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The fuel segments for the high burn-up integral rod behaviour test IFA-597 were taken from fuel rod 33-25065, which was irradiated in the Ringhals 1 BWR for approximately 12 years. The irradiation of this rod and its sibling rod 33-25046 was performed in two stages. During the first irradiation, 1980 to 1986, the rods were part of Ringhals assembly 6477 and an approximate rod averaged burn-up of 31 MWd/kgUO2 was reached. The rods were then placed into fuel assembly 9902 for a second period of irradiation from 1986 to 1992. The location of the fuel rods 33-25065 and 33-25046 in this assembly were in positions 9902/D5 and 9902/E4 respectively. A final rod averaged burn-up of 52 MWd/kgUO2 was achieved. The burn-up at the location of the Halden segments was estimated as 59 MWd/kgUO2, well beyond the the formation of High Burn-up Structure (HBS) formation at the pellet rim. At the rim, the burn-up was estimated as 130 MWd/kgUO2.
After commercial irradiation, PIE was performed at Studsvik. Inner and outer clad oxide thickness measurements were 42 and 5 microns respectively. The measured cold rod diameter varied between 12.20 and 12.25 mm, thus only a small amount of creepdown had occurred from the original diameter of 12.25 mm. Cold gap measurements were taken by diametral compression of the clad onto the fuel. The stiffness changes twice during these measurements, the first (relocated gap) associated with the onset of pellet fragment movement, the second (compressed gap) when the fragments are together and the pellet is compressed. For these rods, the compressed diametral gap was measured as 30 microns. This is in agreement with the pellet and cladding being in contact during the final irradiation cycle, i.e., at ~12 kW/m. FGR measurements were made after puncturing and values of 2.5%-3.3% were calculated from the extracted gas. The uncertainty is due to different methods of calculation. Ceramography showed a normal crack pattern and no evidence of grain boundary porosity in the central regions of the pellet. The back scattered electron image of the pellet periphery showed an area of high porosity as would be associated with re-structuring caused by formation of the HBS. The depth of this region was typically 200 microns with traces of HBS at greater depths.
After transfer to Kjeller hot laboratories in Norway, three segments were cut from rod D5 33-25065. All were from regions of uniform burn-up as measured by axial gamma scanning. The sections were identified as D5:1, rod 7, D5:2, rod 8 and D5:3, rod 9. Whilst drilling rod 7 to insert a thermocouple, the drill broke and thermocouple insertion was abandoned. The rod was finally sealed with a clad elongation detector (EC). Rods 8 and 9 were successfully drilled and thermocouples inserted. They were then sealed with pressure transducers (PF).
Rods 8 and 9 were loaded into positions 2 and 5 in IFA-597.2 (second loading) and irradiated in Halden for some 20 days in July 1995. After a small number of power ramps, rod 9 failed and the assembly withdrawn. During this time, useful data were generated on centreline temperature as a function of power.
Rod 9 was removed and replaced by rod 7. The assembly was returned to the reactor as IFA-597.3 (third loading); the irradiation started in January 1997 and continued to May of that year having accrued a further ~2 MWd/kgUO2. Data obtained included, centreline temperature as a function of power and burn-up, (rod 8), FGR from the increase in rod internal pressure (rod 8) and clad elongation (rod 7).
The assembly was discharged and transported to Kjeller for PIE. FGR of 12.6% and 15.8% were measured from puncturing and gas extraction from rods 7 and 8 respectively.
Further PIE was carried out on rod 8 after it had been transported to Studsvik. Here, additional ceramography was performed, axial gamma scans, radial scans of selected fission products and the radial porosity distribution.
On account of the high burn-up of these rods, the data are particularly valuable for modelling fuel temperatures, FGR and PCMI.
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Keywords: burnup, cladding, fission gas release, fuels.