3. DESCRIPTION OF EXPERIMENT
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.
The experimental programme consisted of refabricating 8x8 BWR segment rods, which had been base irradiated to a burnup of about 35 MWd/kgU in Swedish power reactors, and pressurizing the rods with a gas mixture to simulate high end-of-life internal pressure due to fission gas release. The refabricated test rods, as well as a reference rod, were irradiated in the R2 reactor at fairly constant linear heat ratings of ~22 kW/m, for times in the range of 1200 to 2400 hours.
The effective test matrix was somewhat reduced due to the failure (leakage) of some of the test rods, so that only one high overpressure rod , (rod RX, internal overpressure relative to system pressure approximately 14 MPa) and one low overpressure rod (rod R4 2-5 MPa overpressure) were successfully tested together with the unopened reference rod (rod RR). The test rod with the highest overpressure had a measured diametral cladding outward creep strain of 6 microns after 733 hours, and 11 microns after 1634 hours irradiation, with no apparent primary creep. This in fact exceeded the expected pellet diameter increase attributable to fuel matrix swelling, since the average solid fission product swelling rate of about 0.9% per 10 MWd/kg U measured in the fuel would have only caused a fuel pellet diameter increase of 3.2 microns. The measured cold gap (both the compressed and relocated pellet to clad) in the high overpressure rod increased by 15 microns during the test irradiation. However, noise analysis measurements showed that fuel-clad mechanical contact during constant LHR operation was maintained. They also gave no indication of any significant deterioration in the gap conductance. The hydride orientation was not determined in reference material, so that hydride reorientation in the high overpressure rod could not be assessed. No difference in the cold pellet-clad gap, and no indication of cladding hydride reorientation, could be ascertained for the test rod with a lower overpressure, compared to the non-pressurized reference rod. The measurement precision was too poor to determine clad creepout in the low overpressure rod.
The fission gas release and fuel microstructure results did not indicate any enhancement in the fuel temperature as a result of overpressure, for either the low or high overpressure rod.
The project thus confirmed that there is no unexpectedly fast creepout in BWR fuel rods after stress reversal due to end-of-life rod overpressure, and that a fuel rod can be operated at a linear heat rating of at least 22 Kw/m for more than 2 months above the pressure giving a clad creepout rate equal to the solid fuel swelling rate, without any apparent detrimental effect.