3. DESCRIPTION OF EXPERIMENT
The STUDSVIK ROPE II Project was an internationally sponsored research program, with the aim of studying the performance of PWR fuel rods with an end-of-life rod internal pressure exceeding the system pressure.
The project focused on clad creepout as a function of overpressure, time dependent changes in the fuel rod thermal conductance (in particular behaviour possibly indicating 'clad lift-off'), and general performance during extended operation with rod overpressure.
A Siemens-KWU segmented rod (14x14) from the Obrigheim reactor, with a burnup in excess of 40 MWd/kg U, was made available to the Project as one set of three test rodlets (K rods). The other set of three test rods (A rods) were refabricated from an ABB Atom 17x17 rod (Westinghouse type) irradiated to a similar burnup in the Ringhals 3 reactor.
Three rodlets were 'refabricated' from the ABB Atom rod. They were filled with a simulated fission gas. For two of the rodlets the fill pressure was chosen to give high and intermediate overpressure at power. For the third rodlet the fill pressure was low giving underpressure at power. One of the Siemens-KWU segments was used unchanged as test rod. Two other segments were "refabricated" and filled with the simulated fission gas
mixture to give high and intermediate overpressures.
The six rodlets were irradiated in the R2 reactor for eight cycles under typical PWR conditions. The first and the last cycles were used for noise measurements. The six intermediate long term cycles were the main part of the experiment for studying possible changes in the fuel rodlets due to operation with high inner overpressure. The cladding diameter changes were monitored by profilometry in the shutdowns between the 18 day operating cycles.
After the irradiation nondestructive and destructive examinations were performed on the rodlets in order to see if the R2 irradiation had resulted in fuel or clad changes. The major results were
1. The experiment was successfully performed as planned in its entirety.
2. There was no fuel rod failure.
3. Good overall performance of fuel and cladding during extended overpressure operation was shown by detailed PIE.
4. Clad creepout was measured corresponding to rod overpressure.
5. Although the clad creepout exceeded the pellet diameter increase expected due to solid fission product swelling there was no indication of a decrease in the gap conductance in the overpressure rods.
6. The clad creepout did not influence the fission gas release during the R2 irradiation.
7. Mechanical contact between fuel and cladding was maintained during overpressure operation at a constant LHR of about 20 kW/m.
8. Some hydride reorientation in the cladding of all the overpressure rods was observed - corresponding to overpressure.