Cool results from the red-hot cast testing
The casting of the insert of one final disposal canister requires almost 20 tonnes of melted iron. The pour point of the melted iron exceeds 1,300 degrees Celsius. For years already, inserts have been cast for development purposes at the Metso foundry in Jyväskylä, and they have been studied carefully through international cooperation. The cast testing of the inserts has reached the required material characteristics.
Development Engineer Leena Nolvi of Posiva, what is the purpose of the insert of the final disposal canister in the entire final disposal solution?
"As indicated by its name, the insert is installed inside the copper canister. It provides the mechanical strength for the canister. The spent fuel of TVO's and Fortum's active power plants require different inserts: a BRW inner part for the spent fuel of the existing plants at Olkiluoto and a VVER insert for the spent fuel of Loviisa plants."
What is the cast process of the canister's inserts like?
"Approximately 18 tonnes of iron is melted to manufacture a BWR insert and about 15 tonnes for a VVER insert. The pour point of the melted iron is more than 1,300 degrees Celsius. The casting is performed by pouring the melted iron from the ladle into the sink on the mould, and then conducted through channels to the mould. The casting takes about two minutes.
Before removing the mould, the piece is allowed to cool down in the mould until the temperature is about 300 degrees Celsius. This takes approximately five days.
Once the mould has been removed, there is a waiting period of about a day before the cleaning of the piece begins. After cleaning, the piece is milled to its final dimensions. A milled BWR insert weighs 13.1 tonnes and a milled VVER insert 10.1 tonnes.”
How many inserts have been manufactured?
"Nodular cast iron inserts for canisters have been cast at the Metso foundry in Jyväskylä in up-running casting since 2009. The up-running casting method was adopted to improve the material characteristics. Four BWR inserts have been manufactured with the up-running casting method in Finland. Otherwise, BWR inserts have been manufactured right from the start of the canister development work.
The first VVER insert was cast in autumn 2010 and another one in early summer 2011. The goal is to cast one more VVER insert by the end of 2011."
How are cast canisters studied and what kind of results have been reached so far?
"Mechanical testing, such as tensile strength testing and investigations of the microstructure are performed on the piece in Jyväskylä. A calliper test is also performed there, that is, a prototype of the fuel rod is used in order to test that the rod will definitely go in.
After this, the inserts are transported to Oskarshamn in Sweden where ultrasound scanning is performed on them to verify the integrity of the insert and the straightness of the fuel channels. The mechanical characteristics of both inserts are good.
The required material characteristics of BWR inserts have been met, but reaching the required straightness of the fuel channels in the BRW insert still requires further development. The manufacturing of a BWR insert is slightly more demanding than that of a VVER insert due to the square pipes and more extensive length of the steel cartridge.
Based on initial results, the most recently manufactured VVER insert met the manufacturing requirements. The tension remaining in the insert after casting has now been studied for the first time. The studies are conducted in Bristol in the UK. Actual study results are expected during 2012, but based on initial results, it seems that the inserts do not have any major tension."
What kind of international cooperation has Posiva carried out in cast development?
"Posiva has conducted cast testing with SKB in Sweden and Germany. SKB is responsible for final disposal in Sweden. We carry out development work related to canister manufacturing technology in partnership with SKB because our canister designs are similar."
How are the cast testing results utilised?
"We are preparing a final disposal canister production line report for the construction license application of the final disposal plant. The report addresses the canister design, manufacturing and inspection as well as the welding of the cover.