Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

an independent expertise and to the education of

young specialists in this safety-critical field. Furthermore,

the generated know-how is made available

to ENSI for expertise work and on-call projects.

Figure 1:

New electro-mechanical

loading system for

EPFM tests in hightemperature

water.

3. Performed Work and Results

3.1. Sub-Project I – Environmental

Effects on Rapid Fracture

Fracture toughness and tearing resistance are material

properties, which not only depend on microstructure

or loading conditions (e.g. strain rate

or constraints) but are also strongly influenced by

the environment in which the cracking occurs. Except

for temperature and irradiation, the effect of

environment on fracture behaviour of PPBC has

not been taken into account in the nuclear power

industry. There is now growing experimental evidence

that the fracture resistance of most structural

materials might be degraded by reactor coolant

(hydrogen) effects in the LWR operating regime

[3–7]. Apart from Ni-base alloys, it is unclear if the

environmental effects at the higher temperatures

correspond to a real reduction of toughness or to

rather fast SCC crack growth, which appears as

sudden fracture, or if they are related to plastic

collapse because of violation of small scale yielding

conditions, loss of constraints and dK/da effects in

small-sized specimens.

Hydrogen pickup in structural materials in LWR

occurs due to contact with hydrogen containing

reactor coolant (hydrogen from radiolysis and intentional

additions) and corrosion reactions. The

hydrogen level reaches equilibrium bulk concentrations

of several ppm within a few weeks or

months at 300 °C, which is high enough to affect

their mechanical properties [3–7]. Although

the hydrogen content in primary pressurised PWR

water is significantly higher than in BWR coolants,

similar or even higher concentrations of absorbed

hydrogen occur in BWR components, especially

in crevices/cracks with aggressive occluded crevice

chemistry.

This sub-project aims to establish the role of the

environment and hydrogen on the fracture and

mechanical behaviour of LAS and SS in the LWR

temperature regime and identify critical combinations

of metallurgical, environmental and loading

conditions, which may result in significant environmental

and hydrogen effects. A literature survey at

the beginning of the project shall summarise the

state-of-the-art and help to identify knowledge

gaps and the systems with highest safety concern.

Within the first project year the available literature

was collected and a first screening performed. ENSI

was informed on the interim results at the semiannual

project meeting [8]. A detailed analysis will

follow. Furthermore, an upgrade of the loading

system in one of our high-temperature water loops

with a higher load capacity of 100 kN (Figure 1)

and new data acquisition and control system was

performed. The system is fully operating since

June 2012 and is used for elastic-plastic fracture

mechanics (EPFM) tests in high-temperature water

and SCC tests with DMW specimens in high

K I -range of up to 100 MPa⋅m 1/2 . Additionally, a

successful application was made to the PSI Fellow

Program (EU-Cofund) [4]. The awarded Post-Doc

S. Roychowdhury from BARC (India) has experience

with SCC, environmental effects on fracture

and high-temperature water loops and will work

on this sub-project for the next two years. This will

allow a more systematic and extended experimental

study [4] than originally planed and is thus a big

asset for the whole project.

3.2. Sub-Project II – Environmental

Effects on Fatigue

The possibility of reactor coolant effects on fatigue

of LWR structural materials is undisputed, but

their adequate implementation in fatigue design

and evaluation procedures is still not satisfactorily

solved. This sub-project aims to contribute to

the experimental basis for such Code modifications

and is a logical continuation of the work in

KORA-II [9]. The special emphasis in SAFE is placed

ENSI Erfahrungs- und Forschungsbericht 2012 131

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