Copper canister lid welded – everything went as planned in the matryoshka
Lid welding tests on the copper canister intended for the final disposal of spent nuclear fuel mean good news to Posiva. Posiva has not yet selected the final method for the closure of the final disposal canisters. Instead, the development work is still in progress. Electron beam welding is Posiva's reference method in the licensing.
What did the copper canister lid closure welding tests carried out last year indicate?
"Based on the testing results, it can already be stated that the electron beam welding (EBW) method can be used, without interruptions, to close the canister intended for final disposal," says Timo Salonen, Development Engineer at Posiva. The testing did not find anything that would cause problems for the welding process. The vacuum level and temperatures inside the canister increased more than expected but did not cause any disruptions to the welding. Component installation was performed as planned, and the equipment functioned as intended.
The vacuum level required by the welding between the copper canister and the insert was met very easily. The actual tension began when the welding of the lid on the canister was started. If the vacuum level would have decreased excessively, welding would have been interrupted and the closure would have failed. A slight increase of the vacuum level was identified during welding between the insert and the copper canister, but the approximately 40-minute welding cycle functioned without disruptions.
The challenge for the implementation was that the measurements had to be made inside the vacuum chamber inside which there was another vacuum tank, i.e., the copper canister, and inside the canister there was the canister insert, and inside the insert there was air pressure. All this started to resemble a matryoshka.
What was the goal for the closure testing in 2011?
"The goal was to find out what kind of impact the canister insert has on the canister closure, and to ensure the tightness requirements for the insert during welding.
In closure welding, possible disturbing factors must not compromise the weld quality. In other words, the production processes must not be sensitive to disruptions."
Why does Posiva carry out welding development work?
"The goal is to obtain a weld that meets the requirements for long-term safety. The main requirements are mechanical durability and corrosion endurance. The reliability of the process and the ability to provide good quality are also important.
Regarding production technology, it is essential that for welding, copper parts of a sufficiently high quality can be manufactured using conventional workshop methods. The weld design must be such that it is easy to implement non-destructive tests (NDT). It must also be possible to carry out welding in the encapsulation plant by taking applicable official regulations into account.
So far, the welding development has focused on the development of the actual welding process. The goal has been to achieve a flawless weld and a high disruption tolerance for the welding process."
For how long has welding development work already been carried out?
"The welding development work was already started in the early 1990s, that is, actually during the time of TVO's nuclear waste office, before Posiva was established.
The cooperation in Finland commenced as a joint project by Outokumpu Poricopper and Posiva, where Poricopper implemented the development work related to the copper canister welding and manufacturing. The then aviation motor maintenance unit of Valmet (the current Patria Aviation) was involved in the canister welding development.
Initial testing had been implemented and two lids had been welded by 1997. Based on these experiences, Posiva and Poricopper jointly welded a full-scale canister of two plates in 1998–2000, and a lid was also welded to it. The canister is on display at the Olkiluoto visitors' centre.
In 2001–2003, the cooperation continued with Poricopper (the current Luvata).
In 2003, Posiva and Patria Aviation signed a long-term partnership agreement, where Patria's electron beam welding equipment was modernised and the efficiency was increased to meet the level required for Posiva's work. The actual welding development work commenced at Patria in autumn 2004. About 50 copper canister lids have been welded so far.
How will the development work continue from now on?
"The canister included in the testing last year will be disassembled and the lid weld will be delivered to a non-destructive test. In addition, test samples of about 10–20 cm will be taken from the lid and used as a basis for investigating the weld.
The equipment of Patria Aviation does not enable a full-scale closure test. The equipment for such a test can be obtained from Central Europe when necessary.
In future, the performance of the welding process will be specified further, that is, welds of a good quality will be produced in a reproducible way, and the weld characteristics will be specified further particularly in regard to flawlessness and residual stresses of the weld. The application for a construction license and the selection of the welding method are the next milestones."