The high burn-up disk irradiation test, IFA-655.1: results after three cycles of irradiation

At high burn-up (greater than 60 MWd/kg U), the region encompassing the outer 100 μm or so of fuel pellets exhibits a different structure from that in the bulk of the material - the high burn-up structure (HBS). The role of this rim region in reactivity transients is not well understood. The high burn-up disk irradiation test in IFA-655 commenced in August 2001, with the objectives of studying properties of the HBS in UO2 and MOX fuels. The test is divided into two phases: a steady base irradiation period (up to 100 MWd/kgHM, to produce fuel with the HBS) and a fast power transient sequence (to study fuel elongation and rod internal pressure). The materials being irradiated are UO2 and MOX disks, operating at temperatures of 500 and 700 C. Measurements include fuel temperature, elongation and open porosity (quantified by the surface to volume (S/V) ratio), and rod internal pressure. After three cycles of irradiation the rig has now reached an average burnup of 52.61 MWd/kgOxide. Several short-lived fission gas measurements have been performed each cycle. Only MOX rod 12 has shown significant rises in the S/V and recoil R/B values. This is likely to be connected with the increase in fuel temperature, and associated fission gas release, which occurred after the reactor restart in late December 2002. To date four 'direct' 13I measurements have been performed on fuel rods 7 and 11. Both rodlets operate at similar temperatures ∼700 C and currently show similar values for the release fraction, although there is some over estimation of 13I release by the scaling method compared to the direct measurements. So far the pressure data (rods 1-6) indicate that no significant FGR has taken place in any of the fuel rods. U pto a burn-up of approximately 10-15 MWd/kgOx, all rodlets show evidence of fuel column shrinkage caused by densification. The amount of which depends upon grain size and operating temperature. All fuel rods have similar fuel swelling rates (0.2-0.3 %/10MWd/kgOx) with the exception of heterogeneous MOX rod 6 (0.7 %/10MWd/kgOx). (Author)