RD&D-Programme 2004 - SKB

2004
2005
Process model
welding methods
Verification
SR-Can
Identification of
important parameters.
Development of
process system.
Investigation
of parameters
and interaction
effects.
Determination of
process window.
Choice of
welding method
NDT Reliability
(BAM)
Evaluation
Method
development
Demonstration
Evaluation
Data for SR-Can
Evaluation of
NDT. Detection
capability and
measurement
accuracy. Interaction
of different
systems.
NDT, metallography,
mechanical
and long-term
properties.
Statistical
methodology,
testing plans.
Trial fabrication
and NDT under
serial production
forms.
Statistical
methodology.
Figure 6-1. Logical connections and time sequence between the milestones in sealing technology.
For the first two points (verification and demonstration of the welding methods), statistical
methodology will be used for test planning and evaluation, and data from the programmes will
be used in the safety assessment. For more detailed information, see sections 6.2 and 6.3 /6-1/.
The logical connections and time sequence between the milestones are shown in Figure 6-1.
6.2 Electron beam welding
During the past two decades, SKB has developed the technology for electron beam welding at
The Welding Institute (TWI) in Cambridge, UK /6-2/. The development work at the Canister
Laboratory began in 1999. Technical details regarding this system and process development
have been reported through 2001 /6-3/ and 2003 /6-4/.
An important part of the development work is determining how often discontinuities occur in
the welds and under what conditions they are formed. The goal is to design the welding process
in such a manner that the risk of getting defects in the weld is very small. As mentioned in
SKB’s plan of action, see Appendix A, the preparatory work for an application for a permit
to erect an encapsulation plant is focused on a copper canister with a wall thickness of five
centimetres.
6.2.1 Welding equipment
In electron beam welding, a gun is used to generate the electron beam. The gun was developed
by TWI and is designed for use at reduced pressure. The design is unique and consists of an
indirectly heated diode gun and a switch mode power source. Table 6-1 shows technical data
for the welding machine. Figure 6-2 shows a schematic drawing of the electron gun.
The cathode is heated and starts to emit electrons. The electrons are accelerated to about 2/3 of
the speed of light by a high acceleration voltage. The electrons pass through a hole in the anode
and on through the gun. The electron beam is focused by the upper lens coil and passes through
the nozzles N1 and N2, after which it is focused on the workpiece by the lower lens coil. The
upper alignment coils and the lower alignment coils centre the beam exactly in the centre of
the small bores in nozzles N1 and N2. Thermocouples monitor the temperature of the nozzles
N1, N2 and N3. The signal from each thermocouple has two limits: one for alarm and one for
system trip /6-4/.
66 RD&D-Programme 2004