Supervisor at Vattenfall: Bertil Josefsson.
Dimensional stability is an important aspect of fuel mechanical design and licensing of new fuel designs for nuclear power plants. Dimensional changes within the reactor can affect the safety margins against overheating of the cladding and the pellets, therefore it is crucial that the dimensional changes are kept to a minimum.
The profits per produced kilowatt hour continue to decrease for the Swedish nuclear power plants. Some reactors are even operated with a calculated loss. To reduce fuel costs and thereby improve the profit some reactors are operated with higher uranium enrichment, so that the assemblies can reach a higher average burnup. This increases the neutron fluence to the pellets, cladding an spacers. Fluence is the parameter that most strongly affects dimensional changes within the reactor.
This master thesis was carried out at Vattenfall Nuclear Fuel in Solna during 2015. The purpose of this study is to evaluate the different dimensional changes in the fuel designs operating in Ringhals 2 ,3 and 4. The gained results may improve possibilities to verify if fuel assemblies with observed rod bow are safe for continued operation.
Inspection videos called visual inspections and 4-camera videos are captured each year in each of the three reactors. From these videos the distance to the top and bottom plate was measured as well as the magnitude of the rod bow. The results show that Ringhals 2 fuel designs R2L2b and R2L1 make it possible for rod growth through the uppermost grid. This has been achieved by lesser grid spring force and at the present burnup no severe rod bow is observed for either of the fuel designs. In Ringhals 3 and 4 the fuel designs R34L1 and R34L2 show similar trends in rod growth and have the same magnitude of rod bow though it appears in different spans in the assembly.
A comparison of the major rod-to-rod gaps and the adjacent rods axial growth has led to the conclusion that rod bow is driven by manufacturing differences within the rods and spacers.