cracking (SCC) in dissimilar metal welds

(DMW) and basic studies on SCC initiation in LWR

environments [3]. The technical background and

the objectives of the individual sub-projects were

discussed in detail in the SAFE project proposal

[3]. SAFE also contributes to the maintenance of

Table 1:

Topics of

sub-projects of the

SAFE research program.

Sub-project Topic Share

SP-I Environmental effects on rapid fracture and tearing resistance 25%

SP-II

Environmental effects on fatigue initiation & short crack growth in

stainless steels & Ni-alloys under PWR & BWR/HWC conditions

30%

SP-III SCC in dissimilar metal welds and Alloy 182-RPV interface region 20%

SP-III SCC initiation in austenitic Ni-base alloys & low-alloy steels 25%

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ENSI Erfahrungs- und Forschungsbericht 2012