RD&D-Programme 2004 - SKB

Conclusions in RD&D 2001 and its review
SKB’s conclusions in RD&D 2001 can be summarized in the following points:
• Acceptance criteria will be established for all parts of the canister, including the welds. It
must be possible to determine by means of the testing methods whether the acceptance criteria
for e.g. the welds are satisfied. SKB is producing a derivation of the acceptance criteria.
• The consequences of larger defects than those stipulated by the acceptance criteria are being
examined. Established acceptance criteria regarding defects must be able to be verified by
NDT.
• The ongoing work of choosing suitable equipment and methodology is continuing.
• The lessons learned from the trial fabrication of all canister parts will be applied to and
influence the further development of the factory. The work of investigating and establishing
acceptance criteria and testing methods will make it possible to specify modified equipment
for NDT and other quality inspection more precisely.
SKI regards the ongoing work with design premises for the repository and acceptance criteria
for the canister as very important. Delays in this work may delay other parts of the canister
work. The design premises should be a controlling factor for many activities. They may need to
be revised after the consequence analyses that must be performed to show that design premises
and acceptance criteria are adequate.
The work with acceptance criteria must be given high priority.
Newfound knowledge since RD&D 2001
Work is being pursued to gather data as a basis for assessing the defects that can arise in the
various fabrication steps for fabricating copper canisters and cast iron inserts, and finally for
sealing the finished canister in the encapsulation plant. A project is being pursued in cooperation
with the German Bundesanstalt für Materialforschung und -prüfung (BAM) on the reliability of
nondestructive testing for inspecting sealing welds.
Based on knowledge about corrosion of copper in the deep repository environment and
knowledge from the fracture toughness measurements on copper, we can formulate preliminary
criteria for permissible discontinuities in the copper shell, see section 5.1. In summary, a coherent
discontinuity from the surface inward, a so-called surface-breaking discontinuity, does not
pose any threat to the canister’s integrity in the deep repository over long periods of time. The
corrosion assessments also show that a copper cover of a few millimetres is fully adequate to
guarantee a canister lifetime of over a million years /5-4/. There is still some uncertainty regarding
how a possible influx of oxygenated water during future ice ages will affect corrosion. Our
judgement is that a copper cover of 1.5 centimetres will provide full corrosion protection, even
allowing for uncertainties regarding the consequences of ice ages. SKB has set 3.5 centimetres
in radial extent as a preliminary criterion for the largest permissible discontinuity in both sealing
welds and the rest of the copper shell. This means that the minimum permissible copper cover is
1.5 centimetres at a copper thickness of five centimetres.
As mentioned in section 5.1, a project is under way concerning probabilistic analysis of
canister strength. In parallel with this project, computer simulations are being done at the
Swedish Foundry Association to identify those areas in the canister insert where the probability
of casting defects is greatest. A survey of actual casting defects in inserts and testing of methods
for nondestructive testing has also been commenced. These projects will provide a basis for
judging the damage resistance of the insert, which in turn provides a basis for establishing
acceptance criteria.
Programme
The work of probabilistic analysis of canister strength will be concluded during 2004. The
results will serve as a basis for the optimization of technical specifications and the formulation
of acceptance criteria.
48 RD&D-Programme 2004