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Author:Luumi, Lauri
Title:Effect of weld defects on mechanical properties of spent nuclear fuel canisters
Hitsausvirheiden vaikutus käytetyn ydinpolttoaineen kuparikapselin mekaanisiin ominaisuuksiin
Publication type:Master's thesis
Publication year:2014
Pages:70      Language:   eng
Department/School:Insinööritieteiden korkeakoulu
Main subject:Koneenrakennuksen materiaalitekniikka   (K3003)
Supervisor:Hänninen, Hannu
Instructor:Vilaca, Pedro
Electronic version URL: http://urn.fi/URN:NBN:fi:aalto-201408212440
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto     | Archive
Keywords:FSW
EB
DIC
copper
FSW
EB
DIC
kupari
Abstract (eng):This thesis is a part of the KYT-project which studies the safety of the disposal canister for spent nuclear fuel in Finland. The current plan is to encapsulate the spent nuclear fuel elements into a canister with a cast iron insert and a copper overpack. The canister is the primary barrier isolating the nuclear waste from the environment and it has to be welded. Two welding methods have been considered, friction stir welding (FSW) being the primary choice and electron beam welding (EBW) as back-up.

Creep tests indicate that fracture strain for copper reduces with time and that strain localizes especially to EB-welds. The canister needs to stay intact at least 100 000 years in the repository. With enough reduction of creep strain and high enough strain localization to the weld, creep may threaten the long-term integrity of the overpack.

The effect of weld defects on mechanical properties of the overpack and strain localization is studied by conducting tensile tests with digital image correlation using mock-up welds and defected specimens. Strain rates 10-5 and 4*10-7 1/s are used.

The results indicate that the lower strain rate has a negative effect of about 4 % on tensile properties. For FSW, the strain values are almost twice as high compared to EB. In the EB-weld specimens, strain localized immediately to the weld. Two different EB-welds with defects were tested and one showed no reduction of tensile properties while the other one did. A wormhole defect in an FSW-weld reduced strain values by 40 - 50 %.

The general assumption has been that any stress concentration will be blunted by the ductile copper and not significantly decrease the mechanical properties. However, according to the present study, the defect type appears to have a significant effect on whether it reduces tensile properties.
Abstract (fin):Tämä diplomityö on osa KYT-projektia, joka selvittää käytetyn ydinpolttoaineen loppusijoituskapselin turvallisuutta Suomessa. Nykyisen suunnitelman mukaan käytetty ydinpolttoaine sijoitetaan valurautaiseen sisäosaan kuparivaippaan. Tämä kapseli on ensisijainen este ydinjätteen ja ympäristön välillä ja se pitää hitsata tiiviiksi. Kahta hitsausmenetelmää on tutkittu: kitkarappihitsausta (FSW) ensisijaisesti ja elektronisuihkuhitsausta (EBW) varalla.

Virumiskokeet osoittavat kuparin murtumisvenymän alenevan pitkillä koeajoilla, ja että muodonmuutos paikallistuu etenkin EB-hitseihin. Kapselin tulee säilyä tiiviinä vähintään 100 000 vuotta. Pitkäaikainen turvallisuus voi vaarantua, jos virumisvenymä alenee liiaksi ja venymä paikallistuu hitsiin voimakkaasti.

Hitsausvirheiden vaikutusta kanisterin mekaanisiin ominaisuuksiin ja venymän paikallistumiseen tutkittiin vetokokeilla ja optisella venymämittauksella ehjillä ja hitsausvirheitä sisältävillä näytteillä. Käytetyt venymänopeudet olivat 10-5 ja 4*10-7 1/s.

Tulokset osoittavat matalamman venymänopeuden alentavan mekaanisia ominaisuuksia 4 %. Venymäarvot ovat melkein kaksi kertaa korkeammat FSW-hitsatuille näytteille kuin EB-hitsatuille. Venymä paikallistui välittömästi EB-hitseihin. Kahta hitsausvirheellistä EB-hitsiä testattiin, joista toisen ominaisuudet olivat alemmat kuin ehjän, mutta toisen eivät. Madonreikä tyyppinen vika FSW-hitsissä alensi venymäarvoja 40 - 50 %.

Yleinen oletus on ollut, että jännityskeskittymät aiheuttavat sitkeässä kuparissa vain tylpistymistä eikä huomattavaa mekaanisten ominaisuuksien laskua. Tämän tutkimuksen perusteella hitsausvirhetyypillä on merkittävä vaikutus mekaanisten ominaisuuksien heikkenemiseen.
ED:2014-08-03
INSSI record number: 49534
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